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Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 06 Sep 2010 - 08 Sep 2010
- Location: Zurich, Switzerland
- ISBN: 978-90-73781-88-7
- Published: 06 September 2010
141 - 153 of 153 results
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Improving the Understanding of Sub Lagoon Hydrogeology with AEM: The Venice Lagoon Example
By A. ViezzoliA comprehensive investigation of the exchange between surficial waters and groundwater in transitional environments, and hence the mixing of waters characterized by a different salt concentration, is an issue of paramount importance considering the ecological, cultural, and socio-economic relevance of transitional environments. Acquiring information, which can improve the process understanding, is often logistically challenging, and generally expensive and slow in these areas. This applies both for punctual, invasive borehole measurements as well as for ground-based non invasive geophysical surveys. Here we investigate the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy. The quasi-3D modelling of the SkyTEM data by the spatially constrained inversion (SCI) methodology allows to accurately distinguish several hydrogeological features, both underneath the lagoon and the farmland beside it. Hydrogeological features resolved are, for example, the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom, and areas of probable submarine groundwater discharge. While the AEM data show a high degree of coherence with available ancillary information, both form wells and from other geophysical techniques such as high resolution off shore seismic, they improve greatly the understanding of the hydrogeology in the Venice lagoon, both at large scale and in details. The research highlights the great potential of AEM to improve significantly the hydrogeological characterization of subsurface processes in lagoons, wetlands, and deltas worldwide
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Optimizing an Existing Inversion Code
Authors C. Kirkegaard and E. AukenAirborne ElectroMagnetic methods (AEM) have been used for geophysical surveys for several decades, in particular due to its ability to cover large areas on a limited budget and in relatively little time. Ever since the method was first deployed the tendency has been for the typical survey size to be growing at a very rapid rate, in particular during the last decade. AEM surveys of today are often in the order of tens of thousands of flight line kilometres, producing huge datasets that needs to be inverted. Performing a full non-linear inversion of such datasets is a tremendous computational burden and consequently, these datasets are very often inverted using approximate inversion schemes. Several full non-linear inversion codes for AEM data is reported in the literature, however, most of these codes operate on single soundings only and are not optimized for large datasets. We discuss how the existing constrained inversion code em1dinv has been optimized for modern parallel computing and show performance results. The parallel optimized code is capable of handling very large datasets and provides a significant reduction in the total inversion time. Furthermore, we show how the optimized code eliminates inversion artefacts inherent of the previous implementation.
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Seismic Dispersion Analysis Feasibility for the Subgrade Investigation: Measurement, Experimental and Numerical Modeling
Authors D. Leparoux, L. Laguerre, F. Treyssede, P. Côte and V. FerberThe study focuses on the feasibility of the seismic guided waves dispersion analysis for investigating the subgrade layer made of a treated soil, in the case of under construction roads, before the shallower pavement layers are built. Such a non destructive geophysical method could help to qualify the project acceptance concerning this earthworks phase. In this context, the issue deals with a two layers medium case where the investigated subgrade, whom the top is the measurement surface, lays above a low velocity zone, i.e. the natural soil. The first feasibility stage involved field experimental measurements. A clearly main mode was extracted from the dispersion diagram and has been inverted to a S wave velocity profile leading to a global information concerning the subgrade. In a second feasibility stage dispersion diagram has been studied through a reduced scale experimental modeling providing a well controlled data set that have been compared with theoretical results firstly calculated without any source effects and secondly by tacking into account the source effects through a new numerical approach. The results show other modes distinguishable in the diagram that could be taken into account in the case of a dispersion diagram inversion.
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Static Correction Determination Based on Delay Time Inversion
Authors T. Muhammad, H. Perroud and D. RoussetTo obtain a high resolution seismic image,static correction plays a vital role.Static correction is applied to mitigate the effect of irregular topography and weathering velocity.Inadequate static correction will result in seismic data smearing and false anomalies.We present a data driven technique that performs the delay time inversion based on the first arrivals for static correction computation. Based on this technique we do not require the explicit velocity information and the subsurface depth model.So it perform all the necessary steps for static correction computation in time domain thus prevents from certain errors that might propagate otherwise due to time to depth conversion.We applied the time-delay inversion method on the real field data and compared the results with other methods i.e. delay-time and generalized reciprocal method.Generalized reciprocal method requires the explicit velocity information and optimum distance.Change in any one of the parameter will change the apparent reflector velocity and depth model. While using delay time inversion we do not require velocity information and optimum parameter.Moreover delay time technique can be generalised to several weathering layers as compared to the generalised reciprocal method which is valid for only one layer.
