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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 - 20 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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