Tank modelling of azimuthal resistivity surveys over anisotropic bedrock with dipping overburden

Abstract

ABSTRACTRotational or azimuthal resistivity sounding is frequently employed for determining the electrical anisotropy of the subsurface, from which the orientation of fracturing, which might give rise to the anisotropy, is interpreted. However, symmetrical 4‐electrode arrays, such as the Wenner, Schlumberger and square, are ambiguous and will also produce an anisotropy‐style signature over dipping strata or a gradational lateral change in rock resistivity. This problem may be overcome by use of a 5‐electrode offset‐Wenner array. Simple tank modelling of an anisotropic bedrock overlain by an isotropic overburden demonstrates that rotational offset‐Wenner sounding will clearly indicate whether observed anisotropy is real or whether it is merely due to the similar and ambiguous effects of a varying overburden thickness.