Topographic effects in resistivity and induced-polarization surveys

Abstract We have made a systematic study of dipole-dipole apparent resistivity anomalies due to topography and of the effect of irregular terrain on induced-polarization (IP) anomalies, using a two-dimensional (2-D), finite-element computer program.A valley produces a central apparent resistivity low in the resistivity pseudosection, flanked by zones of higher apparent resistivity. A ridge produces just the opposite anomaly pattern--a central high flanked by lows. A slope generates an apparent resistivity low at its base and a high at its top. Topographic effects are important for slope angles of 10 degrees or more and for slope lengths of one dipole-length or greater. The IP response of a homogeneous earth is not affected by topography. However, irregular terrain does affect the observed IP response of a polarizable body due to variations in the distance between the electrodes and the body.These terrain-induced anomalies can lead to erroneous interpretations unless topography is included in numerical modeling. A field case demonstrates the importance of including topography, where it is significant, in interpretation models. A technique for correcting apparent resistivity for topographic effects uses the finite-element program to compute correction factors.