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Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
101 - 120 of 139 results
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Non Invasive Characterization of a Small Mountain Catchment for Hydrological Purposes
Authors M. Rossi, G. Vignoli, R. Deiana, G. Cassiani and M.T. PerriThe description of catchment response during storms and droughts is a fundamental issue in geosciences. In particular, the subsoil characterization is a crucial step towards understanding hydrological processes in mountain regions and for the calibration of models that can describe and predict water flow patterns. Non-invasive (or minimally invasive) geophysical techniques are helpful to provide spatially extensive data, whereas traditional borehole-based sampling is often limited because of the localized nature of such measurements and the disturbance induced to samples. The aim of this study is the characterization of a small-scale mountain catchment to obtain detailed knowledge about its hydrogeological and hydrological system behavior. The experimental site is located at Carrè near Vicenza (North-Eastern Italy) in the pre-alpine hill region named Bregonze. For this purpose we used several geophysical techniques: Electrical Resistivity Tomography (ERT), Seismic refraction survey, Seismic surface wave investigation. The former techniques highlight the shallowest subsoil discontinuities (from 0 to 2 m); the latter one provides knowledge about the underlying layers and the associated lateral heterogeneities. The combination of geophysical methods can help characterize a complex hydrogeological system, at least from the structural viewpoint.
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Electrical Methods, Tools for Water Resource Assessment in a Semi Arid Areas – Case Study in Hebron Mountains
More LessNear-surface geophysical methods are widely non destructive techniques used in applications related to geological and environmental problems. In this study, a multi- electrode geo-electrical method has been employed to localise water outputs in the shallow sub-surface (15 -20 m.). This method has been used to characterize shallow features corresponding to a series of water outputs and to describe their geometry. Measurement campaigns have allowed obtaining several subsurface surveys to extract maps of the electrical resistivity. These surveys have been made in the southern part of Hebron mountains (Bani Naim-Yaqeen, Bani Naim-Bireen, Doura and Freideis), Palestine. They consisted of 12 electrical profiles using different electrode array configurations (Wenner, Schlumberger, dipole-dipole and pole-dipole), and 18 time domain electromagnetic soundings (TDEM). The interpretation of two dimensional (2D) pseudo-sections has been made using least square inversion which allows acquiring a full description of the electrical resistivity contrast around the several water outputs under studies. The applicability of this tool indicates that the Electrical Resistivity Tomography (ERT) was able to characterize the subsurface layers, to locate water storage and explain the flow paths of groundwater. Such information can play a significant role in the sustainable development.
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High-resolution Aquifer Vulnerability Mapping Using Longitudinal Electrical Conductance
Authors Y. Díaz-Acha, H. Himi, M. Elgettafi, J.C. Tapias and A. CasasThe non-saturated zone plays a crucial role protecting shallow aquifers against pollutants. Nevertheless, the high-resolution characterisation of the hydraulic properties of soils and sediments that cover the aquifer is difficult to be achieved, since direct investigations making use of samples obtained from boreholes and trenches are faced with both methodological and financial constraints. High-resolution geophysical methods like electrical resistivity tomography can be of great help in detailed groundwater vulnerability studies considering the well-known relationship between the electrical resistivity and hydraulic permeability of non-consolidated sediments. The geophysical parameter used in this paper is the longitudinal electrical conductance defined as the ratio between the thickness and resistivity of each geoelectrical layer. This parameter has a similar form to the AVI formula frequently used for assessing groundwater vulnerability. In spite of the fact that geophysical derived vulnerability indexes are quantitative and continuous, without the subjectivity inherent of indexes derived from other classical vulnerability methods, five categories ranging from very low to extreme vulnerability have been generated. Given the electrical resistivity variability of the sediments in the studied area, the resulting vulnerability map is very easy to be understood for any non-expert people involved in land-use management.
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Helicopter-borne Electromagnetics and Geoelectrics to Investigate the Fresh-water Lenses of the North Sea Island Borkum
More LessHelicopter-borne electromagnetic (HEM) measurements were conducted over the North Sea island of Borkum to map the size of the fresh-water lenses. Additionally, geoelectrical DC measurements were carried out at locations where data of 13-17 years old Schlumberger soundings exist. Based on the previously measured data possible changes of the fresh-saltwater boundary should be determined. The lateral extension as well as the thickness of the fresh-water lenses of Borkum could be mapped successfully with HEM. At several locations the depth of the fresh-saltwater boundary determined by HEM was confirmed by the results of the Schlumberger soundings. No significant changes of the fresh-saltwater boundary could be investigated at the locations where previous Schlumberger soundings exist. Considering direct push and borehole data, the HEM inversion models could be determined more precisely and additional to the fresh-saltwater structure thin clay layers could be revealed. Based on the HEM inversion results the potential volume of the fresh-water resource was estimated.
