Borehole deviation effect in electrical resistivity tomography

Abstract

In the analysis of resistivity tomography data, boreholes are assumed to be drilled vertically and have no borehole deviation with depth. However, such a condition is seldom encountered in real field situations. Moreover, in two-dimensional (2-D) tomographic imaging, it is assumed that the boreholes belong to the section to be imaged, while this is often far from the case in real situations. Given these discrepancies between assumptions and reality, unrealistic subsurface images may be obtained due to the improper handling of borehole deviation. In this study, the borehole deviation effect in 2-D resistivity tomography is analyzed. The effect is analyzed using a 3-D finite element forward modeling program to handle a general case of 3-D borehole deviations. As a further step, the borehole deviation has been included in the inversion process, which is implemented by assuming that the subsurface structure is 2-D within a 3-D resistivity tomography inversion code. By inverting the resistivity data obtained from a site where boreholes are significantly deviated, it is shown that the borehole deviation should be taken into account to obtain accurate subsurface images. The enhancement was also demonstrated with the 3-D ERT imaging of field data set.