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Application of Mobile Geophysical Methods for the Examination of Areas of Landslide Processes Formation and Development
Authors S.P. Levashov, N.A. Yakymchuk, I.N. Korchagin, Y.N. Pischaniy and D.N. BozhezhaThe results of practical application of the non-traditional geoelectric methods of forming a short-pulsed electromagnetic field (FSPEF) and vertical electric-resonance sounding (VERS), as well as georadar sounding on destroyed by the landslide process area of Kiev-Odessa highway are given. It is shown, that the presence of the zones with raised filtering of ground waters is the main reason of roadway erosion and landslides formation. The detection and mapping of underground water flows and areas with the raised soil moistening can be realized operatively by complex of used geophysical methods. The areal survey by FSPEF method allows finding and mapping operatively the underground water flows of natural and man-caused origins and the zones of soils moistening. The bedding depths and thicknesses of water-saturated layers are defined in cross-section by VERS sounding. This complex of geoelectric methods can be used also for regular monitoring of the engineering-geological condition of environment on landslide-dangerous areas of roads.
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3D GPR Measurements for Fractures Detection in Selected Post-mining Regions in Poland
More LessThe paper presents the results of 3D GPR measurements carried out over shallowly located drifts. The aim of georadar investigations was to analyse the influence of mining activity on the rock-mass. Geophysical measurements were conducted in two sites with different geological conditions, i.e. over the drift made in gneiss and over the excavations made in sandstone. All excavations were located shallowly, surrounding geological media had low attenuation and reflection coefficients on the boundaries between hosting media and the voids (drifts) were high. Despite of mentioned facts there were recorded no reflections from the roofs of drifts. This interesting effect was additionally analysed in the paper.
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Investigation of Near-surface Anomalies Using the Refraction Seismic Method
Authors T. Engelsfeld, F. Šumanovac, V. Krstic and N. PavinIn this paper we investigate the influence of near-surface anomalies on the first arrivals of seismic waves. The basic two-layer model is modified with the following anomalies: (1) circular cavity positioned in the upper layer, (2) buried circular object positioned in the upper layer, (3) part of the border between the layers is deformed into concave shape. All anomalies are characterized with the seismic velocities that are lower than the seismic velocity of the neighboring region. First arrivals are calculated by numerical modeling of 2D eikonal equation. Numerically obtained traveltimes are presented in the time-distance graph and are also used to construct the graph of wavefronts. In all cases, the shape of the time-distance graph is characterized by a peak point which is a consequence of slower propagation of seismic waves through the analyzed anomalies.
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Joint Inversion of Seismic Refraction and Electrical Resistivity Tomography to Investigate Sinkholes
Authors B. Gebregziabher, T. Günther and H. WiederholdA joint inversion of seismic and resistivity data was carried out for site investigation to delineate subsurface expression of dissolution induced sinkholes and assess the risk for future development of the study area. The results of the 2D seismic refraction tomography are compared with results of the 2D electrical resistivity tomography and show good compatibility at a depth of about 20 meters. The results of cluster analysis of the joint inversion indicate that the subsurface section of the study area consists of four layers; dry sand, saturated sand, till/clay and saturated chalk layers. The joint inversion results and the lithology from boreholes confirm that sandy sediments are dominant on top of the chalk close to the observed sinkholes and to the discontinuities in reflection seismic sections instead of till/clay which is dominant towards North. Most of the discontinuities and observed sinkholes are located close to the boundary between the dry sand and the shallow till/clay layer. This geologic boundary and the permeability of the sand layer allow for direct groundwater contact with the bedrock (chalk) surface and may lead to the dissolution and formation of sinkholes at the surface.
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Geophysical Prediction and Following Analysis of the Dead Sea Sinkhole Development Through the Ein Gedi – Arugot Area
Authors M.G. Ezersky, L. Eppelbaum and B. MedvedevGeophysical methods - Seismic refraction (SRFR), Electrical resistivity tomography (ERT) and Microgravity were applied to the Dead Sea (DS) sinkhole problem in the Ein Gedi - Wadi Arugot area during the earlier stage of the sinkhole development (1998-2002). They allowed to determine the sinkhole formation mechanism and localize the sinkhole hazardous zones. Following monitoring of the sinkhole development confirmed our suggestions. The numerous boreholes drilling by the Geological Survey of Israel verified the location of the salt edge. The GIS database testifies that during 2003-2009 new sinkholes developed along the salt edge within a narrow 50-70m wide strip oriented approximately in north-south direction (slightly parallel to the shoreline). No promotion in west-east direction (normally to the DS shoreline) was observed. Collapse of sinkholes and their clustering have been occurred within the area of high resistivity anomaly and negative residual gravity anomaly.
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