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8th EEGS-ES Meeting
- Conference date: 08 Sep 2002 - 12 Sep 2002
- Location: Aveiro, Portugal
- ISBN: 972-789-071-7
- Published: 08 September 2002
1 - 100 of 131 results
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Cave detection with GPR and seismic methods
Authors B. Neducza, L. Hermann and M. Pattantyús-ÁIn the last few years building sites have been extraordinarily overestimated on the hilly part of Budapest, where protected caves can be found. Thus, new buildings are built on the unbuilt areas, while on the already built-up houses are enlarged. If we close the swallers we stop the growth of voids and stalagmites. It’s important to know the size, position and depth of natural voids and cavities before building or reconstruction. We used Ground Penetrating Radar (GPR) and shallow seismic measurements to detect these objects.
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Engineering geological and geophysical investigation at the dam site, Cine/Turkey
Authors S. Sirri Seren, J. Kleberger and O. SimsekCine Dam in South West Turkey will be the first major roller compacted concrete (RCC) dam in Turkey and with the height of 135 m one of the highest in the world. When completed it will perform a number of functions such as water supply, power generation and flood control. Excavation works of dam foot print area are almost completed. The dam is located along a narrow section of the Cine river valley. The cross section along the designed dam axis shows a slightly asymmetrical V-shaped profile, with a steeper left than right bank. The area of Cine dam is dominated by gneiss bedrock, which belongs to the Menderes complex. Foliated augen gneiss prevails, with subordinate massive gneiss beds of more homogeneous texture and highly foliated and intensively sheared gneisses with micaceous schists. The rock mass is gently folded, intensively jointed and locally intensively faulted with the most prominent faults dipping almost vertically and persisting several kilometres. Within the faults and adjacent to them the rock mass is largely disintegrated and locally rich in kaolin. Grain sizes in the core zones of faults are dominated by sand to silt. Site investigation for Cine Dam was developed and executed in several phases. Main aim of the geophysical investigation in the dam site area is to define the boundary of the engineering geological units, location of fault zones, which are covered by slope debris and soils, dynamic elastic parameters of the materials in the core zones as well as in the damage zones of the faults, velocities and dynamic elastic parameters of the soil and rock mass to be encountered at both side of the dam foot print area. Geophysical investigation was correlated with the core drill holes, which were carried out during same investigation campaign, and geological mapping results.
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Spectral analysis of surface waves and seismic refraction for the inspection of tunnel walls
Authors O. Abraham, J. Lagarde, Ph. Côte and O. MagninIn many fields, e.g. civil engineering, mining engineering and underground disposal, it is very important to be able to quantify the extension of the rock mass zone disturbed by underground excavations. In multi-layered plan media a combination of Seismic Refraction (SR) and Spectral Analysis of Surface Waves (SASW) can be used to determine shear modulus and compression modulus profiles. Both techniques cannot be transferred to circular tunnels without questioning the limit of a plan geometry approximation. Indeed, in the following we show that the curvature of the tunnel cannot be neglected even for a thin damage zone compared to its radius. Special attention is given to the SASW for this technique will work even if the tunnel walls are covered with a concrete lining. Both experimental and numerical results are presented. The error introduced by the curvature on the SR interpretation is of the order of precision required for the investigation but its influence does not show clearly on the experimental data. For the SASW, the influence of the curvature on the experimental data cannot be ignored for the global trend of the dispersion curves is greatly changed. The experimental SASW dispersion curves are matched with numerical dispersion curves calculated with the finite element code CESAR_LCPC in an error and trial process.
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Multimodal interpretation of surface wave data
Authors S. L. Valentina, S. Claudio and F. SebastianoSurface Wave Methods become quite popular tools for geotechnical characterization since they supply the stiffness profile of the sites with a cost effective testing procedure. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted (Nazarian and Stokoe, 1984; Park et al., 1999). Independently by the chosen procedure, the inversion is mainly carried out assuming that the experimental dispersion curve is actually the first Rayleigh mode. It is a rather diffuse commonplace among geophysicists that, for normally dispersive site, the first mode is in fact prevailing and that only for inversely dispersive site the role of higher Rayleigh modes has to be taken into account (Tokimatsu et al., 1992). By using a multi-modal modelling it can be easily shown that higher Rayleigh modes contribute to the effective dispersion curve not only for inversely dispersive stratigraphies but also in presence of quite high velocity contrasts. Beside this, since one of the main advantages of surface wave methods is to be effective even in case of velocity inversion, when for instance seismic refraction fails, it is quite important to overcome the limitation due to the influence of higher modes, considering them in the interpretation process. Furthermore it can be also shown that the analysis of higher modes can help to obtain much reliable information also in cases of normally dispersive sites (Foti et al., 2002). As far as concern the dispersion curve interpretation some further considerations are deserved. The simplest approach is to transform the dispersion curve from the phase velocity vs. frequency into the phase velocity vs. wavelength domain and to attribute the velocity values (multiplied by a factor of 1.1) to a depth approximately equal to one-third or half the wavelength. This approach, quite popular in the professional practice of surface wave testing is usually considered an approximated approach but can be easily demonstrated that it is in fact erroneous because the energy distribution with depth can strongly vary depending on stiffness contrasts among layers. A more rigorous approach, which is also widely diffuse, is to invert the experimental dispersion curve considering it as the first Rayleigh wave mode. This procedure can be effective and supplies reliable results only in those cases in which one can be sure that the first mode is prevailing within the considered frequency range. Otherwise it can lead to some degree of error in the estimation of the stiffness profile. The experience carried out at more than 50 different sites, where surface wave test results could be compared with other reference data, has shown that, in the majority of the encountered cases, to perform a reliable interpretation the influence of higher Rayleigh modes could not be neglected. Multi-modal interpretation involves the possibility of recognising experimentally the modal curves but this is usually impossible by using traditional acquisition and processing approach. Another possibility is to invert the experimental dispersion curve but, since it depends also on the acquisition layout, it has to be considered as an apparent curve (Foti et al., 2000). Furthermore its pattern can change dramatically also for quite small changes of the model parameters. For this reason is not easy to use the apparent dispersion curve in an iterative inversion process and trial and error inversion (Tarantola, 1987) can be preferred.
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Development of a new fast remote controlled 3D geoelectrical monitoring system for subsurface surveillance
More LessDuring recent years the dc-geoelectric method has been used widely to investigate the structure of landslide areas, thus gaining the status of a state-of-the-art-method in civil engineering for this However so far the dc-geoelectric method was hardly used to detect structural changes with time in active landslide areas, and no application is known to us where this method was used as a permanent online monitoring system with a sampling interval of several times per day.
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Detection and characterisation of underground cavities using high resolution seismic reflection (HRSR)
Authors L. Driad and B. PiwakowskiOld underground mining works are subjected to risks of collapse of which the assessment is a complex process. In the case of inaccessible underground mines, the difficult task for the risk assessment is to classify the already collapsed zones (no further risk) from still stable structures which may collapse in the future. The knowledge of the characteristics of the underground cavities and hosting rock mass can provide relevant information. However, accurate underground cavities detection and characterisation based on geophysical techniques are still a scientific challenge in the subsoil prospecting domain. Among these techniques, the high resolution seismic reflection is the most successful in the frame of underground cavities detection. The study presented in this paper, is a part of an extended research program aiming at prediction and controlling ground motions induced by underground cavities. The objective is to identify the most appropriate geophysical technique to localize and characterize the underground cavities at variable depths (several meters to 300 m). Thus, to meet these goals we have investigated the high resolution seismic reflection technique (HRSR). The selected test site is located close to Gréasque municipality in south France (figure 1). In this region, the underground works undertaken during the last two centuries consist of two mines: coal mine and cement stone quarries1 (marl limestone). The geological setting is characterized mostly by coal layers and massive limestone formations of the upper cretaceous with alternated coal strata of variable thickness dipping westwards.
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Experience of karst localization using magnetic resonance soundings
Authors A. Legchenko, J. M. Baltassat, Y. Albouy, J. M. Vouillamoz, M. Bakalowicz and W. Al-FaresThe main advantage of the Magnetic Resonance Sounding method (MRS), compared with other geophysical tools for water prospecting is that the MRS is sensitive only to subsurface water (Schirov, et al., 1991). Inversion of MRS field data reveals the water content and the relaxation time T with r being the coordinate vector (Legchenko, and Shushakov, 1998; Legchenko, et al., 2002). In a porous medium the relaxation time T is proportional to the mean pore size 1 (Kenyon, et al., 1997), where and V are the surface and volume of pores respectively.
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Magnetic resonance sounding method applied to catchment study
Authors A. Legchenko, J. M. Baltassat, C. Martin, H. Robain and J. M. VouillamozLogs of the water content and relaxation times T and derived from Magnetic Resonance Sounding (MRS) measurements reveal the depth and thickness of subsurface water-saturated layers. Furthermore, using experience acquired through Nuclear Magnetic Resonance (NMR) logging (Hinedi, et al., 1993; Kenyon, et al., 1997), MRS data allow estimation of the effective porosity and hydrodynamic properties of aquifers. Once calibrated (definition of empirical constants C and for a given geology), MRS provides the effective porosity )z(w)z(*2)z(T1wpC )z(wC)z(wMRS=φ, (1) the permeability )z(T)z(wC)z(k12pMRS=, (2) and the transmissivity ∫=zdz)z(kTMRSMRS, (3)
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GPR Tomography for mapping soil moisture and roots
Authors Sh. Hanafy and S. A. al HagreyFor high resolution georadar tomography inverting of arrival times or amplitude data yields a distribution of electromagnetic velocity or an attenuation-related parameter that images anomalous subsurface features, e.g., voids, moisture content or clay heterogeneities. Tomographic reconstruction technique is widely used in geophysical and geological field investigations (Bregman, et al., 1989; Daily & Owen, 1991; Cai & McMechan, 1999). Solving the tomography problem depends mathematically on solving a system of linear equations iteratively (Peterson et al., 1985). The principle of the tomographic method shows a typical ray path from the transmitter (Tx) to a receiver (Rx) Ri (Fig. 1). Multiple Tx and Rx locations yield a number of such rays crossing the intervening material in different directions. The region between Tx and Rx lines are discretized into cells and the slowness sj of the cell j is assumed to be constant over the area covered by a single cell. The recorded travel time can be expressed as integral over the ray path Each of these integrals, in discrete form, becomes one equation in the linear tomographic system that is to be inverted for velocity and/or layer shape (from travel times) or for attenuation (from amplitudes). The linear system of equations has the form: t=As, where t is the GPR time vector, A is the distance matrix connecting Tx-Rx locations, and s is the GPR slowness vector. We used this principle to develop a new GPR tomography algorithm (SeismoRad computer program) for inverting surface and crosshole data (Hanafy, 2002).
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Application of electrical imaging for borehole siting in hardrock regions of India
Authors R. Barker and T. Venkateswararao and M. ThangarajanAround seventy-five percent of India is underlain by granitic and gneissic basement rocks with a thin covering of weathered material. Many of these areas are semi-arid to arid and do not have high enough rainfall to support agriculture throughout the year. Here, groundwater is not difficult to find as the water table is generally within 10 m of the ground surface; however, the yield of boreholes is frequently low and supplies do not have long-term sustainability, so it is important to locate boreholes in the most productive areas possible. One of the methods found to increase the efficiency and effectiveness of borehole siting investigations is geophysics, with electrical imaging becoming an increasingly popular survey technique. Here we present the results of trials of a manual imaging system applied to groundwater investigations in the Dindigul area of Tamil Nadu and show how electrical imaging can provide much more meaningful and reliable data than is possible with other types of survey. It is the first use of electrical imaging in borehole siting investigations in India.
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Controlled laboratory experiment to test the validity of time-lapse electrical imaging
By J. MooreAs legislation imposes tighter constraints on the identification and remediation of contaminated land, so the demand for reliable, detailed and preferably inexpensive techniques with which to monitor subsurface conditions has increased. The electrical resistivity method is a particularly appropriate technique to implement in the study of groundwater movement and contamination, as it is sensitive to contrasts in saturation and to contrasts in groundwater conductivity. The resistivity method has additional benefits in the monitoring of groundwater conditions as it is non-invasive and provides continuous spatial information, properties lacking from all traditional hydrogeological monitoring techniques. However, interpretation of resistivity data can be ambiguous, as many subsurface models of resistivity distribution can give rise to similar results measured at the surface. A means of negating the need for a definitive interpretation of resistivity data is to examine only the changes in resistivity over time. This concept of time-lapse electrical imaging involves repeat measurements of resistivity at a time interval appropriate to the rate of change of subsurface conditions expected at the study site. It has been shown that time-lapse electrical imaging carried out during groundwater drainage (Berry, 1996) and infiltration (Barker and Moore, 1998; Hatzichristodulu, 1999) events can provide a description of the changes in saturation occurring within the subsurface. However, interpretation of these studies remains qualitative and often difficult to validate. One means of validating the fundamental concept of a technique is through the use of laboratory modeling, as it is possible to select the properties of the media under investigation and to visually record the physical processes occurring during the experiment. This study describes a laboratory experiment, designed to simulate a groundwater pumping test in a porous media aquifer, with concurrent measurements of electrical resistivity. The aim of this experiment is to validate the interpretation of electrical resistivity measurements recorded during subsurface changes in saturation and to establish the scope of this technique for other hydrogeological settings. Initially the tank is filled with a well-sorted sand to represent a homogeneous model. A body of contrasting hydraulic properties is then implanted to observe the effects of heterogeneities.
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Integration of VSP and TDEM data for hydrogeological characterisation
Authors A. Godio and C. StrobbiaThe hydrogeological characterisation often needs great detail and resolution to plan the drilling phase and to evaluate the possible interference between wells. We describe an integrated approach to analyse a complex formation of argillite and sandstone hosting an aquifer of great interested for mineral water supply: Time Domain Electromagnetic and borehole seismic methods are used.. Electromagnetic and electrical tomography measurements from the surface allowed estimating the geological set-up of the area for the optimal positioning of the borehole. The subsequent VSP (vertical seismic profiling) permitted to analyse the main fractured zones inside the sandstone: a full-wave analysis is performed to improve the resolution and reliability of the seismic data. The paper introduces some aspects of seismic data processing to separate and analyse the different wavefields involved in the VSP measurements.
