Three-dimensional imaging of subsurface structures using resistivity data

We have developed a three-dimensional inverse scheme for carrying out DC re sistivity surveys, incorporating complicated topography as well as arbitrar y electrode arrays. The algorithm is based on the finite-element approximat ion to the forward problem, so that the effect of topographic variation on the resistivity data is effectively evaluated and incorporated in the inver sion. Furthermore, we have enhanced the resolving power of the inversion us ing the active constraint balancing method. Numerical verifications show th at a correct earth image can be derived even when complicated topographic v ariation exists. By inverting the real field data acquired at a site for an underground sewage disposal plant, we obtained a reasonable image of the s ubsurface structures, which correlates well with the surface geology and dr ill log data.