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Reconstruction of Groundwater Aquifer Models with Cluster Analysis
Authors R. Kavanda and Z. NyariPrecise detection of the structures of subsurface aquifers is an important environmental geophysical problem. The main point of the geophysical investigations is the determination of the aquifer’s vulnerability with the presence of low permeable protective layer above it. DC resistivity method is suitable for this task. Geological and geolelectric models were createdfor investigation of the four types of typical fresh water resources. Cluster analysis is an effective tool for combined interpretation of measured and non-geophysical data. A non-hierarchic algorithm was developed for the reconstruction of the groundwater aquifers’ structures combined interpretation of DC resistivity data and geological-hydrogeological information.
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Integrated Geophysical Profiles and H/V Microtremors Measurements for Subsoil Characterization
Authors A. Macau, B. Benjumea, A. Gabàs, F. Bellmunt and S. FiguerasThe application of traditional geophysical techniques such as seismic or electrical resistivity tomography (ERT) in broad areas requires large period of time. In this work a methodology is proposed in order to optimize the data acquisition time in the determination of the weathered materials thickness overlaying the unaltered rock in a large area. This methodology is based on combining traditional geophysical techniques and H/V microtremor measurements. The test site is in an area located in the NE part of Iberian Peninsula, close to the Mediterranean coast. Geologically it is mainly made of granitoides (Catalan Coastal Pluton) highly altered on its surface. In this context we found the suitable conditions for the application of this methodology: strong contrast of geophysical (mechanical and electrical) properties. The proposed methodology allows us to characterize the sediment properties and the bedrock depth.
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Effectivity Enhancement of Azimuthal Geoelectric Measurements
More LessFor the hydrogeological characterization of fissured rocks it is very important to know the fissure directions. To determine the dominating directions geophysically, the so-called azimuthal geoelectric measurement had seemed to be a realistic way, until its shortages (false fissure directions in many cases) were revealed. In this paper, we improve the azimuthal measurement, on basis of analogue modelling, by using the Wenner array, which is generally applied in azimuthal measurements. We study how many azimuthal measurements are needed to be carried out to determine the dominating fissure directions (each in different measuring point over the investigated area, instead of one single measurement over the whole area as had been earlier suggested). In the model parameter range, we used to characterise three-directional fissure system, it is possible to apply the anisotropy paradox theory (which is not always the case). As we concluded from the model measurements, in order to determine solely the fissure directions, our improved method seems to be a bit expensive, however it is less expensive than the aquifer test. Involving further arrays, or combining the azimuthal measurements with profile geoelectric measurements, we expect a more reliable solution at a somewhat reduced cost.
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Improving the Progress of Small Scale Magnetic Resonance Sounding (MRS) Measurements
By T. RadicQuestions about precision agriculture, foundation testing and renaturation require fast methods with a high spatial resolution in the near-surface (up to 10 m) subsoil. The water content, the saturation and the pore space distribution are key parameters to characterize the near-surface soil. By measuring these parameters, the Magnetic Resonance Sounding (MRS) method provides an important contribution. Unfortunately, however, measurement progress is too slow. A substantial increase of measurement speed is therefore an essential requirement for a routine application of the MRS method for the mentioned questions. For this, two aspects play a central role: the effective disturbance signal suppression method and the parallelization of the MRS measurement procedure. For both aspects, we have developed and successfully tested a solution. A novel type of reference technique was implemented for effective noise suppression. Parallelization was realized with a multi-channel receiver and a special configuration of the transmitter loop and the receiver loops.
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Surface and Borehole Geophysical Measurements for Fissured Clay Sediments and Water Infiltration Characterization
Authors G. Bièvre, D. Jongmans, R. Béthoux, Y. Orengo and V. ZumboThis works presents the results gained with classical geophysical methods (Electrical Resistivity Tomography and Seismic Refraction) for the characterization of water infiltration within fissured clays in a landside area. Measurements were repeated on a monthly basis. Time-lapse processing of the data showed that electrical resistivity tomography is efficient to image preferential infiltration paths. On an other hand, seismic refraction methods did not allow to detect variations. Other techniques, such as ambient noise cross-correlation, will be tested in the future.