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Water exploration on the Lastove Island (Croatia) by electrical tomography and seismic methods
By F. ŠumanovacWater exploration was carried out at the island of Lastovo, situated farthest from the Croatian Adriatic coast. The investigated area is situated in a tipical karst field, close to the Lastovo town, with the name Prgovo Polje (Prgovo Field). There are several exploited water wells in this area, which produce 2-3 l/s of water, but this is not sufficient because of the growing demands for water. The main goal of this water exploration was to capture more water, minimum of 5-6 l/s. Previous explorations and wells made in the sixtees were aimed at very shallow aquifers, mainly fine-grained sands, but because of the strong pumping, water has become brackish and has to be desalted by adequate technology. With respect to the fact that more water can only be captured from deeper aquifers, target rocks were fractured carbonate rocks in the bedrock of clastics. Since the infrastructure in the area of Prgovo Polje is mainly completed, the first goal was to discover new resources in this area. The secondary goal was to get the water of as low a salinity as possible, since the price of desalinization technology depends on the water salinity.
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The use of ERT for the characterization of a thermal karstic aquifer
Authors A. Bratus and D. Nieto YabarThis is a case history of an ERT survey in a karstic aquifer characterized by the presence of thermal water having a temperature > 35° C. The aim of this survey was to define the best position of a well for exploiting thermal water, in order to restore the old thermal bath. ERT was used to define: o The depth and the morphology of the carbonated bedrock o Different aquifer features o Thermal water concentration The positioning of the well defined by the ERT survey result was successful. Thermal data logging agreed with the ERT model.
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Analog modeling of streaming potential during well-pumping
More LessElectrokinetic potentials, also called streaming potentials (SP), are natural potentials between sets of points on the surface or within the soil. They are produced by the flow of a fluid, generally water, in a porous media. SP being the only surface method able to give indications on the direction of flow, it is used in several applications involving water movement, as in volcanic and geothermic areas, dam and lake leakage, landslides and well-pumping. The electrokinetic phenomenon, the relative motion of ion species producing an opposite conductive current, is well-known, as is the double layer model of Helmholtz-Smoluchowski. Furthermore, from the mathematical point of view, the thermodynamical set of equations allows the distribution of the SP to be modelized (Revil et al. (1999a, 1999b)). Although the overall theory is known, as soon as mineral species are involved, the subject becomes much more complicated. Many laboratory surveys (e.g. Ishido et al. (1981)) had been conducted in order to understand which parameters, in what manner and to which extent, act upon the production of SP. But after more than forty years of research, the authors still do not agree completely on the results. On a larger scale, field measurements, even less is understood. The main reason is that there is no way to know the exact values of the influencing parameters and, moreover, they are likely to change over distance and time. Besides, few field cases are presented in the literature, so it is difficult to know how other research groups cope with these problems. There is a gap between the laboratory and the field measurements that has been rarely studied (e.g. Sato et al. (1998)). We are trying to fill it partially by the analogical study of a scale model of a pumping-well.
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Electric and electromagnetic response patterns of fault zones in granites
More LessGroundwater percolation in granitic terrains is highly conditioned by fault zone type structures where the rock has been disrupted to a point as to increase the overall porosity and local permeability. This is particularly the case where a surface sedimentary or weathered layer is nearly absent. For practical purposes that is the situation in granitic terrains in central Portugal where the weathered layer is no deeper than 3 m. In such a situation the variations in geophysical parameters may be considered to be related to variations in the bedrock itself. The geophysical methods considered below include “classic” electrical sounding, multidirectional electrical sounding and VLF/VLF-R profiling.
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Integrated geophysical/hydrogeological study in the Danish-German border region – An Interreg project
Authors R. Kirsch, S. Thomsen, W. Scheer, W. Voss and R. FriborgAs groundwater reservoirs are crossing national boundaries transnational efforts are required to ensure a sustainable use and an effective protection of these water resources. This is the background of a Danish-German project for the mapping of aquifers and covering layers in the border region Sønderjylland / northern Schleswig-Holstein (Fig. 1). This project was funded by the European Community under the INTERREG-programme. The use of geophysical methods to establish a geological model of the project area is shown in this paper.
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Analysis of helicopterborne electromagnetic data for hydrogeological investigations
Authors N. B. Christensen and R. J. TølbøllTraditionally, frequency and time domain airborne electromagnetic (AEM) systems have been used for mineral prospecting, often in parts of the world with little environmental electromagnetic disturbance. However, the increased focus on hydrogeological investigations in many parts of the world has led to a growing interest in the possibility of using airborne systems for such purposes, even in densely populated areas (Sengpiel and Siemon 1998). This raises questions about the resolution capabilities of AEM systems and their sensitivity to disturbing influence from ambient electromagnetic noise and the presence of man-made structures such as power lines, buried cables, and fences. The data quality of earlier AEM systems was such that they were mainly used as "bump detectors" capable of indicating the presence of good conductors. Quantitative interpretation of the data was often not warranted (Huang and Fraser 1999). Traditionally, helicopterborne frequency-domain electromagnetic (HEM) data have been processed to produce iso-resistivity maps using lookup-tables, and transformations of the apparent resistivity and the centroid depth obtained from the table lookup have been used to produce pseudosection images. With the general improvement of HEM systems quantitative interpretation has become an option (Sengpiel and Siemon 1998). In general, the objectives of a hydrogeophysics survey are; detection of non-permeable boundaries of a potential aquifer, often coinciding with the clay-sand boundaries, discernment of internal structure in the aquifer and mapping of near-surface capping clays reducing the vulnerability of the aquifer. This paper presents quantitative analyses using one-dimensional (1D) models of the resolution capabilities of a modern HEM system with 5 frequencies. Because a frequency domain ground system equivalent to the HEM system does not exist we have chosen a profile oriented, multi-electrode DC geoelectrical system (CVES) with a comparable depth penetration and compared the resolution capabilities of the HEM system to that of the CVES system for a number of hydrogeologically relevant models.
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First experiments on solid transport estimation in river-flow by fast impedance tomography (SOTEBIT).
Authors L. Sambuelli, G. Lollino, G. Morelli, L. Valentina Socco and L. Bidoneayleigh (1892), studied the effect, on the overall conductivity, of particles with conductivity σ included in a medium with conductivity σ0 ; Fricke (1924) extended the Maxwell's formulas from spherical to ellipsoidal particles. Geophysics and chemistry literature is reach of studies dealing either with the conductivity of saturated grainy materials or with the conductivity of suspensions: from the work of Archie (1942) to the model of Wyllie and Southwick (1954); from the one of Hanai (1960) up to those more recent of Waxman and Smits (1968) taking also in account for the clay content. More recently the papers of Clavier et al. (1977), introducing the dual water model; of Sen et al. (1981), working with the complex dielectric; of Bussian (1983), Garboczi and Douglas (1995) and Chinh (2000) can be cited. The interest on the subject is still high, as the many papers constantly published on scientific geophysical journals demonstrate, and a consistent scientific background on the relations among fluid, grain, mixture conductivities is available. Moreover the recent, fast development of hardware and software related to electric impedance tomography (EIT) allows for the possibility of performing fast tomography acquisition and processing (Barber and Brown, 1984; Barber and Seagar, 1987) so that the so called “process tomography” has become a leading field of research (Dickin and Wang, 1996). In as much as process EIT allows for fast resistivity or capacity imaging of sections through rapid flows (Xie et al., 1992), to test the possibility of getting resistivity images crossing the rivers and containing information on the solid transport distribution becomes a worthwhile research target. This paper reports the first tests performed within a real scale model with quasi-static flow to verify the possibility of Solid Transport Estimation By fast EIT (SOTEBIT).
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Avoiding miss-interpretation of GPR measurements in forested areas
Authors H. Paasche, N. Ormuz, J. van der Kruk and J. TronickeGround-penetrating radar (GPR) is a common method for investigating the shallow underground. Subsurface reflections are usually associated with variations in dielectric properties. Unfortunately, reflections and diffractions from above-surface objects may contaminate GPR data. We have conducted a dual-component GPR survey across a shallow aquifer situated beneath a forest. Strikingly different reflection patterns are obtained for the two antenna orientations. To understand better the observed patterns, we first describe key characteristics of the dipole radiation emitted by GPR antennas. This is followed by the results of a simple modelling exercise designed to determine the effects of diffractions from numerous trees on GPR data recorded with the two antenna configurations. Based on these results, it is then possible to distinguish between subsurface and above-surface features recorded in a field data set, thus leading to a more reliable image of the shallow subsurface.
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Combined high resolution magnetics and GPR surveys of the roman town Flavia Solva
Authors S. S. Seren, A. Eder-Hinterleitner, W. Neubauer, S. Groh and P. MelicharArchaeological problems as considerations on the extent, the structure and the preservation state of an archaeological site can be solved very quickly and cost effective by archaeological prospection. The use of both prospection methods, high resolution magnetic and ground penetrating radar (GPR), became very successful in investigating Roman sites in Austria. High resolution magnetic prospection delivers not only information about the extent and structure of a Roman town, but also important indications of former wooden- and burned archaeological features. Prospecting 30.000 square meters a day even large sites can be surveyed within reasonable time applying magnetic
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Analysis of high resolution GPR time-slices from archaeological sites in Italy
Authors S. Piro, D. Goodman and Y. NishimuraRecent developments in 3-D high-resolution multi-image processing and contouring, as opposite to destructive testing, have greatly contributed to improve the quality of geophysical information in archaeological reconnaissance surveying. Nowadays non-destructive ground-surface geophysical prospecting methods, which involve detailed physical and geometrical reconstruction of hidden ambient, are increasingly used for the investigation of archaeological sites. Ground Penetrating Radar (GPR) offers very high resolution sounding capability with detection of features of the order of a few tens of millimetres thickness at ranges of several metres. In this work, the results of two high-resolution GPR surveys carried out in order to detect superficial walls and floors, in two archaeological sites in Italy, characterised by different geological environmental conditions, are presented.
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Archaeological prospection of clay-brick walls using resistivity methods and GPR at San Julián Bay, Argentina
Authors E. Lascano, A. Osella, M. de la Vega, S. Buscaglia and J. LanataIn this work we present the geophysical prospection done in Florida Blanca archaeological Site, in San Julián Bay, Santa Cruz province, Argentina. Ground Penetrating Radar and the resistivity method were applied to find very shallow clay-brick archaeological structures. An optimization in the resolution of both methods was necessary to detect the low contrasts in the electrical response of the soil and the clay-brick walls. Profiles were carried out in the zone of interest and outside it to compare results. A periodic distribution of low contrast shallow anomalies was found with both methods indicating the presence of the walls of the houses.
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Intensive survey, geomagnetic, excavation and GIS for the intrasite study of the Roman site of Terlamonte I (Teixoso, Covilhá, Portugal).
Authors A. J. M. Silva and P. C. CarvalhoThe Roman site of “Terlamonte I” is lies in the natural region of “Cova da Beira” (East Central Portugal), belonging to the Zêzere’s river basin side system, tributary of the Tejo river. The region was intensely occupied in the Roman period, certainly due to its mining wealth in tin and gold, to its remarkable agricultural capacity and to its geo-strategic position on the Roman road system of the province of Lusitania. The “Centum Cellae tower” and the “Orjais” temple are certainly the most important of the standing Roman monuments of the area (Frade, 1998). The site of “Terlamonte I” stands on a gentle slope, about 500 m from the river Zêzer, which flows down in the valley. The landscape, now loosely settled, being a prairie drained by many little lines of water, running in fields of moderate declivity. The landscape’s monotony is broken by the presence of oak woods in spots, where more often the coarse porphyroide granite crops out. Many streaks of quartz, sometimes "brechóides", cross that granitic mass which is the geological substratum (Teixeira, 1974). Trial excavation on this site began in the spring of 2000, under the direction of one the authors of the present paper (PCC), as part of a research project on the “Roman’s occupation the ‘Cova da Beira’ region” (enrolled in the “National Archaeological Program: 2000-03 – IPA”). The excavations fell upon the main building area and, as a result of these, the type and chronology of the site were determined. The “Terlamonte I” is a rural site dated to the 1st and 2nd centuries AD.
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Magnetic interpretation of gradient field in archaeological prospection
Authors F. Almeida, A. Silva, J. Alarcão and P. CarvalhoArchaeological magnetic exploration had been intensively used to investigate different occupational periods in various geological environments. Today, magnetic acquisition is very rapid, processing data can be easily done and large areas can be surveyed making this method one of the most popular among archaeologists. When magnetic material properties contrast in low noisy magnetic environments mapped images can be obtained showing linear patterns that can be sometimes architectural related. Notwithstanding the archaeological importance of the surveying plan view, generally, the image only shows a kind of projection over the surface where the measurements were done and depth is not directly evident as well as horizontal localization is biased. Much more information can be obtained from the anomalies if their wavelength and earth magnetic induction direction and inclination are taken into account. Quantitative approaches are known like reduction of the magnetic image data from the geographical coordinates of the measurements to the Magnetic North Pole, and direct or inverse magnetic body models. The self-explanatory images generally obtained in archaeological geophysical exploration may justify why in this area of Humanity/Science, quantitative methods are sometimes neglected that lead the field archaeologist to dig with no planned depth and right emplacement. The objectives of this communication are: to define qualitative rules for interpretation and to show, for point anomalies, an easy and rapid method based on the inversion of the location and depth of two opposed poles (bi-pole).
