Anisotropic apparent resistivities arising from non-homogeneous, two-dimensional structures

Abstract

Variations in the electric and magnetic fields are calculated for a non-homogeneous, two dimensional structure representing a sediment-filled graben surrounded by crystalline rocks. The apparent resistivities calculated along the cross-section profile for the electric field parallel (ρ_E) and perpendicular (ρ_H) to the strike are anisotropic. The two apparent resistivities, ρ_E and ρ_H, at a given point outside the graben, parallel each other over a broad range of periods. They do not show a convergence at long periods. Thus, it is not necessarily an indication of non-homogeneity at depth, when ρ_E and ρ_H are different at long periods. In this case, the apparent resistivity anisotropy is caused by a near-surface distortion of telluric currents. Although the ρ_E data are less disturbed by the non-homogeneity, it does not seem justified to interpret the ρ_E data in terms of Cagniard models.