Topographic responses in magnetometric resistivity modeling

Abstract

The magnetometric resistivity (MMR) topographic responses due to earth topography were simulated using a finite-element method. An algorithm was de­ veloped and the computer program was verified by comparison with analytic responses for half-space and contact models. The topographic responses for differ­ ent rugged surfaces were computed, and the model results indicate topographic effects can affect MMR sounding interpretation. In general, MMR topographic responses do depend on surface form; the more rugged the ground surface is, the larger the MMR topographic anomaly will be. These topographic effects will de­ crease as the distance between the source (and/or receiver) position and the high relief area is increased. We only address the problem of determining MMR anomalies over a two-dimensional (2-D) topography. A numerical example illustrates an effective means of reducing the terrain effects for a 45-degree dipping fault model incorporating a 45-degree ramp surface, suggesting that the finite-element modeling technique does provide a means of determining topographic correction for MMR sounding data. authors provide a way to reduce a great deal of topographic noise in electric surveys. Rather than measuring the potential field by the de resis­ tivity method, the magnetometric resistivity (MMR) method measures the magnetic field produced by the noninductive current flowing from grounded current electrodes. Based on the similarity between the two geoelectric methods, we have to evaluate the topographic effect on the MMR sounding. Until now the only available model study on the terrain effect in MMR surveys was demonstrated by Oppliger (1984) by using the surface integral equation method. In this paper, a 2-D modeling of topographic effects for the MMR survey is studied using the finite-element method instead of a finite-difference approach (Pai and Edwards, 1983; Tseng et al., 1985). The results provide the user of the MMR method with a guide of what to expect in some simple field situations. These results are also complimentary to those presented by Oppliger (1984). Although, 2-D modeling is useful only under certain circumstances, we hope the modeling technique described may extend its application to estimating and removing 3-D effects in MMR surveys. In the modeling process, a nonhomogeneous element is used to calculate the MMR topographic anomaly. The accuracy of the numerical solutions is examined by comparing them with analytical solutions, and results show that the rugged topog­ raphy can produce significant effect on the MMR measure­