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The airborne ramses radar experiment for archaeological prospecting in a temperate climate region
Authors R. Chapoulie, M. Martinaud, P. Paillou, D. Barraud and P. DreuilletIn less than ten years, microwave remote sensing techniques have provided unexpected insights in the Earth surface structure and processes, and triggered the development of entirely new research fields. Using space-borne Synthetic Aperture Radar, SAR, theoretically three characteristics of the Earth surface can be observed : the slope (topography), the surface roughness (vegetation, geology, ocean surface), and the dielectric properties (nature of soils, moisture content). Low frequency SAR (L and P-bands) represents a promising future for remote sensing, since it allows to investigate the sub-surface down to several meters and penetrates the vegetation cover. In particular, P-band radar can achieve penetration capabilities which could be used for accurate mapping of sub-surface characteristics such as moisture content or geological structures. The region near Bordeaux in France was chosen as a suitable laboratory site for fieldwork validation of the new P-band RAMSES airborne SAR, the so-called PYLA’01 experiment (Paillou et al. 2001). Several flights were performed over the region during April and May 2001. This experiment was planned within the “low frequency radar working group” set up by the French space agency CNES in order to explore potentials of low frequency radar for sub-surface moisture detection, biomass evaluation, mapping of the ocean bathymetry, mapping of ocean salinity and for archaeological prospecting. The results will be used to promote a future P-band RAMSES flight foreseen in Egypt for the end of 2003, which will validate potentials of low frequency SAR in an arid context. The data quality of the PYLA’01 experiment is high, with no UHF interferences. Moreover calibrated data are available. The aim of this article is to present the first results of this airborne radar campaign over an archaeological site.
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Non invasive controls of balcony corbels by means of GPR investigations
Authors P. L. Cosentino, D. Miraglia, L. Romano and M. ScopellitiThe aim of this research is to optimize a non-destructive control procedure, studied and already used by authors, carried out with electromagnetic method (high frequency GPR) to control the security of balcony corbels of historical and monumental masonry buildings, built since many centuries ago until the incoming of reinforced concrete.
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Geophysical researches in the Roman villa del Casale (Piazza Armerina, Sicily)
Authors P. L. Cosentino, P. Capizzi, R. Martorana, D. Miraglia, S. Pellerito, V. Sanfratello, G. Anselmo, C. Genovese and P. Messina“Piazza Armerina” is a very important archaeological site in Sicily and many excavations have been carried out during the last century. One of the most important findings is the well known “Villa del Casale” (Fig.1). Fig. 1 – Aerial photography of “Villa del Casale”. The excavations started in 1929; took back in the '40s and more deeply in the ‘50s and they aren't finished yet. "Villa del Casale" was built between 330 and 360 A.C. and it had its period of shine during the 4th and 5th century, when disappeared under the mudflow. The villa is 3500 square meters large, and it was once Maximilian Hercules’ haunting house. He was one of Diocletian's collaborators and both of them managed the Roman Empire. Its interest is mainly due to the mosaics of the floor, among the largest and most beautiful ones of the Roman epoch. Techniques, style and the figures call to mind the floors of Northern Africa, and there are certainly works of African artists. The Villa can be divided into four main groups: main entrance and spa; peristyle with dining room and guest room; private rooms and basilica; triclinium and elliptic courtyard. They are not on the same level because the structure was built following the topographic profile.
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Comparative application of gravimetry ground penetrating radar and high-resolution seismic reflection surveys for the detection of shallow archaelogical vestiges
Authors P. Palacios, J. Cartel, C. Macedo, E. Loiseau, B. Piwakowski, M. Gouyegou, J. Mania, D. Roger, F. Loridant, P. Herbin and C. LouvionThe city of Bavay, in the North of France, has been, since the Roman Empire, an important place for trade, and the main crossroads before the Boulogne’s harbour. Its population was, at that time, mainly composed of rich traders and of a garrison force. Bavay had an extended waterworks system that was supplied by a 30 kilometers long underground aqueduct built in 96 A.C. This is one of the only monuments of that kind known in the North of France. The location map of this aqueduct is shown in Fig.1 The little part of this monument that has been discovered showed that the aqueduct was constituted of a brick and stone masonry. Its section is approximatively 1 m². It is burried at depth varying from 1 to 2 meters in a clay layer which is followed by limestone. A cross-section of it is shown in Fig.2 The goal of this paper is to check the possibilty of the detection and the localisation of the aqueduct. Between the various geophysical methods, three methods were chosen in order to test their availability for the aqueduct detection: gravity, ground penetrating radar (GPR) and high resolution seismic reflection (HRS). We point out that the tested methods are non destructive methods, compared with the classical commonly used destructive archaelogical exploration. Other methods like electro-magnetics, electrical coring were tested in the past and did not give any promising results. The experiments were conducted in the site where the position of the aqueduct was partially suspected. The test site was chosen as a 8 x 12 m rectangular area (Fig.3).
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Integrated geophysical methods for the characterisation of an archaeological site (Massenzio Basilica - Roman forum, Rome, Italy)
More LessThe Massenzio Basilica was built by Emperor Massenzio in the AD 308 until AD 312, it is one of the biggest Monuments of the Roman age after the Coliseum. It spreads in 100x65 square meters and 40 meters high. Now only an aisle remains to evidence the size of the Monument. A general study has been performed in the frame of co-operation between Superintendence of cultural heritage of Rome and University of Rome with the aim to have better knowledge of the Monument both historic artistic as structural point of view. At first bore-hole investigation was performed then to improve and increase the knowledge of the site the study was implemented with a geophysical survey. To characterise the soil where the Monument was built we used electrical resistivity tomography (ERT), vertical electrical survey (VES) and seismic refraction technique, to obtain information about the foundation structures ERT surveys and ground penetrating radar (GPR) were used. The data obtained with the bore-hole investigation were interpolated with the results of 15 VES performed with Schlumberger array having maximum distance between the electrodes AB=100m. The VES were located inside and outside the Basilica (Fig.1). Electric data were inverted by using the steepest descend method (Koefoed 1979). The interpretation of SEV performed inside the Basilica indicated the presence of three layers, characterised respectively from the top to the bottom by a resistivity of about 200ohm.m 2 meters depth, 20ohm.m 16 meters depth and 5ohm.m. The first layer was identified with the concrete floor of the Basilica, the resistivity values of second layer were ascribed to silt clay materials typical of fluvial lacustrine formation and the third layer was identified as clay formation. Resistivity curve of Mas15 with the interpretation is shown in the fig.2. To confirm the results obtained with the VES a multielectrode dipole-dipole array was performed inside the Basilica, (line C).
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Electrical resistivity by electromagnetic slingram mapping, electrical 2D and 3D imaging, and electrostatic logging: A tool for studying an old waste landfill
Authors R. Guérin, P. Begassat, Y. Benderitter, J. David, A. Tabbagh and M. ThiryMany brownfields and mine sites contain acid waters which are responsible for an important mobility of the metallic ions in the environment. To plan site remediation, it is necessary to locate, estimate and describe with non-destructive methods the areas where contaminated materials are present. The aim of this study, funded by ADEME, is to define a geophysical measurement protocol which can be used on contaminated landfills in order to: i) map the distribution of contaminated materials, ii) locate the water table and its seasonal variation, iii) evaluate the water mineralization and its spatial variation, and iv) identify the water leak toward the river drain.
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Dam hazard inspection by nuclear and resisitivty measurements
Authors H. Lindner, R. Käppler, V. Patzold and W. ThießenThe task is to explore hydraulic conductible ranges in a dam. This object is situated near the Rhine, Germany, for flood prevention. On the one hand it is necessary to detect these ranges which are destabilising the dam. On the other hand, this knowledge is important for technical reconstructions. The exploration of such embedments was done by resistivity measurements in a first step. We validated the indications by nuclear geophysical soundings in a second step. In an experiment hydraulic conductivity coefficients were computed.
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Environmental seismic measurements on inland waters
More LessThe latest development of electronics and computer sciences made possible to create special devices to solve special engineering problems. Environmental tasks on inland waters require methods and equipment with low penetration but very high resolution, in many cases less than 1 cm. Because the expression “ultra high resolution” is often used for boomer seismic sections let’s call the VHF seismic method simply sonar. The source, receiver and the data acquisition system was tested systematically on Lake Balaton (Hungary) in connection with the Limnogeologic Project of the Hungarian Geological Institute. Two case histories are presented to show the capabilities of the method in environmental applications.
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Geophysical measurements in volcanic areas: vulcano and lipari
Authors R. Supper, A. Römer, B. Meurers, R. Di Maio and K. AricAs part of the European Community project “TOMAVE” airborne and ground geophysical measurements were carried on the islands of Vulcano and Lipari. In this talk new results and interpretation of ground geoelectrical, seismic and gravity measurements are presented.
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Gradient and Mid-Point-Referred measurements for multi-channel 2D resistivity imaging
More LessResistivity imaging now becomes more and more popular in electrical exploration, due to its ability to efficiently and effectively produce images of the subsurface as a result of the availability of automated data acquisition systems and efficient user-friendly inversion software. One of the major limitations of the technique today is the time-consuming measurement process that tempts data acquisition teams to reduce the data density in order to save expensive field operation time, which in turn can be devastating for the imaging quality. In this paper, two electrode configurations suitable for multi-channel-recording, called gradient array (GD) and the midpoint-potential-referred measurement (MPR), were numerically examined for 2D resistivity imaging. These electrode configurations are well suited for multi-channel data acquisition systems, so that many data-points can be recorded simultaneously for each current injection, so as to reduce fieldwork time very significantly without compromising the data density. First, numerical experiments were conducted to examine the spatial resolution and surveying efficiencies for different data acquisition schemes. The results are compared with the results achieved for the same models with the Wenner-α (WN), Schlumberger (SC), dipole-dipole (DD) and pole-dipole (PD) arrays (Dahlin and Zhou 2001). Then field experiments were done, and results from at a waste dump are presented here.
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Geophysical investigation of the large séchilienne gravitational movement, The Alps (France)
Authors J. M. Vengeon, D. Jongmans, M. Jeannin, M. Wathelet, A. Giraud and O. MericMajor gravitational movements are common in the metamorphic formations of mountain ranges and show various types of failure, such toppling, sagging and translational or rotational sliding. The different failure processes are mainly governed by the characteristics of the discontinuities (foliation, schistosity, faults and fractures) affecting the mass (Antoine et al., 1990). In the Alps, most of the gravitational movements have probably been initiated or reactivated after the retreat of glaciers some 10,000 to 15,000 years ago, and have evolved at very different rates, depending on the initial geological and topographic characteristics, as well as on the other factors contributing to lower the stability (presence of water, earthquake ground motions, climatic cycles). The instability process progresses through periods of stabilisation and reactivation and leads to slope failure after decades or centuries. The Séchilienne movement, which is located in the French Alps near Grenoble, is affecting the right southfacing bank of the Romanche river (figure 1) The slope is homogeneously made of micaschists with subvertical foliation at right angle with the valley (except in the upper part of the slope where it is folded with an axis inclined of 30° in the north direction) and is affected by 3 sets of subvertical fractures. The main family runs ENE-WSW and delimits vertical slices in the rock mass. It is clearly distinguished by several hundred meters long depressions in the morphology associated with scarps of several meters high (Vengeon, 1998). Some of these depressions are 20 m wide, attesting the long duration of the gravity-induced processes. The slope angle is about 40° in the lower part of the hill (elevation between 330 m and 950 m) and decreases to 20° between 950 m to 1100 m (Mont Sec). Near the crest, a 20 m high scarp which is followed on a distance of several hundreds meters reveals an upper collapse. The non-freshness of the scarp and the absence of glacial erosion sign and deposits show that this movement is relatively old and occurred after the last ice age. The part of the slope which exhibits signs of current instability is located in the middle of the hill, at an elevation between 700 m and 850 m, and involves a rock volume estimated to about 3 million cubic meters (Giraud et al., 1990). This area was extensively instrumented since 1988 (Evrard et al., 1990) and the measured displacements are globally oriented in a SSE direction, perpendicular to the strike of the main fractures, and dip downhill between 10° and 20°. The movement rate varies from a few cm/year to a few dm/year. Besides geological surface observations and displacement measurements, a 240 m long gallery was excavated in 1993-94 at the elevation of 710 m. It showed a succession of blocks delimited by highly fractured and sheared zones parallel to the main fracture set. No sign suggesting the presence of a sliding surface has been observed (Vengeon, 1998). Numerical modelling with the discrete element method was able to retrieve the main field observations and suggest that the movement at Séchilienne is controlled by the main discontinuities cutting the mass into blocks and includes toppling and local sliding, evolving through progressive damaging to a potential large sliding of unknown characteristics (Vengeon, 1988). The data available at Séchilienne have led some authors to propose that the hill could be affected by a massive movement, delineated to the East by the active zone and to the North by the Mount Sec scarp. No western limit is clearly visible in the topography and the thickness of this moving mass is unknown. Consequently, the volume estimations for a rock avalanche scenario are highly variable and poorly constrained, ranging from 3 to 20 hm3 (Giraud et al., 1990 ; Antoine et al., 1994) with a global mass movement between 50 to 100 h m3. The aim of this study is to try to get information at depth over the movement area by using geophysical techniques and to confront the results with existing geological data.
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Geophysical surveys for detecting subsidence and collapse areas at Cardona salt diapir (Catalonia, Spain)
More LessSubsidence and collapse are major problems associated with salt mining, causing damage to surface structures. These processes can be associated with different processes as compaction of natural sediments, ground water dewatering, natural terrain deformations and underground material exploitation. Many of these factors are natural and have a long period occurrence, while human intervention can start or accelerate some processes. Consequently, to avoid adverse impacts of subsidence and collapse, reliable prediction is essential. Barla and Jarre (1991), Bell (1992) and Marino (1999) have conducted studies dealing this subject. Around the Cardona salt mine major problems related to this facts occurs and geophysical methods have been applied in order to gain information about the hazardous areas. Gravity and EM methods have been carried out to detect potential collapse areas, while GPS levelling has been performed for subsidence control.
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Monitoring of leakage in embankment dams through resistivity measurements – A 2.5D modelling study
Authors P. Sjödahl, T. Dahlin, B. Zhou and S. JohanssonThe study aims at improving current long term monitoring routines at two embankment dams in the north of Sweden and increase the understanding when interpreting existing data from these dams. For this purpose special software was developed as a tool for modelling resistivity distribution in earth embankment dams. The study comprises evaluation of the influence from 3D effects due to the specific dam geometry. Moreover a comparison between four different electrode arrays is carried out.
