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Designing surface-to-borehole electrical resisitivity tomography surveys using the frechet derivative
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 8th EEGS-ES Meeting, Sep 2002, cp-36-00064
- ISBN: 972-789-071-7
Abstract
Crosshole 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.