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Assessment of Improved Measurement Technology for Magnetic Resonance Sounding
Authors M. Mueller-Petke, S. Costabel, G. Lange and U. YaramanciThe technique of Magnetic Resonance Sounding as unique hydrogeophysical prospection method has shown several improvements in data processing, inversion and interpretation during the last years. Along with these improvements, detailed innovations on instrumentation have always been demanded to support the use of this developments. Latest developments in MRS instrumentation now promise to fulfill these demanded hardware developments with a second generation MRS instrumentation. Therefore, a field comparison was carried out comparing this second generation instrumentation (GMR) and the standard MRS device (Numis) at field condition. This field compain includes direct comparison using exactly the same configuration, i.e. loop size and position, while only the device is changed. Different well known and documented test sites (Nauen and Haldensleben) with both different geology and noise properties to test S/N improvements and comparison to further geophysical methods, i.e. DC geoelectrics, GeoRadar and borehole measurements. First results of this field comparison clearly support the demanded hardware developments. The noise cancellation technique has been successfully proved. The multi-channel digital signal detection provides fast and accurate data and opens new field of research.
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Transmissivity Estimation from Sounding Data of Holocene Tidal Deposits in the North Eastern Part of Cuxhaven, Germany
Authors M. Attwa, T. Günther, M. Grinat and F. BinotThis paper introduces transmissivity estimation for Holocene tidal flat deposits in Cuxhaven area by using various approaches. A new empirical relation is discussed and derived from normalized aquifer resistivity (ρ-). Thirty electrical soundings are conducted and interpreted using 1D inversion. A spectral induced polarization (SIP) sounding point is derived from 2D profile to calculate the transmissivity using Börner model. The DC geoelectical parameters (Dar-Zarrouk) are estimated and interpreted qualitatively in the terms of hydraulic conductivity, transmissivity, clay content, grain size distribution and potentiality of the aquifer. We calculate quantitatively the transmissivity using the empirical equations suggested by Kosinski and Kelly (1981), Niwas and Singhal (1985) and Yadav (1995). These empirical relations are developed to a new empirical equation with expression of the form of (ρ-)A * h * B, where A and B are constants and h is the aquifer thickness. This equation depends on the best fit between the actual and estimated transmissivity values. The estimated transmissivity values from various approaches are compared with those derived from the grain size distribution of samples, at the four well locations, and we observe a reasonable agreement between them. Thus, the locations of high hydraulic conductivity could be determined.
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Density Variation Modeling Based on Resistivity and Soil Moisture Data
Authors T.M. Hokkanen and N.P. SzaboThe need of the real time (or with short time period) soil moisture monitoring has become more important nowadays. Many research groups are developing new monitoring techniques and methods for that globally. We established a study field for applying resistivity measurements (RES) together with capacitive soil moisture (CP) data to determine the bulk density function of the soil of the study area. According to results it is possible to define a density model as a response of temporally and spatially changing the moisture of the soil. Relevant equations are presented in this paper.
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Airborne Geophysical Investigations for Hydrogeological Purposes in Northern Germany
Authors H. Wiederhold, G. Schaumann and A. SteuerAn airborne geophysical investigation project is carried out in 2008 and 2009 by the Leibniz Institute of Applied Geophysics (LIAG) in close cooperation with the Federal Institute for Geosciences and Natural Resources (BGR). Both, frequency domain as well as time domain electromagnetic airborne systems are used in these surveys. Emphasis is placed on the mapping of fresh-water-saltwater interfaces in coastal aquifers, the mapping of submarine freshwater discharges and saltwater intrusions due to saltdomes. Results are presented showing freshwater lenses in the North Sea coastal area and the influence of saltdomes on the groundwater quality.
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Application of PNN (Pulse Neutron) and EM (Electromagnetic) Measurements for Ground Mapping of Polluted Areas
Authors P. Buckup, P. Dietrich and M. BuckupThe combined PNN and EM technologies offer new perspectives for control and mapping of polluted soil areas. Polluting components of different character are detected and quantified. By mean of time elapsed measurements changes of pollution are controlled in-situ. There are new perspectives for complex data processing, related to problems in near surface geophysics.
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Seismic Characterization and Monitoring of Fresh Concrete Using Acoustic Waves
Authors P. Senechal, S. Garambois and C. BordesThis study focuses on the seismic characteristics and behaviour of fresh concrete (water-cement ratio >2) designed for consolidation of soils, which was not really studied in the past. The propagation of acoustic waves, which constitute a potential non-destructive imaging tool in the future, was monitored and characterized using a specifically-designed experimental device during eight hours in the laboratory, to assess the evolution of the mixed material as a function of its maturity. This study highlights the very low values of P wave velocities in such a material, as well as the corresponding S waves velocities, their associated large attenuation and dispersion in the [0.5-6 kHz] frequency range. It has been found that attenuation remains the lowest when concrete mixture is younger than 90 minutes and that no velocity dispersion was detected. The study of velocity and attenuation at early age shows that the concrete has a fluid mixture with grains in suspension, and presents an elastic behaviour which stays poorly compressible during the first eight hours.