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Application fo geophysical methods for the evaluation of thermal centers and their structural controls in Boku, Nazareth, main Ethiopian rift
Authors T. Haile, G. Ranieri, T. Alemayehu, A. Vernier, E. Gavaudò, G. Barbieri and G. GhiglieriThe Boku thermal centres are located in the tectonically active axial part of the East African Rift system where the east and west rift margins are the narrowest. More specifically, in the part of the rift system known as the Main Ethiopian Rift (MER) where acidic and central volcanoes are erupted along NE-SW tectonic lines forming a region interspersed with fault scarps, volcanic scoria cones and collapsed segments (Figure 1). The topography is dominated by sunken strips of land between a series of normal faults forming part of the so called Wonji Fault Belt system. The area is also characterised by high geothermal gradient and thermal anomaly associated with the acidic volcanic centres. Steam temperatures as high as 72 °C has been recorded at the currently active thermal centre, although the steam temperatures has also been observed to show variations within a given day. Studies conducted in the area earlier specially in Aluto volcanic centres, which are currently being developed for power generation, and adjoining areas within the rift have shown that pyroclastc deposits are the major thermal aquifers. The main geological feature in the area, called the Boku caldera and its surroundings represent a typical example of central eruption from where peralkaline acidic and pyroclastic deposits have been erupted. The caldera shows rims of irregular geometry, indistinct at some places but well marked on the eastern side of the Boku. The prominent topographic high in the area is the Boku ridge which rises to an elevation of 1875 meters a.m.s.l. in the centre of the region. In a few places in the area, manifestations of steam of very high temperatures from the ground have rendered the area to be usable as a traditional steam bath centre. The major area currently used for the purpose, the Boku, has a small scale infrastructure connected to Nazareth town by gravel road and serving the local population as a bath/healing centre. Due to its geographic location connecting parts of Ethiopia between Addis Ababa and the large sections of the country in the east, Nazareth town is one of the fast growing urban centres of Ethiopia with future potential to be the centre of economic and industrial growth. It has recently shown large growth in population and has a future which will make it a trade and economic centre. The potential of the Boku to be involved in the future development of the town both as a source of recreational and healing centre and in the future for the supply of hot water to the town and the surroundings is therefore apparent.
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The effects of tree roots on the electrical properties of triassic clay
More LessA large percentage of housing in England is built on clay foundations, where clay shrinkage and swelling can lead to building subsidence and heave that result in insurance claims. As volumetric change due to absorption of water into clay mineral structure is a characteristic of some clay minerals, it follows that buildings situated on clay geology may be susceptible to heave and subsidence. This may be strongly amplified if trees are in close proximity, as root-induced changes in moisture content will also cause volumetric changes. The problem of clay and root induced subsidence really became recognised in the UK in the 1970s after the drought of 1975 and 1976 that affected the whole of North-western Europe (Driscoll 1983).
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Comparison of 1D laterally constrained inversion and 2.5D inversion of CVES resistivity data with drilling data as apriori information
More LessData collection with a multi electrode system, such as Lund Imaging System from ABEM, is usually arranged so the dataset is suited for inversion using 2.5D inversion algorithms (Loke and Barker, 1995). This data may not always be suitable for 1D inversion. However, with a 1D inversion approach it is possible to decide the number of layers to be used in inversion and also put in apriori information, where this is available, to lock layer boundaries. This approach is likely to give a result where depths to layers are better determined than with a 2.5D approach. This paper aims to compare inversion with a 2.5D least squares algorithm, Res2Dinv, and the 1D Lateral Constrained Inversion (LCI) algorithm (Auken et al., 2000). The comparison is based on a large dataset collected with a multi electrode system in Lockarp in Southern Sweden (LTH, 2000). There are a large number of geotechnical investigations available, such as auger and core drillings, which can be used as apriori information in the inversions. The geology in the area is sedimentary and consists of Quaternary deposits underlain by Danian limestone. The Quaternary deposits consist of four individually different units. On top there is a half to two meters of high resistive sediments. The second and fourth unit consist of low resistive clay till. In between these two units a third unit consisting up to three meters of high resistive intermorainic sediments can be found in parts of the area. The resistivity data was originally acquired for 2.5D inversion. For LCI inversion the data was divided into individual soundings, one every meter, and all soundings with midpoint within a four meter distance were gathered into one sounding. This results in a dataset with soundings every four meters containing exactly the same amount of data as the original dataset. This means that a sounding might contain more than one data point with the same electrode configuration.
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Interurban - geophysical assessment of water dynamics and solute transport in urban soils
Authors M. Müller, O. Mohnke, J. Schmalholz and U. YaramanciUrban soils are subject to strong variations of environmental conditions, like water flow, solute transport or heat budget. The research project INTERURBAN has been formed to clarify the dynamics of water and materials at urban locations with special consideration of the spatial heterogeneity, the organic soil substance, and soil-biological transformation processes. In order to achieve these objectives, we work on four interdisciplinary questions: 1st The evaluation of spatial heterogeneity and its interaction with sub-processes; 2nd The influence of biotic and abiotic factors on the characteristics of the organic substance; 3rd The recording of water- and material transport from heterogeneous urban locations and application to selected locations; 4th The recording of the interaction between biotic and abiotic factors and their participation in the dynamics of the release of pollutants. These goals of the research group define the target for geophysics: The development of a noninvasive conception to map the water distribution and important structural parameters. In addition to standard methods, like geoelectrics and Ground Penetrating Radar (GPR), Spectral Induced Polarization (SIP) and Surface Nuclear Magnetic Resonance (SNMR) will be used. SNMR is the only method that allows to determine water content and pore size distribution directly from the surface and therefore a special focus in the project.
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GPR Mapping toluene infiltration in a sand model
More LessThe high resolution GPR technique of 1.5 GHz antenna is applied for mapping infiltrated dense non-aqueous phase liquid DNAPL (here toluene) in a partially saturated sand medium with time. A model box (83*9.5*61 cm3) is filled with a medium sand which is saturated with water at the lowermost 18 cm. The model contains five prismatic objects of fine (objects 1, 4) and coarse sand (objects 2, 3, 5). A toluene is infiltrated from a single driplet at the centre of the upper surface. Georadar measurements (1.5 MHz) from the external surfaces show that mapping toluene is possible in the partially saturated zones of objects 1, 2 and 4 and hardly in the other areas. The weak reflections within the low saturated region may be caused by a weak electromagnetic impedance contrast at the interface to the toluene zone.
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Preliminary result of resistivity measurements for the name project (Nitrate from aquifers and influence on carbon cycling in marine ecosystems)
Authors B. Ludovic and D. ChapellierThe NAME project is an European research project which gathers hydrogeologists, geophysicists, geochemists and marine biologists belonging to different institutions: • DHI Water & Environment, Denmark; • Netherlands Institute of Ecology, Netherlands; • Max Planck Institute, Deutschland; • Ribe County, Denmark; • Technical University of Denmark; • University of Lausanne, Switzerland. The role of nitrogen in eutrophication of coastal marine environments is well documented (Horwarth et al., 1996). It is also well known that the application of fertilizer on agricultural fields causes nitrate concentrations in groundwater to increase (Postma et al., 1991). The nitrate bearing-nutrient that enters the marine environment in this way is monitored by measuring their fluxes in outlets of rivers and estuaries. Furthermore a significant part of the nitrate is removed by denitrification in the estuarine system (Nixon, 1996). However, at extensive sections along the European coast, the groundwater is discharging directly into the sea by upward transport throw the sea bottom. The importance of this direct nutrient flux into the marine environment is virtually unknown both with respect to its quantitative importance and with respect to the processes that take place at the freshwater-seawater interface. Given the steady increase in groundwater nitrate concentrations (Howarth et al., 1996), there is a strong need to investigate and document this matter and to evaluate its significance for coastal zone management. To illustrate the potential significance of processes occurring at the freshwater-seawater interface for the impact of direct discharge of nitrate containing groundwater on the marine ecosystem, two imaginary end-member scenarios may be envisioned: • In the first scenario nitrate bearing groundwater emerges unaltered through vents in the sea bottom. In this case there is a direct input of nitrate into the marine environment which may seriously affect the state of eutrophication in the coastal marine ecosystem. • In the second scenario, the freshwater is thought to mix dispersively with seawater below the sea bottom and in this case the emerging nitrate is reduced to free nitrogen while oxidizing organic matter that is deposited from above. The two scenarios described above are purely speculative, basically because of our current ignorance concerning real world processes. A recent research interest in this subject has lead to some reports describing the seepage of groundwater through the sea bottom (Moore, 1999; Hussain et al., 1999; Bussman et al, 1999). Generally, however, the current literature reveals a disturbing lack of knowledge concerning the quantitative significance of nitrate-bearing groundwater directly discharging into the marine environment and the biogeochemical processes involved.
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Georadar estimation of ground pollution degree as a result of leakage from main pipelines
Authors A. Dolgiy, Аn. Dolgy, V. Zolotarev and V. MarkulisThis report discusses the georadar technology application to problems occurring during operation with main pipelines. Two of the authors - V. Markulis and A. Dolgiy represent an operation service of a Latvian-Russian joint venture on transportation of oil and derivate products by main pipelines LatRosTrans Ltd (figure 1 – pipeline layout). The others two authors - V. Zolotarev and Аn. Dolgy represent Radar Systems Inc., Latvia.
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Induced polarisation survey at a waste site in southern sweden
More LessTime-domain induced polarisation and apparent resistivity pseudosections were measured on a former waste site in Southern Sweden, just outside the city of Kristianstad. The landfill is still in service, except an approximately 400 m x 400 m area, which has been covered by a rather inhomogeneous layer of clay and clay till. Since the landfill is to become a green leisure area, the aim of the study was to be able to assess the thickness of that covering layer with good precision in order to guide the rehabilitation process. A grid of auger drillings, down to a maximum depth of 3 m and placed approximately every 50 m yielded punctual information on the stratigraphy of the material covering the waste, and was used as reference. Apparent resistivity and chargeability pseudo-sections were measured to get a continuous picture of the distribution of the material.
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Signal optimization for electrokinetic metal decontamination: sample and fiels scale model experiments
Authors F. Cherubini, G. Losito, A. Trova and R. AngeliniAs well known, the decontamination of polluted soils by heavy metals is an environmental problem that concerns wide areas of the planet. The electrokinetic decontamination (Acar Y.B. et al., 1993; Acar Y.B. et al., 1995; Alshawabken A.N. et al., 1999) is a recent "in situ" method, but in the current technology it requests high levels of power (some MW), with consequent security and cost problems. So the main purpose of our research has been to find an experimental procedure that requests low voltage levels to obtain the metal mobilisation.
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An application of magnetic and ecletromagnetic methods for environmental research
Authors P. Barsukov, E. Fainberg and E. KhabenskyIn 1941 when the German armies invaded Sevastopol (Crimea), the Soviet sappers blew up the arsenals of the Black Sea fleet stored in the limy massif galleries known as the Inkerman galleries. However, when undermining only small part of the arsenal has been detonated, the massif was strongly destroyed and the basic part of the arsenal has remained under blockages galleries. According archive data the area of the galleries at the moment of explosion was more than 1 hectare, there were from 10000 up to 30000 tons of ammunition, mainly large aerial bombs, sea mines, torpedoes, charges for ship artillery of the big calibers, etc. At present, there are some crevices in the destroyed limy massif through which it was possible to penetrate into the blockages of the arsenal and to find kept ammunition on the area about 40 m2. The find state of the ammunition is quite satisfactory, that allows using the mine clearing and the further recycling technologies. At the moment of explosion the vaults of the galleries were destroyed and sank on 10-15 m. The rock massif has intensively cracked and turned to a congestion of limestone blocks in the volume up to 1000-5000 m3. Cracks are filled with the clay formations and water supplied. Intensively progressing karstic processes in cracked limestone create danger of explosive objects of the arsenal corrosion, which is situated within the city boundaries. On territory of Inkerman galleries SINECO Company carried out measurements of a full vector of magnetic field on a uniform mesh 5m×5m. Several powerful sign-variable anomalies were revealed there, however their locality and huge gradients (up to 200-400 nT/m) testified to the small size and small depth of bedding (not more than 1-3 m). As the goal of the magnetic survey was the detailed mapping of the ammunition’s value, the decision to involve in researches TEM-FAST sounding technology was accepted. This technology is used successfully to solve the engineering, geological, hydro-geological and environmental problems. Primary goals of the research were the following: 1) Mapping of the ammunition on the depth of 10-30 m from a surface; 2) Three-dimensional mapping of sluggies in the limestone massif. For the solution of these tasks TEM-FAST 48 HPC (AEMR ltd.) system with 25m×25m antenna has been used. This system defined transient characteristics of the media in a range of times from 2-4 μs up to 16 ms, both in one-loop configuration, and in configuration loop-in-loop. Preliminary full-scale test has shown that well conducting layers of limestone screen metal objects even of 5 ton aerial bomb size which practically are not visible at depths more than 10 meters. However the super paramagnetic effect (SPM), caused frequency dependence of a magnetic susceptibility of metals χ(ω), is confidently registered even for much big depths. When measurements of transient characteristics of an electromagnetic field the voltage on receiving antenna U(t) in case of SPM-effect [1] is inversely to time U(t) ~1/t1+δ (|δ|<<1). In regulation E (t) = t*U (t)/I the response of the media at t > 100 μs weakly depends on time E(t) ~ t−δ. At the same time the induction signal caused by attenuation of the curl currents in rock, attenuates much faster: E(t) ~ 1/tn, where n ~ 1-3. Examples of the transients for two sites: P-38 and P-44 in the above regulation are given in the fig. 1.
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Eigenimage analysis of shallow seismic sections
More LessThe appearance of seismic events related to various subsurface objects is to a large extent defined by their coherency on seismic sections. In particular, shallow subsurface inhomogeneities can be expected to appear as the areas of poor coherency. Identifying and separating coherent and non-coherent events on the sections can be performed using the technique of eigenimage analysis. This technique is based on the singular value decomposition (SVD) of the sections represented as data matrices. A similar approach is implemented in the Karhunen-Loeve (or principal component) reconstruction method (Hemon and Mace, 1978). The technique has been successfully applied to various geophysical problems (Jones and Levy, 1987; Freire and Ulrych, 1988; Jackson et al., 1991; de Franco and Musacchio, 2001).
