A Method to Detect Displacements of Borehole Electrodes through Electrical Resistivity Tomography

In this work a new geophysical methodology is presented which is able to detect any displacements of buried electrodes that are permanently installed in boreholes. These displacements can seriously affect the quality of the measurements and the subsequent resistivity inversion image. The exact knowledge of the electrode displacements will either help in correcting the electrode coordinates and the geometric factors of the measurements associated with these electrodes or just to reject all corresponding erroneous measurements. During this method pole-pole type measurements are conducted, where the current is injected in every buried electrode in the borehole and the potential is measured on a number of electrodes that are spread out along a line on the ground surface. A finite-difference least-squares non-linear inversion algorithm with damping constraints has been developed in an effort to recover the accurate coordinates of the borehole electrodes. Although a priori resistivity model resulted by a surface ERT or by any other geological and geophysical information can be used to constrain the optimization algorithm it is also shown that that the lack of this knowledge does not affect the resulting electrode positioning. The validity and effectiveness of the approach was tested through synthetic modeling and real data.