Application of 3-D resistivity tomography to delineate subsurface structures

Abstract

We have developed a three-dimensional (3-D) resistivity inversion code for resistivity tomography, where resistivity data are measured using electrodes installed in several boreholes as well as at the earth surface. The algorithm is based on finite element approximations for forward modeling and an ACB (Active Constraint Balancing) algorithm is adopted to enhance the resolving power of the smoothness constrained least-squares inversion. Resolution analysis with numerical examples shows that 3-D resistivity tomography is a promising tool to get a high-resolution 3-D image of subsurface structures. We have also shown that the topographic effect should be incorporated in the inversion to get an accurate 3-D image without artefacts. In the application of the method to the field data set acquired at a granite quarry, we could successfully delineate the 3-D attitude of a fault or fracture at the site. Moreover, we have shown that attributes of subsurface structures can be accurately defined by combining 3-D resistivity tomography and the borehole radar reflection method.