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The use of constraints in 2D and 3D resistivity modelling
Authors M. H. Loke and J. W. LaneElectrical imaging surveys are now widely used to map areas with complex subsurface geology. The many applications include groundwater exploration, mineral exploration, environmental and engineering studies (Dahlin, 1996; Griffiths and Barker, 1993). While 2D surveys are more commonly used due to time and cost limitations, 3D surveys are also used in very complex areas such as in mineral exploration surveys (White et al., 2001). A cell-based inversion model is normally used to model such complex structures. The subsurface is divided into a large number of rectangular cells and an inversion algorithm is used to determine the resistivity of the cells. The inversion of resistivity data gives non-unique results. Thus some assumptions are made about the subsurface so as to limit the range of plausible models. The regularized least-squares optimization method where constraints can be easily added into the inversion procedure (Ellis and Oldenburg, 1994) is frequently used. The effects of constraints on the inversion results are discussed in the following sections.
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P-S Wave separation in 3-D elastic seismic data
By R. SunIn geophysical exploration, reflected P- and S-wave have different velocities but are always coupled in solid earth. Receivers sense only specific displacement components without distinguishing P- or S-wave. An accurate velocity distribution is required to image the seismic energy at its reflecting point. A strategy is to separate reflected P- and S-waves before imaging such that the P- and S-wave can be imaged separately. This study intends to develop an algorithm to separate the reflected P- and S-wave in three-dimensional (3-D), three-component, prestack elastic seismic data. The 3-D elastic seismic data is downward extrapolated from the receiver locations into a vertically homogeneous elastic model (with P- and S-velocities) using the 3-D elastic wave equation. During downward extrapolation, divergence and curl of the displacement are computed and recorded independently at a certain depth. The P- and S-velocities in the elastic model are then split into two independent acoustic models. The divergence is upward extrapolated, using the acoustic wave equation, into the P-velocity model and extracted at the receiver locations to obtain the separated P-wave. The curl is extrapolated upwards, using the acoustic wave equation, into the S-velocity model and extracted at the receiver locations to obtain the separated S-wave. Tests are successful on synthetic seismic data computed for simple laterally heterogeneous 3-D models and if the velocities used in the extrapolations are incorrect.
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Pulled array seismic (PAS) – A new method for shallow, high-resolution reflection seismic data acquisition
Authors J. F. Jensen, J. Ringgaard, P. Skjellerup and T. Vangkilde-PedersenAlong with the increasing demands in Denmark for new and deeper groundwater resources, the application of shallow reflection seismic methods has been intensified. While the traditionally low-priced and extensively used electric and electromagnetic methods are used successfully for mapping of the more shallow geology, reflection seismic is an ideal method for mapping of deep (up to several hundreds of metres) buried quaternary valleys and deep tertiary (Miocene) aquifers as well as faults. The cost for carrying out shallow onshore reflection seismic surveys has, however, been a limiting factor towards the use of seismic data in mapping of groundwater resources. Previously, results using a towed land streamer with gimbal mounted geophones and a pipe gun or sledgehammer as seismic source, has been presented by van der Veen and Green, 1998 and van der Veen et. al. 2001. RAMBOLL has developed a new method for shallow reflection seismic data acquisition called Pulled Array Seismic or PAS using a towed trail of conventional geophones and a seismic vibrator as energy source. The new method is considerably faster and more cost-effective compared to traditional shallow reflection seismic data acquisition, and the data quality is fully comparable also to deeper conventional seismic data. Since the first commercial data acquisition with this new method took place in August 2000 there has been a substantially interest for the method. RAMBOLL has thus acquired 106 km of Pulled Array Seismic on 28 lines for 6 different clients in Denmark and southern Sweden.
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Model recognition by 1-D laterally constrained inversion of resistivity data
Authors E. Auken, N. Foged and K. I. SørensenSeveral 2-D “minimum structure” inversion algorithms (de Groot-Hedlin and Constable, 1990) are available for interpretation of profile oriented resistivity data (e.g. Oldenburg and Li, 1994; Loke, 1995). For very large data sets these algorithms are intensive to run and the models are often hard to interpret in terms of a layered geological model which is the most suitable model type in sedimentary environments. As an alternative to the 2-D “minimum structure” inversion algorithm, we have developed the 1-D Laterally Constrained Inversion (1-D LCI) algorithm. The LCI algorithm is parameterized and uses a series of laterally constrained 1-D models in the inversion. The 1-D LCI algorithm has for some years been used for some years by routine to invert multi electrode data (Wisén and Auken, this conference) as well as Pulled Array Continues Electric Sounding (PACES) data (e.g. Sørensen, 1996). The aim of this abstract and presentation is first to present a statistic study of the ability of the LCI algorithm to recognize a geological model, second to present the LCI algorithm itself. PACES data are used to map the upper 25 – 30 m of the geology for a detailed, regional mapping of protective clay caps of aquifers (Christensen and Sørensen, 1998). With the resolution capabilities and the equivalence problem of resistivity data in mind it is important to know not only the geological/hydrogeophysical model but also the probability of the model. One approach to analyze the quality of an inverted model section is to generate data over some simplified models, invert them and compare the inversion results to the original model. The decisive factor doing this is how representative the forward model is. A more thorough approach is to generate data from a large number of stochastic model sections which reflect the expected geological environment in a given area of investigation. Subsequently, the data must be submitted to the same data processing and inversion process as field data. Based on a comparison between the original model and the inverted model, it is possible to calculate a statistical measure for the quality of the inverted model.
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2-D Laterally constrained inversion (LCI) of resistivity data
Authors A. V. Christiansen, E. Auken, K. I. Sørensen and T. Smith2D laterally constrained inversion (2D LCI) is a new approach to interpretation of continuous resistivity data using an inversion scheme similar to that used for 1D LCI. The only difference is that the forward code in the inversion scheme is 2D. This enables a layered 2D inversion in sedimentary areas where the layering has been disturbed in a degree, that a 1D model is inadequate, but a layered model is still applicable. The layered 2D inversion provides a full analysis of the model, and can include a priori information just as in the 1D case. Preliminary results on synthetic data show that a layered 2D approach enables a more detailed description of 2D structures, which the 1D formulation can not resolve. The model complexity is controlled and balanced by the lateral constraints on depths and resistivities.
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Shallow seismic reflection profiling with a towed geophone system: preliminary results
Authors R. Marques Moura and M. J. Senos MatiasIn recent years some workers have been experimenting with towed receiver array systems in an effort to increase the productivity of land seismic reflection acquisition. The works of Eiken (1989) and Van der Veen (1998) are key references. To our knowledge, there is at least one firm (in Denmark) that provides this type of acquisition service on a regular basis. Based on some of these promising results and due to our need in arranging a time efficient method for profiling we decided to design and build our own towed array system.
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Modelling of the phase behaviour in the SNMR-Signal
Authors M. Braun, M. Hertrich and U. YaramanciSurface Nuclear Magnetic Resonance (SNMR) is a fairly new geophysical method for groundwater exploration and aquifer characterization (e.g. Yaramanci 2000). The NMR-Experiment is conducted by a coincident transmitter and receiver loop at the surface exciting the protons of water molecules underground with the Larmor frequency of the earth’s magnetic field. Conducting a SNMR measurement with increasing excitation intensity yields a complex sounding curve, where the amplitude of the relaxation signal is determined by the numbers of protons i.e. the water content and the phase is sensitive to the subsurface conductivity.
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Weighted F-K for surface wave analysis
By C. StrobbiaTests based on surface waves (SWM) consist of three steps: acquisition, processing and inversion. The acquisition is the observation of Rayleigh wave propagation in time and space: the vertical velocity at the free surface is usually recorded in two or more points at known distances from the source. To assess the dispersion characteristics on a wide frequency range, the source has to produce either a single broad-band signal or a series of monochromatic signals over a wide frequency band. The processing extracts from field records the information about Rayleigh waves dispersion. The inversion estimates site mechanical properties as a function of depth from the experimental dispersion characteristics. The amount of information extracted by the processing and its uncertainty, which strongly depend on the acquisition parameters, affect the reliability of the final result. Several processing techniques can be used; the basic principle is the same and some of them are formally identical. Nevertheless their different sensitivity to coherent and random noise leads to peculiar quality assessment and filtering capabilities. I compare here different processing techniques for dispersion curve extraction, and propose a new algorithm based on f-k processing and coherence between signal pairs. The procedure reduces the effects of noise by discarding low coherence signals and allows a great increase of the useful frequency band. Moreover non-uniformly spaced arrays can be processed and poor quality signals are filtered out automatically.
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Mapping a quartz vein using three electrode arrays at Santo Domingo, Nicaragua
Authors A. Mendoza and T. DahlinThe multi-electrode electrical resistivity method has become one of the most efficient geophysical methods used in a wide range of applications, which is also suitable when disturbances in the ground are to be avoided. The selection of the right electrode configuration can provide better effectiveness, depending on the target being investigated. In this work a case of the use of three electrode arrays for vertical quartz vein localization is presented. The investigations were carried out within the framework of a multi disciplinary research and training programme funded by Sida/SAREC, with the aim of mapping groundwater systems in an area contaminated by mining activities. The study area is close to the village Santo Domingo, central Nicaragua, were gold mining has been an important economic activity for over 140 years. Three Continuous Vertical Electrical Soundings (CVES) were performed crossing El Cuatro gold-bearing quartz vein. This is one of the more representative geological structures in the area because of its characteristic tectonic settings, steep topography and its hydrogeological significance for groundwater occurrence.
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Hitem – A high moment / high production TEM system
Authors J. E. Danielsen, E. Auken and K. I. SørensenDuring the nineties the transient electromagnetic method (Nabighian and Macnae, 1991) became the method preferred for mapping the extension of aquifers in the sedimentary environments in Denmark. The most commonly used system has been the Geonics PROTEM 47 system with a 40 x 40 m2 transmitter loop. More than 45,000 soundings have been carried out by use of this system. However, for many tasks its penetration depth is insufficient and a wish rose to develop a high moment TEM system (HiTEM) with the same model resolution capabilities and similar production rates as the 40 x 40 m2 system. The development of the HiTEM system to its present state took about 3 years. The system is capable of transmitting up to 75 A in a 30 x 30 m2 transmitter loop. This offers a magnetic moment of the system about 15 times larger than the conventional 40 x 40 m2 system and a penetration depth up to 300 m compared to the 120-150 m of the PROTEM 47 system. Since spring 2001 more than 2,000 HiTEM soundings have been carried out. Several array configurations were tested, as significant technical difficulties arise when high currents are transmitted in a small loop and the Earth response is measured in the centre of the loop. For this reason and because of expected IP-effects (Flis et al., 1989), the receiver coil was moved outside the loop. This configuration is more complex to handle in the data interpretation process than the central loop configuration, and it took some time to gain knowledge of the strengths and limitations of the configuration. Thorough 3-D modelling of various arrays and model scenarios (Toft, 2001) showed that the offset configuration is extremely sensible to small-scale, near-surface inhomogeneities at early times. In addition, the offset of the receiver coil has to be known with an impossible accuracy in a practical field situation. All this led to the characteristic HiTEM sounding which includes both a central and an offset loop measurement. Early times are measured in the central loop configuration, and late times are measured in the offset loop configuration at 2.4 A and 75 A, respectively. Both sensitivities and locations of the arrays differ, but they are interpretatively the same sounding. A traditional joint inversion of the two data sets often results in a poor data fit due to the model inconsistencies between the data sets. The solution to this has been the Mutually Constrained Inversion (MCI) approach. In a MCI inversion two models, one for each data set, are constrained to each other, and the MCI algorithm inverts the two data sets concurrently and produces two similar models. The hardness of the constraint serves as a handle which the interpreter can use to meet certain requirements: the harder the constraint, the more similar the out coming models, but the more consistent data sets are asked to provide a satisfying fit to data.
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First gravity surveys of the EEZ programme in Galicia Margin (Spain)
Authors P. Llanes, J. Álvarez, A. Muñoz-Martín and A. CarbóIncluded in the systematic mapping programme of the Hidrographic and Oceanographic Research Plan for the Spanish Exclusive Economic Zone (EEZ), gravity surveys have been carried up in Galicia margin. Using gravity data collected during the cruise carried out in 2001 aboard Hesperides Research Vessel, we have made the Free Air anomaly and Bouguer anomaly maps for this area. Sea bottom terrain correction has been calculated for all readings, -45.60 as maximum value and 36.86 mGal as medium value have been obtained. Typical oceanic Bouguer anomaly values, between 70 and 320 mGal, can be observed in the map, being located the highest values in the southwestern area.
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Tomographic formation mechanical stability estimation using borehole radial gradient of bulk modulus distribution
Authors C. Pretzschner and H. LindnerKnowledge of mechanical stability of the near borehole formation is fundamental in: geotechnical investigation • • • defining optimal locations for deviation of horizontal drilling paths reducing risks of borehole collaps through scattering of formation particles in the the borehole while production (sanding) The respective information may be obtained by laboratory measurements directly on cores or in situ as a result of borehole-caliper evaluation or sonic-log interpretation. Normally the borehole-surface is destroyed by the drilling process (microfractures, breakouts) and the measured travel times will be influenced by a medium of lower-velocity. Because of increasing velocity in higher penetration r the seismic energy is traveling as diving waves through the formation in this case. Using a multi-array sonic-probe and low shot-point distances, the high density of travelpaths through the formation allows imaging the formation velocity distribution by an In-Borehole Tomographic Reconstruction (IBTR). These 2D-images especially reflect the sensitive mechanical behaviour in the transition from mud to formation besides an accessible higher vertical resolution and exclusion of caliper effects (Fig. 1).
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Contribution of well logging to hydrogeology investigation
More LessA study area – “The Bohemian Cretaceous Basin” forms the north-eastern part of Bohemia (Fig.1). The main reason for selecting the central part of this structure as an object of the study was its significance for water management, in terms of important natural groundwater resources of mineral water in the vicinity of the Poděbrady spa. The aim of the study is to synthesise available data, to assess natural groundwater resources and to keep the groundwater safe. Furthermore, the goal is to propose ways of rational groundwater development, to consider the impact of groundwater overexploitation in the Poděbrady spa, as well as that of industry, agriculture and landfills on the Poděbrady vicinity environment. This study tends to summarize the results of different earlier logging measurements (62 boreholes) together with surface geophysical interpretation and by this way to yield important information for hydrogeological conclusions.
