Difficulties in determining electrical anisotropy in subsurface investigations

Surface electrical and electromagnetic methods have a limited resolution capability for determining the conductivity structure of the earth. In one-dimensional modelling a collection of many thin layers is frequently considered as one composite layer, which is then macro-anisotropic. Neither galvanic methods nor inductive methods alone can resolve the anisotropy of the ground, but a joint inversion of galvanic and inductive data may do so. The necessity of including the coefficient of anisotropy in the joint inversion of galvanic and inductive sounding data is demonstrated. An analysis is made of the combined use of geoelectrical and transient soundings to resolve the coefficient of anisotropy of a subsurface layer for varying thickness, resistivity and coefficient of anisotropy. It is found that the coefficient of anisotropy is well resolved only for layers that are many times thicker than the overburden and for coefficients of anisotropy that are not too small. The ability of the joint inversion of geoelectrical and transient sounding data to resolve macro-anisotropic layers is tested using realistic earth models determined from electrical logs.