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3D PSTM Using Weighted Multipath Summation for Imaging Shallow Subsurface
Authors S. Keydar, V. Shtivelman, P. Trachtman and M. EzerskyA new method for imaging shallow subsurface using 3D prestack time migration is proposed. The method is implemented as a very fast, fully automatic, macromodel-independent procedure. The migration algorithm is based on a multipath summation of seismic traces. Contrary to the conventional techniques requiring information on migration parameter (velocity or radius of wavefront curvature) which is usually estimated by a time-consuming optimization procedure, the proposed multipath approach does not require exact values of the parameter since applies spatial summation over a wide range of the parameter values. Furthermore, performing the multipath summation with image-dependent weighting can significantly improve the resulting images. The method was tested on synthetic and real data. The results of the testing show that the obtained time migration images are usually similar to those obtained using the conventional Kirchhoff migration. In cases of varying velocities the results of the weighted multipath migration may be even superior to those of the conventional migration.
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Intra Source Array Offset and Static Difference Compensation by Optimal Vibrator Sweeping
Authors A. Jimenez, S. Savescu, A. El Aziz Khairy Abd El Al and P. ScholtzDuring a seismic reflection measurement vibrators are used in groups, where all the members emit the same sweep simultaneously. Since there are small offset differences within the source array (the vibrators cannot be placed at exactly the same location and the pattern of vibrators provides filtering of coherent noises) the recorded signals are distorted by the slightly different arrival times of the individual vibrator signals. Also intra array static differences exist due to the changing properties of the nearsurface. These effects can be observed in the loss of energy, which is more pronounced at higher frequencies. For a seismic survey we can calculate optimal sweep start delay times for each vibrator within the vibratory group at each vibrator position. These optimal start delay times are applied to control the vibrators’ sweeping during the field acquisition. We show, that down to a target depth, the signal summation is improved and there is less high frequency loss, the resolution increased. We have also investigated the dependency of the arrival time differences on the survey parameters and the frequency-amplitude decay phenomena. The suggested method could give more signal amplitude at high frequencies with no special effort compared to the conventional technique.
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Amplitude Correction in Separating P- and S-waves in Elastic Seismic Data
More LessThe P- and S-waves in elastic seismic data recorded at the earth’s surface can be separated by implementing downward extrapolation followed by divergence (dilatation) and curl (rotation) calculations at a decomposing depth. The ratio between P-wave amplitude and S-wave amplitude (P-S ratio) in the form of displacement (pre-separation) and that in the form of dilatation and rotation (post-separation) are different because dilatation and rotation are different physical quantities. This difference can be eliminated by multiplying a correction factor, which is the ratio of S-velocity to P-velocity, to the separated S-wave (rotation), and the P-S ratio after separating P- and S-waves is then preserved.
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Preliminary Results of the Local Seismic Response Analyses for the Structural Design of the Tremestieri Port, NE Sicily
Authors F. Pizzolo, M. D'Amico, S. Cutrì and A. BottariWithin the geophysical surveys for the construction of the intermodal platform Tremestieri port’s (Messina, NE Sicily), we have performed active seismic methods (cross-hole and down-hole) that allowed to estimate the Vs profiles from the analysis of the wave-fields generated by artificial controlled sources. Passive seismic methodology (HVSR), based on single station measurement, has been used to estimate the site amplification concerning to the resonance frequency of the sedimentary cover from the horizontal to vertical spectral components ratio of the environmental noise. In addition, the subsoil was sampled in order to obtain a good litostratigraphic control of the investigated terrains. The achieved results have been employed in the present work to define the design seismic input and to characterize the soil instability levels in a region that has been struck in the past by moderate and large earthquakes.
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Spatial Distribution of Weathering Using Seismic Refraction – Further Results in the Vicinity of Canelas, NW Portugal
More LessIn the recent years the ever growing concern of policy makers with respect to natural hazards has led researchers and practitioners to seek more reliable, precise and time efficient techniques map zones prone to hazards. Mass movement is a hazard common to many mountainous areas around the world which can pose a serious threat to the population living within the area as well as to the structures, property, environment and crops. Several geophysical methods assume a relevant role in monitoring and surveying unstable slopes. Our aim was to show that seismic refraction can meet the demands of a time efficient and cost effective method to map this vulnerability. We performed a series if profiles with the aim of determining distribution of rock weathering thickness and velocity distribution within the vicinity of Canelas (NW Portugal). The results of each profile were analysed to give thickness of geotechnical soil as well as average values for both near surface and deeper part of each profile. The results were georeferenced in GIS and thus the estimation of the spatial distribution of the parameters allowed a means of observing some correlation with the previously mapped features of the area, namely the geology and topography.
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