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Designing surface-to-borehole electrical resisitivity tomography surveys using the frechet derivative
Authors L. Marescot, S. Palma Lopes, R. Lagabrielle and D. ChapellierCrosshole resistivity imaging is increasingly used in civil engineering, groundwater and environment investigations: the cost of equipment is low and the field measurements are easy to perform. Such arrays are able to yield valuable information on the variation of resistivity between the boreholes (e.g. Daily and Owen, 1991). Resistivity imaging using buried electrodes permits a greater accuracy and resolution than what can be obtained with surface arrays. The strong imprints of near surface inhomogeneities are reduced whereas the resolution at depth is increased since the sensors are closer to the structures of interest. Nevertheless, crosshole resistivity imaging surveys are frequently prohibited on the working site. There is either one single borehole available or the distance between two boreholes is too large for crosshole resistivity investigations (the distance between the boreholes should be comparable to the length of the boreholes). In this case, surface-to-borehole (or borehole-to-surface) resistivity measurements are performed by placing a current source on the surface (or downhole) and measuring the potential field in a borehole (or on the surface). Geophysicists are probably more familiar with surface arrays since they have been using Wenner, Schlumberger or dipole-dipole configurations for many years. Resistivity surveys with non-conventional arrays (e.g. surface-to-borehole arrays) are more rarely carried out. This prompts us to find a tool for designing such surveys.
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Multifrequency microwave diffraction tomography: Application to ground imagery by transmission
Authors R. Tarnus and X. DérobertAmong the important number of geophysical methods dedicated to ground analysis, those based on the propagation of electromagnetic waves at radar frequencies have gained a lot of interest these last ten years. Their non-destructive capabilities in addition to the technological advances in antenna design and signal processing played an important role in their success, especially for the Ground Penetrating Radar, which is commonly used from Field Geophysics field to Civil Engineering, and also for tomographic applications [1]. The multifrequency microwave diffraction tomography, presented in this paper, is based on the measurement of the scattered electric field for several frequencies after it has diffracted in the ground. There are several applications for this technique, such as human body imaging [2]. Applied to ground analysis, its goal is to provide an image of the ground that is relevant of its electromagnetic properties. The technique is applicable, in theory, in reflection from the surface as well as in transmission around walls, or from boreholes [3]. In the latter case, we would obtain an image of the vertical section of ground between the two boreholes, enabling to reduce the number and therefore the cost of the investigations. This technique has already been studied trough several PhD theses in the LPC network (Laboratoires des Ponts et Chaussées) [4,5]. In the present work, the interest will be mainly focused on the transmission mode from boreholes, with an approach of numerical modeling using laboratory experimental results. The goal is to develop a cross-hole microwave imagery technique that could be applied to the localization of objects like cavities or utilities.
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Use of geophysical borehole logging techniques and depth sampling methods to characterize the hydrogeochemical heterogeneity of the aveiro cretaceous multilayer aquifer
More LessThe Aveiro Cretaceous aquifer is a multilayered coastal aquifer formed by several hydrostratigraphic units with distinct grain sizes and mineralogical compositions, and with different hydrogeological properties. As is common in the region, most boreholes exploiting the aquifer have increasing depths towards the coast following the aquifer thickening and are multi-screened, pumping water from the different aquifer layers with known distinct hydrochemistry and residence times. The use of conventional methods of groundwater sampling from pumping multiscreened boreholes in the Aveiro Cretaceous multilayer aquifer inevitably leads to water mixing between the different aquifer layers, making the understanding of the water quality patterns more difficult, and eventually, leading to misinterpretations. To better address this problem or this uncertainty, it was decided to select seven non-equipped boreholes within the study area for detailed geophysical logging studies and depth sampling. The integration of lithologic logs, geophysical logs and depth sampling hydrogeochemical results allowed to confirm the borehole construction and to observe the vertical variation in the formation, aquifer and fluid properties (temperature and salinity). Flow logs showed that the Furadouro sandstone formation is contributing with most of the water supplied by the aquifer, with a ratio for the transmitted flow that could be as high as 9:1 when compared to the other aquifer layers. The depth sampling allowed the hydrogeochemical characterisation of each aquifer layer confirming the most permeable and productive layers as the ones with youngest and lowest salinity waters.
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Electromagnetic fields generated by vertical current in volcano Etna
Authors E. Fainberg and B. Singere based on electrical methods, magnetotellurics, transient electromagnetics, and other electromagnetic methods. The source of the electromagnetic field in these methods is located either immediately at the Earth surface or above it. Methods using external sources to monitor tectonic processes are classified as active methods (Svetov 1992). Alternatively, passive methods of monitoring rely on measurement of electric and magnetic fields caused by volcanic or tectonic processes inside the Earth. A number of electrokinetic, piezoelectric, thermoelectric, and other physical and chemical phenomena can be responsible for generating electrical currents in the interior of a volcano. Electric and magnetic fields induced by these currents are known to be measurable. Electrical structure of the Earth near a volcano is always laterally heterogeneous. Interpretation of measurements made in such conditions is a challenging task. On the other hand, as shown by Fitterman (1979) and will be further discussed in this publication, the magnetic field of the internal origin is often observed at the Earth surface only because the Earth is electrically heterogeneous. The magnetotelluric field caused by ionospheric and magnetospheric currents obscures the internally generated field. A significant suppression of the magnetotelluric noise can be achieved by using linear relationships between components of the electromagnetic field at different locations on the Earth surface. As shown by Svetov (1992), a reduction of the magnetotelluric noise by the orders of magnitude can be accomplished by finding the transfer functions that relate the electromagnetic field at an observation site to the magnetic field at the reference site (base), and using these transfer functions to forecast the magnetotelluric noise at the observation site from the magnetic field measured at the base during the monitoring periods. In this approach, the transfer functions are determined during "seismically quiet" periods. During the “monitoring” periods, the transfer functions are used to eliminate most of the magnetotelluric noise from the signal recorded at the monitoring site. The sites must satisfy rather different conditions. The monitoring site must be located in the area where the signal of the anticipated internal source is sufficiently strong to be registered. The reference site should satisfy the exactly opposite requirement, i.e. the field of the internal source should remain at a negligible level even during periods of seismic activity. On the other hand, the monitoring and reference sites must remain within the coherency distance of the magnetotelluric field. Both the internal and magnetotelluric fields are scattered by geoelectric heterogeneities. It can be expected that features of the subsurface conductivity distribution, relief, proximity to the seashore, as well as presence of active faults in the Earth crust may have a direct effect on the level and structure of the internal and magnetotelluric fields. The main purpose of this publication is to demonstrate on the example of volcano Etna that modern approaches to 3D modeling, including the so-called generalized thin sheet modeling, allow for a reliable simulation of effects of lateral heterogeneities on the field of the internal source, thus allowing for developing recommendations for the optimal allocations of the observational and reference sites.
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Geophysical surveys for the investigation and monitoring of landslide areas
More LessDuring recent years several large landslide hazards took place in Austria. One of the largest happened at Sibratsgfäll, where about 70 million m3 and an area of 1,4 km2 was involved in a hazardous mass movement, that disturbed 13 buildings completely. Starting on the 19.May 1999 after three days of heavy rainfall, the whole area of Rindberg started to move with a maximum speed of 10m/day. Immediately after this event, a multi-disciplinary research program was started to investigate the structure and triggering cause of this landslide and to develop a geoelectrical monitoring system to register pre-alarm critical values. The research program included: • GPS measurements to monitor actual movements • Airborne geophysical measurements involving measurement of electromagnetic field, gamma ray, magnetics, infra red temperature and soil humidity • Detailed geological and hydrological mapping • Ground geoelectrical surveys on selected lines • Development of a geoelectrical monitoring system, with additional hydrological and meteorological monitoring parameters for calibration purposes Within this talk result of the structural investigations, focusing on the geophysical work, as well as the first results from a new geoelectrical monitoring network will be presented.
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Slow active fault detection and imaging using multiple geophysical methods
Authors F. Nguyen, S. Garambois, D. Chardon, D. Jongmans, O. Bellier and D. l’HermitteDespite its low to moderate seismic activity, most of Western Europe is characterized by a relatively high seismic risk due to the concentration of human and material properties presenting high-vulnerability. These regions have been affected by destructive earthquakes like the Ms=6 Lambesc earthquake, France, 11th June 1909, generating VIII-IX intensity values. Consequently, detecting tectonic structures that may lead to destructive earthquakes in such areas characterized by slow deformation rates, weak seismicity and dense vegetation coverage requires a multi-disciplinary approach which includes applied geophysics. The present geophysical study has been conducted in Provence (France), in the area touched by the 1909 Lambesc earthquake (Champion et al., 2000). The Provence domain is located at the transition between the Pyrenean and the Alpine mountain belts. It is bounded by the Nîmes fault to the west and by the external alpine thrusts to the east (Figure 1). Provence is affected by numerous east-west trending thrusts that have either northward (Sainte-Baume, Etoile, Ventoux-Lure) or southward (Alpilles, Lubéron, Costes-Trévaresse) vergences, and by regional NE-SW to NS trending strike-slip faults (Nîmes, Durance, Salon-Cavaillon faults).
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A combined geophysical survey on an active debris flow
Authors F. Bruno and F. MarillierThe Peillettes landslide in the western Swiss Alps is a complex landslide threatening villages of the main Rhone Valley. The slope is between 25 and 30 degrees. Trees cover a large part of the unstable zone which extends between elevations of 1000 to 2100 m; its width is 1000 m about. This slope is a known unstable zone and it was the subject of many studies, considering the potential impact on the neighbouring communities. In spite of many works to increase drainage of the run-off water, the zone remains unstable and it is reactivated after each particularly rainy period. The lower part of the landslide is the most active of the instability. Here a debris flow called le Grand-Tsâble is located few meters away from the closest country cottages. The goal of this work is to evaluate the contribution of several geophysical methods and in particular we will discuss the results of combined geophysical surveys: high-resolution P and S wave seismic reflection (PHR and SHR) and georadar (GPR). We focus on imaging the subsurface of the active debris flow and comparing the results of each method, which are often complementary.
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An underground railway project in Lausanne, Switzerland, as urban micro-gravity test site
Authors P. V. Radogna, R. Olivier and P. LogeanA new underground railway, named M2, is planned to be constructed within the next decade in Lausanne, the fifth densest populated city of Switzerland. In 1994, a preliminary geological report of the quarters chosen by the administration had shown the general conditions and features of the lithology of interest. The involved geology constitutes alpine molassic bedrock, an overlaying quaternary glacial fill and an overburden of loose soil. The choice of the corridor had to consider the exact thickness of these geological units. It was necessary to find out whether the best effective cost ratio would be an easier, but deeper, tunnel in a favourable lithology (Bedrock) or the technically more difficult tunnel in a superficial lithology (Glacial fill). Nearly 750 existing well loggings over the city, conducted for geological superficial surveys, test pits and geological maps were used to build a map of the depth to bedrock. Its surface was positioned with a precision of 3 to 5 m. The depth is varying between 2 and 15 meters. Once the corridor was chosen, the needed precision was increased. The requested precision in depth determination was about 1 meter. For this reason, more drillings were executed in 1999 and 2001 along the chosen corridor. The free availability of all data by the Road Service of the canton Vaud was an opportunity for testing a micro-gravity survey, integrated by other geophysical techniques, along the 6 km metro corridor.
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Geoelectric measurements in claystone for exploration of anomolous zones
Authors S. Kruschwitz and U. YaramanciUnderground excavations provoke micro- and macrofractures in the vicinity of drifts and galleries. A detailed knowledge of this excavation damaged rock zone (EDZ) is indispensable for the planning and dimensioning of geotechnical barriers for underground ultimate waste disposals in particular that for chemical, toxic and radioactive wastes. The host rock as well as the artificial, geotechnical barrier should avert any propagation or trespassing of virulent and radioactive substances. Still different geophysical methods are tested and adapted to find out the best assessment strategy for each kind of host rock. Seismics [1], microacoustics [7] and geoelectrics [8, 9, 10] turned out to be the most promising methods in claystone. Since the beginning of 1999 also complex valued geoelectric measurements underground in tunnels the have been successfully conducted
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Coastel erosion monitoring by means of geophysical methods
Authors G. Ranieri, R. Deiana, G. P. Deidda, S. Erriu and M. NuvoliOne of the most frequent problem in the islands is given by the erosive coastal process, due to the atmospheric agents and to the wavy motion. Moreover human activities favor and accelerate the modifications of the sea tides and the superficial erosive processes. Often, in fact, the losses from water-pipes or the construction of buildings and roads, modify soil structure, limiting and varying the natural groundwater flow and exposing the most alterable rocks to the action of erosion by waters and winds. In the present work the “physical condition” of the Promontory of Balai (Sardinia-Italy) is examined (fig.1). This zone is exposed to predominant winds in the island and wavy motion. Recently the construction of docks, near the harbour of Porto Torres, has produced acceleration of the erosion phenomena. The area is of particular interest for the presence of an old church (S. Gavino a Mare, XI a.C.) and for a built-up village. The final aim was to establish the sinking and settling risk. In order to know the evolution of the erosion phenomena in the inner part of the promontory, boreholes, electrical tomographies, seismic tomographies and a microgravimetric survey, have been executed.
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Nuclear magnetic resonance (NMR) properties of unconsolidated rocks and synthetic samples
Authors M. Müller, U. Krüger and U. YaramanciThe nuclear magnetic resonance (NMR) technique is used in geophysics mainly for well logging and laboratory applications. Consequently, the available investigations focus on the NMR properties of solid rocks, because of their significance as hydrocarbon reservoir rocks (Kenyon, 1992). In recent years Surface NMR (SNMR) has become available for hydrogeological applications (e.g. Shirov et al., 1991, Yaramanci et al., 1999). The need for well founded interpretation of SNMR field data has steeply increased the interest in NMR properties of unconsolidated rocks. In our work we therefore analyzed the NMR properties of synthetic and natural unconsolidated samples to enable an enhanced understanding and interpretation of SNMR data.
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Some features of the subsurface geoelectric structure in the sinkhole occurance sites along the dead sea shore line.
By M. EzerskyHazardous sinkholes started to appear in alluvial fans and unconsolidated sediments along the Dead Sea coast in 1990, when a small group of sinkholes 2 to 15m in diameter and up to 7m deep were formed in alluvial sediments in and around the middle of the main asphalt road close to Neve Zohar. Since then hundreds of sinkholes have appeared along the western coast from the Attraction Beach in the north to Nahal Hamar to the south of the Dead Sea. There are currently three theories explaining the sinkhole forming mechanism in the Dead Sea region. This could be a pseudo-sinkhole phenomenon (piping) for alluvial fan gravel holes, or mass mobilization and suspended clay transfer hypothesis for mud holes or salt layer dissolution mechanism. However, while none of these explains the sinkhole phenomenon completely, all the investigators agree that sinkhole formation is a dynamic process. It follows in time and could be accompanied by variation of the medium properties (density), salt layer or saline clays dissolution and/or fines washing out and transfer. The presence of fresh or salty groundwater plays an important role in sinkhole formation. Numerous geophysical investigations carried out in recent years were mainly aimed at detecting the sinkholes formation mechanism at depths of tens meters (salt layers, fresh water and large voids detection). In the present study we have used geoelectric methods to achieve a better understanding of the subsurface geoelectric structure at the sinkhole development sites, taking into account that electric parameters (such as resistivity or conductivity) are very sensitive to medium properties and their variations in time, for example, high porosity or voids including medium manifests the same as a high resistivity anomaly zone. In contrast, the water (different degree of salinity) as well as zones including clay appear as low resistivity anomalies. First, we reinterpreted previously obtained and published results of geoelectric surveys at the Neve Zohar, Nahal Hever southern and Ein Gedi Holiday Village sinkhole development sites. We then performed geoelectric mapping of the Ein Gedi Holiday village and palm tree plantation areas using the Continuous Vertical Electric Sounding (CVES) method (Ezersky, 2002) .
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Preliminary geological and geophysical investigations on a complex karstic area
Authors M. T. Carrozzo, G. Leucci, S. Margiotta, S. Negri and L. NuzzoThe growing urbanization of karstic areas and the diffuse utilization of chemical products in agricultural increases the probability of soil and groundwater pollution. The relevance of these problems in karstic areas forced decision-makers to produce a number of laws and regulation on this subject and to demand an increasing number of environmental assessment studies in those areas. The research we are at present carrying on is to define the impact of agricultural practices in a complex karstic area of Salento (southern Apulia, Italy), placed among Carmiano, Copertino, Leverano, Salice Salentino and Veglie villages (Fig. 1).
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Application of the seismic reflection method to abandoned mine detection in Óbidos, Portugal
Authors J. Carvalho, L. Torres, C. Galhano and L. Mendes-VictorThe municipality of Óbidos wanted to investigate the suitability of an area in Casal Caxinas, for construction of one or two story building. The surrounding area was an active gypsum mine, now closed for a few decades. An old mine map exists but serious doubts are cast over its validity since, for example, no gallery connects the shaft well to the extensive net of galleries, and some references seem misplaced. Several areas a few tens of meters from the planned construction site collapsed but the map shows no galleries at the site. The spot has been covered by several meters of mine wastes, that show some irregularities and fissures that can be caused by natural subsidence due to gypsum withdrawal or by galleries collapse. To verify the landfill competence and thickness and detect the eventual presence of old mine galleries, a reflection seismic survey was carried out. The water table was expected to be a few meters deep and the fact that the landfill materials were clayish prevented the use of georadar. The depth of the galleries (over 20 meters deep) and the short space available in some areas of the construction site (less than 60 meters) also did not allow the use of geolectric methods. A grid of seismic reflection lines was realised and the results interpreted with a few geothecnical mechanical soundings. Several meters of alluvium were detected beneath 5.5m of waste material, while the water table showed at about 7 meters depth. None of the soundings exceeded 20 meters and so no cavity was detected but also no signs of decompression in the alluvium were detected, indicating that no collapse occurred, even if galleries beneath the site of drilling exist. To the South of the study area, the presence of galleries is strongly suggested by the seismic data and a new map of the mine is created showing the limit of the galleries several meters further North. However, space is still available for the building construction and though the landfill material competence is weak, construction is possible provided some measures are taken.
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Detection of subsurface cavities by gravity methods
More LessThe detection of subsurface cavities by gravity methods has been investigated in recent years. We use some quantitative interpretation methods to determine the depth and location of the subsurface caves in a mountainous area located in the west of Iran. Key words: Euler deconvolution, analytical signal, Filon frequency, maximum depth, minimum depth.
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Field measurements of spectral induced polarisation on permo-triassic sandstone in the UK
More LessThe Triassic Sandstone is a major aquifer for western England and it has also historically been the location of much heavy industry. As industrial use of local groundwater recedes, water tables are rising and may remobilise contaminants held near the surface in the vadose zone. A robust method to survey for contaminants, and to be able to discriminate between different types, is desired. Measurements of Spectral Induced Polarization (SIP) is a potentially powerful discriminatory technique; however, before investigating the SIP properties of contaminated sandstone, it is important to understand the response of natural uncontaminated sandstones. This paper describes the use of SIP in field measurements on uncontaminated sandstones in various sites in the west of England.
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The spectral induced polarisation response of triassic sandstone from the UK
More LessThe Permo-Triassic sandstones form the second most important aquifer in the United Kingdom (U.K.) supplying about 25% of licensed groundwater abstractions in England and Wales (Allen et al., 1997). Geophysical methods have shown to have important applications in locating and characterising contaminants and determining the hydrological properties of the sandstone. In particular, SIP spectra of Triassic sandstone from the U.K. have already been shown to change dramatically with contamination (Scott and Barker, 2001). Low frequency Spectral Induced Polarisation (SIP) has been used to measure the electrical properties of samples of Triassic sandstone from around the U.K. in order to characterise the different SIP responses of this sandstone. Here we present the work carried out so far.
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Hidrogeological implications of Beja fault location using geoelectrical methods
Authors R. Rocha, A. Andrade Afonso and E. RamalhoDue to widely spread nitrate pollution that reaches the groundwater supply in the Beja area, deeper aquifer zones that may be considered as an alternative to the present situation must be identified. A deep unpolluted aquifer below the unaltered rock was detected using several surface geoelectrical methods: a dipole-dipole survey was therefore conducted, which made it possible to detect the main fracturing zones to a depth of about 600 metres, as well as deep alteration zones corresponding to possible groundwater circulation. More precise studies were then carried out in areas of interest detected during the above-mentioned survey, namely a Rectangular Survey and three Vertical Electrical Soundings (VES). Data interpretation has revealed a good adjustment with local geology and has allowed the determination of the approximate geometry and type of geological formations at these depths. It has also indicated the existence of two zones with potential groundwater circulation: one shallower and more vulnerable to the pollution; and a deeper aquifer, associated with the Beja Fault, less vulnerable to pollution.
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Geophysical surveys in the capoterra coastal plain (Southern Sardinia - Italy)
Authors F. Ardau, R. Balia, G. Barrocu, E. Gavaudò and G. RanieriThe Capoterra coastal plain lies on the south-western tip of the Campidano Graben, few kilometers west of Cagliari, the main town of Sardinia (Fig.1). It was originated by the ancient Santa Lucia river delta and is bounded by salt-works and lagoons on the east, the seashore on the south-east and the Paleozoic basement on the west and south-west. The surface geology of the plain is characterized by Pleistocene and Holocene sand and alluvium; the Paleozoic bedrock is mainly constituted by Carboniferous granite and Cambrian-Ordovician schist (Fig.1). Concerning hydrogeology, to date the dominant theory is that two aquifers can be distinguished: a shallow phreatic aquifer and a deeper, multilayer, locally confined aquifer. Due to the large number of incorrectly drilled water-wells, these two aquifers, laterally recharged through the fractured Paleozoic rocks, are now communicating in several areas of the plain; moreover they are affected by saltwater intrusion coming from the sea and the neighbouring salt-works. In the past, several hydrogeological and geophysical surveys were carried out in the Capoterra plain , e.g. Balia et al., 1994; Barrocu et al., 1994; Godio et al., 1999; Montaldo, 1966; Sciabica, 1994; Vernier, 1999. Referring to most recent works, Sciabica (1994) afforded the the mathematical modeling of the aquifer system based on hydrogeological and hydrogeochemical data, while Vernier (1999) conducted a more general synthesis including all available data. However the overall information on the Capoterra plain is still inadequate for a reliable modeling of saltwater intrusion and its evolutionary trend, and very important aspects such as stratigraphy of Pleistocene-Holocene sedimentary cover, relationships between phreatic and confined aquifer and depth to Paleozoic basement must be further elucidated. Hence a new campaign of geophysical surveys including a gravity survey and then some seismic reflection profiles, properly designed on the basis of gravity interpretation, and TDEM soundings has been recently planned.
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Using geophysical methods for detecting seawater intrusion and waste plumes in the river Foix delta (Catalonia, Spain)
Authors D. Vázquez, R. Lázaro, M. Himi, N. Yilo and A. CasasGeophysical surveys, particularly electric and electromagnetic methods, have demonstrated to be useful for mapping inorganic conductive plumes and delimitating seawater intrusion into coastal aquifers (Frohlich et al., 1994). In the study area groundwater is affected both by salt-water intrusion and toxic industrial wastes. Therefore, the purpose of this study was to test a cost-effective geophysical methodology to detect saltwater in the shallow subsurface using electromagnetic ground conductivity and electrical imaging measurements to determine the immediate sources of leachate plumes flowing in the area.
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Geoelectrical survey at the water barrier of Elva river at the area of Havelberh (N. Germany)
Authors P. Sotiropoulos, P. Tsourlos, K. Buckup and G. SiderisA 2-D geoelectrical survey was carried out at locations at the water barrier (dam) of the west bank of river Elba at the area of Havelberg (Sachen Anhalt), Germany (Figure 1). The target of the geophysical survey was to provide lithological and hydrogeological information about the surveyed water barrier area and its surroundings. The localization of lithological variations (i.e. shale lenses) is of particular interest since these are associated with potential vulnerability zones in case of river flood. This information will be of use to geologists and engineers who are currently plan works, which will increase the safety of the dam. The sediments in the region consist of sands, mixture of sand and clays and lenses of shale. The dam is elevated on average 2.5m above ground and has relatively steep slopes.
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Location of buried valley aquifers using transient electromagnetic soundings
Authors K. I. Sørensen and E. AukenIn Denmark, 99% of the water supply is based on groundwater abstraction. Increasing problems with the groundwater quality due to urban growth and agricultural contamination in 1995 caused the Danish Minister of Environment to approve a plan for improving ground water protection. In July 1998, the Danish Parliament decided to launch an ambitious plan to significantly intensify the hydrogeological investigation of the Danish groundwater resources. This plan runs for the next 10 years and includes all areas characterized as groundwater recharge and aquifer areas, a total of 16,000 km2 or 1/3 of the country. In Denmark, a large number of aquifers are composed of Quaternary sand and gravel deposited in deep valleys eroded in Tertiary clays. The valleys are often covered by an inhomogeneous moraine cap. In order to protect the aquifers, it is necessary to delineate the buried valleys and to locate permeable sandy areas in the inhomogeneous clay cap. It is generally observed that variations in hydraulic conductivity of these formations are closely related to the variations in electrical formation resistivity. Numerous investigations during the nineties showed that the application of electrical and electromagnetic methods (Christensen N.B. and Sørensen K.I., 1998, McNeill, J. D., 1990, Meju, M. A., 1999) was successful and provided the hydrogeologists with detailed information on location of the buried valleys and the inhomogeneities of the protecting surface clays. In Denmark, the largest investigation of this kind so far is the transient electromagnetic (TEM) mapping of the municipality of Aarhus (Sørensen et al. 2003). The survey area is approximately 120 km2. In the period 1991 - 1998, more than 6000 TEM soundings were acquired with a density of 16 per km2. In 2000, the data were reinterpreted to ensure uniformity using current processing, modeling and inversion techniques. The investigation was carried out by cooperation between the Municipal Water Supply of Aarhus, the County of Aarhus and the Department of Earth Sciences at the University of Aarhus. The Department conducted the measurement, the data processing and the geophysical interpretation of the soundings, and the hydrogeological interpretation was performed by the County of Aarhus. The cost of the survey was approximately 1 million euro. In this presentation we give an overview of the gained experience and the new techniques developed through the investigation period. On a general level, it is development of standards which ensure enhanced data density and reliability, increased acquisition efficiency as well as standardization of mapping methods in a region. The latter provides the precise, detailed mapping needed to determine the extent, vulnerability and water quality of aquifers. The following geophysical developments have taken place: - Development of field methodologies for operating the TEM method in densely inhabited and culturally developed areas (Sørensen et al. 2003). - Development of new and enhanced TEM instrumentation. These are the PATEM method (Sørensen, 2000) and the HiTEM method (Danielsen et al., 2002). - Development of new data processing and interpretation software packages. This is enhanced modeling of instrument transfer functions (Effersø et al., 1999), MCI and LCI interpretation (Auken et al., 2001, Auken et al., 2000) and instrument calibration (Halkjaer et al. 2002).
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A Web based knowledge integration method: Application to data from multiple investigations on cultural heritage
Authors A. Castellano, R. Cataldo, G. De Nunzio, L. Marzo and A. TurnoneThe aim of this work is to suggest a method to explore relationships between features of chemical-physical variables that are distributed unevenly over space. Given a large number of data one could seek patterns that may not be visible without using advanced techniques of query, selection, analysis and display. For example, to access to the great variety of data available from multidisciplinary surveys on works of art, powerful tools are required to manage them. Exchanging and sharing are necessary to reduce time to be spent on low-end activities and to focus instead in higher-level thinking and problem solving. GIS (Geographical Information System) and MapServer, an OpenSource development environment for building partially enabled Internet applications (free available on http://mapserver.gis.umn.edu) are useful to perform the above task. GIS is a combination of robust hardware, powerful software, special data and thinking explorer. Through GIS one can manipulate hardware and software, work on the data, explore relationships between features and investigate important locations and topics. MapServer, beyond browsing GIS data, allows to create "geographic image maps", i.e. maps that can direct users. The same application serves as a "map engine" for other portions of the site providing spatial context where it is needed. This coupled software has never been applied to strongly multidisciplinary studies. In this work we attempt to fill this gap. Public administrators also have discovered GIS as an important tool for a wide variety of tasks; CAD-based maps of historical building can be maintained during many years. Furthermore by adding an intelligent interface to these maps the planning and analysis for safeguard management can be improved.
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Reduction of the archaeological risk by means of geophysical methods. An example from the south-west of peloponnese (Greece)
Authors S. Gaviano, C. Olivas, G. Korres and G. RanieriThe “a priori” knowledge of the physical and characteristics and the geometry of the subsoil is the basic element both for the optimisation of the design of engineering constructions, and the means to avoid or reduce the so called “archaeological risk” which can cause the interruption of engineering works. On the other hand the imperfect knowledge of the subsoil characteristics and the thoughtless or forbidden intervention that can follow, can produce the loss of the cultural goods without the “memory” of the archaeological remains. Practically that is very usual in the “urgency” condition and especially in urban areas and/or in a building construction. In such cases it is very hard to preserve the archaeological structures. It is well known that the subsoil knowledge can be obtained by the application of appropriate geophysical methods. They must be applied with care taking into account the context, the dimensions of the archaeological bodies, the presence of noise sources, the physical properties of the materials. The choice must also be made taking into account the economic aspect: the geophysical methods must be “convenient” in respect to the economic value of quality and cheaper than other methods like archaeological diggings or excavations. Normally in archaeological research it is necessary to acquire substantial data, which must be filtered, processed and interpreted in order to give a detailed reconstruction of the subsoil. All data must be ordered and normalized to give a pseudo 3D reconstruction: The quantity of data can make the direct excavation process long and expansive. Archaeological Heritage has different dimensions and characteristics, so the choice of geophysical methods depends on the type of archaeological structure that it needs to show. Type of measure, sampling rate, profiling distance, have to be compared to archaeological structure. To establish the convenience of Geophysics an economic parameter (Marchisio and Ranieri, 2000) can be applied: Monetary Expected Value of the archaeological heritage is the product of the probability to discover the object and its monetary value. Besides it’s possible to define the intrinsic value of geophysical information and value the quantity of methods to apply.
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Geophysical methods to recognise preexisting structures under an important archaeological site
Authors G. Ranieri, M. Boccone, R. Deiana and R. PorcuThe most characteristic Sardinian constructions are the “nuraghe”, all around the world there are examples only there. The nuragic period began from the Bronz Age to the Steal Age. A famous one is “Su Nuraxi” of the Barumini nuragic site (near Cagliari), that is the bigger between the 7000 nuragic sites in Sardinia. Researchers of several disciplines are studying to know more about their function, their constructive modality and their scope. Near the village there is a temporal stratification that probably continue under the surface soil. In some nuraghe site there are signs of pre-existing culture, with examples of neolithic villages. It would be possible that some nuragic sites were been chosen because of these pre-existing structures. The acknowledgement of those constructions near a such important archaeological site is very hard and often impossible. For the nuragic constructions were made without foundations we have applied geophysics methods, among them the seismic and the electric type, with emitters and geophones on the rest plan. We have used seismic tomography with P and SH waves to recognise probably pre-existing structures. High velocity zones are in correspondence of probably older constructions, while low velocity zone could represent poor resistance zones. The geophysical methods applied on the nuragic site have permitted to characterize the points in which take part with a static restoration, in some of them we have really found visible craks, in others not. These results have shown smaller resistance zones to take part for a static restoration. In Conclusion, the totality of the geophysical methods can be used to perform the static restoration and the electrical tomographies give important information on the groundwater paths, this is very interesting because the surrounding zones are devoid of water.
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GPR Survey to confirm the location of the Ronda castle wall (Andalucia, Spain)
Authors N. Yilo, D. Vázquez and A. CasasWith the increasing importance of the conscience for archaeological sites conservation, non-destructive methods of subsurface analysis are developed and becoming important. With the excavation budgets there are a lot of restrictions, political, economical and strategical ones (Scolar, 1990). Many times it is not possible to excavate, and the remote sensing and the geophysical methods could be a solution. Ground penetrating radar (GPR) is one of the methods that is gaining respect by archaeologists due to its capacity of give a high resolution results, wich allows the prospecting of small structures(Vaugha, 1986) and the location of underground objects at shallow depths, comparing it with others geophysicals technics (da Silva, 2001). We report here the results of a recent survey using GPR to search an Arabic castle wall that at the present is probably under the playground of a school in Ronda (Málaga), where a underground car-parking is projected.
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Non-Destructive testing on masonry structures: A series of different methodologies applied on the cathedral of Lucca.
Authors M. Marchisio, L. D’Onofrio, A. De Falco, V. Baroncini and D. MorandiA very dangerous situation of the West wall of the left transept of the Cathedral of Lucca was discovered by chance in 1994. The masonry structure is a typical three-layered one: the two external faces enclose an internal less regular brickwork, the so-called "muratura a sacco". During the eight centuries of life it underwent numerous interventions and repairs. Unfortunately in one occasion gypsum mortars were used. Due to water infiltration, in several areas the mortar was completely ruined. The damage was discovered by chance. Seismic tomographies were the first quantitative diagnostic tool to show the very critical situation of the structure. Very low velocity zones were very evident: they correspond to very low dynamic elastic moduli, less than 1000 daN/cm2. The tomographies put in evidence many structural details. The heavy loads due to the 30 m high structure of the transept act only on the external layers of the wall structure. The presence of an important portal makes the situation worst. Our description of the situation has been well confirmed by a series of other very accurate tests and historical investigations that were performed in the following years. A series of seismic tomographies were later performed across other parts of the wall of the church. Luckily the rest of the building did not show other problems. Since that period a number of different diagnostic tools were used and tested on this structure. The structures of this building in fact are a very interesting test-site, as there are very different situations in the different sides of the church and because of the very detailed information that were obtained by different methods. A number of microseismic and electromagnetic experiments are shown.
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Application of GPR and ERT in the archaeological site of “Palazzo di Teodorico” in Galeata (Forli’ – Italy)
Authors A. E. Bracci, C. Cesaretti and S. De MariaGaleata is located on the Bidente’s river valley (Tosco-Romagnolo Apennines), in the province of Forlì. The ancient urban area was located on a busy mountain pass. The first evidence of human presence at Galeata goes back to the prehistoric age, but the most ancient settlements are dated between the VI and IV century B.C. In the III century B.C., the Romans occupied the Celtic centre of Mevaniola, which became an important Roman municipality. The collapse of the Western Roman Empire caused the disappearance of the Roman town so the inhabitants moved downhill to what is now Galeata. During the Second World War an equipe of the German Archaeological Institute of Rome, led by Siegfried Fuchs and Friedrich Krischen, made excavations to find Teoderico’s Palace. The goth king raised his palace in order to use it as hunting resort in the beginning of VI century, A.D. The location of some shallower foundations can be partially identified in the area during the harvest time when the crops assume a lighter colour on the ancient walls, the picture suggests plan of the goth palace. The excavations had been resumed in 1998 by the Archaeological Department of the University of Bologna. Stratigraphical excavations revealed the existence of two main building periods: the first related to the Roman age, probably at the beginning of I century B.C., with an extensive installation of a large villa; the second period related to the late-ancient age. The activity of the Italian archaeologists partially confirmed the German excavation results and additionally found other important structures. These structures include a building with a complex planimetry, with polygonal rooms, apses and heated rooms which are probably the thermal area of a late-ancient “Palace” (extensive excavations in this zone are continuing and so the data are not confirmed yet).
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Geophysical investigation of a recent shallow fault
Authors J. B. Edel, G. Herquel and M. BanoThe surveyed area is located in the central part of the upper Rhinegraben, a few kilometres south of Wissembourg, close to the western border. A normal fault has been evidenced in the open sand quarry of Riedseltz. It was an opportunity for 1) practising geophysical prospecting, as part of the final year curriculum of Ecole de Physique du Globe, 2) testing various methods for imaging a shallow fault, 3) locate and follow the fault outside of the quarry. The study was partly supported by the BRGM.
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A First geophysical survey to investigate the Telóes basin
Authors J. M. Lourenço and M. S. MatiasAlong the major geological fault of Penacova-Régua-Verin, FPRV, (north Portugal, fig. 1) there are several tectonic depressions that have been interpreted as the result of recent reactivation of the fault. On the other hand these depressions are very important for water exploitation in a region where water resources are scarce. Generally, these basins are filled with undifferentiated sedimentary materials of unknown thickness. The “Telões Basin”, a grabben structurally controlled by the FPRV, is a typical and important economical example and its 3D study comprises a comprehensive geophysical project including Electromagnetic Mapping, Resistivity Imaging, Seismic Refraction and Gravity Mapping. Here in an account will be given of the Electromagnetic and part of the resistivity survey (Lourenço, 1997) that were the basis for the planification of the other geophysical techniques to be used.
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Joint interpretation of surface waves, refracted waves amd resistivity
Authors D. Orlowsky, R. Misiek, R. Elsen and B. LehmannTechniques to investigate the soil with the help of different geophysical methods increased rapidly during the last decade. Nevertheless, the use of different geophysical methods results often in different interpretations of the investigated area. This paper deals with the joint interpretation of surface waves, refracted waves (full waveform) and resistivity data acquired in an urban area of old coal mining where cavities or zones of loosening are expected in the near subsurface. Special anomaly indices are determined for each geophysical method. A weighting process combined these indices to a “Joint-Anomaly-Indicator” for the whole investigated area. This results in a map which clearly shows those locations with a high probability of cavities or loosening.
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Contribution for a better understanding in soil temperature
Authors M. A. R. Talaia and A. H. AmorimThis study allows us to understand how soil varies in depth and time when its thermal profiles are known. The research analyses the data of a twelve year period and contributes to the development of agricultural planning strategies.
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Geophysical characteristics of precambrian fracture zones - A case study from Rovaniemi, Northern Finland
Authors E. Lanne, J. Lehtimäki, H. Vanhala and U. VäisänenThe recognizing of fracture zones is important in the geological mapping, ore prospecting, ground water studies and in the applications of rock engineering. In environmental accidents fracture zones may also provide flow channels for groundwater contaminants. The aim of this study is to locate and characterize potential bedrock aquifers. The study is methodical and it is continuation of earlier works (Lanne et al., 1998).
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The Molasse fault zone in Lake Geneva, South-West Switzerland, From high-resolution seismic reflection data
Authors I. Chaudhary, D. Dupuy, M. Scheidhauer and F. MarillierThe Oligocene-Miocene Molasse basin along the northern rim of the Swiss Alps is affected by a major fault zone that separates Plateau Molasse to the north from Subalpine Molasse to the south. As a result of alpine compression, thrusts along this zone brought up Subalpine Molasse rocks by an amount of up to 1000 m. On the northern shore of Lake Geneva, near the City of Lausanne, the fault zone is well defined (Fig. 1). However, its extension in the lake and on its southern shore is not well known. Geology on both sides of the lake, however, indicates that major changes in the distribution of Molasse units occur in this region. Locating the Molasse fault zone across Lake Geneva would help to better understand geology and would provide important constraints on tectonic models of the Alps. In order to study the extension of the Molasse fault zone across Lake Geneva, we conducted a 2-D high-resolution multi-channel seismic reflection survey. Molasse units in the lake are overlain by Quaternary sediments up to 300-400 m thick (Vernet et al. 1974). Our data imaged these sediments and detected several faults in the Molasse units.
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Gravity anomaly map of the lower Vouga basin (Aveiro, North - Central Portugal)
Authors F. P. O. Figueiredo, M. J. Senos Matias and M. R. P. V. FerreiraSeveral studies have been carried out in the ‘Baixo Vouga’ area, near Aveiro, Central-North Portugal (Fig. 1), in order to investigate its geology and tectonics. However, so far it has not been presented an overall model for the behaviour of the bedrock (metamorphic and igneous) in the region. This work aims to contribute to the definition of the bedrock topography and deep geological structures in the ‘Baixo Vouga’ region. A gravity survey in an area of 560 km2 in the Estarreja, Murtosa, Aveiro, Ílhavo, Oliveira do Bairro, Albergaria-a-Velha, Vagos and Mira districts was done and a total of 653 gravity stations were measured over a non uniform grid.
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Geophysical investigation of a hydrogeological important fault structure
Authors H. Wiederhold, F. Binot, R. Kirsch, W. Scheer and C. StadtlerThe water supply in the northernmost part of Germany is mainly based on the groundwater reservoirs of Tertiary sedimentary layers. South of Flensburg these sedimentary layers are displaced by the north-south striking Flensburg-Sieverstedt fault zone with a throw of about 600 m at the base of Tertiary. The deeper part of this fault zone is well-known from earlier seismic investigations for hydrocarbon exploration. For an assessment of the groundwater reservoirs the near-surface structural feature of the fault was recently investigated through shallow seismic reflection and gravity surveys.
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Geophysical and atmogeochemical methods revealing old LNAPL contamination history of sandy aquifers
More LessDuring last years a series of papers was devoted to possibilities of geophysical methods in detecting long time residing LNAPL plumes in the granular sediments. Sauck (2000) developed a new conceptual geophysical model for this case taking into the account geochemical processes caused by the bacterial biodegradation of hydrocarbons. The existence of the low resistivity zone below the LNAPL plume is the only chance to estimate the extent of the plume by resistivity survey (multielectrode measurements, vertical electrodes) and ground penetrating radar. We combined the geophysical techniques with the soil vapour survey using the new method and instrumentation (ECOPROBE 5) based on the combination of total PID (photo ionisation detector) and selective IR (infra-red) analysers. The efficiency of this approach is documented on materials from an abandoned military area in the Cretaceous of Bohemia.
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Use of geoelectrical methods to evaluate undustrial contamination near Setúbal, Portugal
Authors E. C. Ramalho, R. Gonçalves, A. Nogueira, L. Torres and C. PenedaThis study consists on the estimation of contaminated soil volumes to be removed or remedied in a small industrial area near Setubal, Portugal, using geoelectrical methods. The area was used for a long period of time as a deposit of iron fillings impregnated with lubricating oil with high Zn content that can jeopardize its environmental quality. As a complement of a preliminary chemical analysis survey conducted to identify the problem, Induced Polarization and DC Resistivity surveys were also carried out, and results were related with Fe concentrations. Zn concentrations were estimated after Fe concentration and critical areas were identified using international standards. Bearing in mind these results, contaminated soil volumes were estimated.
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