Can electrical anisotropy be determined using surface electrical and electromagnetic methods?

Geological forrnations may exhibit anisotropy in two ways. Firstly, forrnations like clay may be intrinsically anisotropic because of the micro-structure of the forrnation. Secondly, surface electric and electromagnetic methods have a limited resolution of the conductivity structure of the subsurface, and in one-dimensional modeling we shall often have to consider the collection of many thin layers as one composite layer, which will then be macro-anisotropic. In both cases it is most often assumed that the conductivity is the same in all horizontal directions, but different from the vertical conductivity. Neither galvanic nor inductive methods alone can resolve the anisotropy of the ground. However, a joint inversion of galvanic and inductive data requires that anisotropy be taken into account and can also resolve the coefficient of anisotropy and thereby contribute to a more detailed description of the subsurface resistivity structure (Jupp and Vozoff 1977). The deterrnination of electrical anisotropy is desirable as it may serve as an indicative parameter for the presence of otherwise unresolved thin layers. From a hydrogeological point of view these may severely influence the hydraulic flow pattern in the ground. Thin clay layers in an otherwise sandy forrnation will lower the vertical hydraulic conductivity considerably, and the presence of thin sand and gravel layers in an otherwise clayey forrnation may serve as fast hydraulic conduction channels for polluted water. In connection with mapping of raw materials anisotropy indicates that the material under investigation is not homogeneous throughout and may thus be of inferior quality. Through the following analyses it is found that a combined use of geoelectrical and transient soundings can resolve the coefficient of anisotropy of a subsurface layer. It is found that the coefficient of anisotropy is only well resolved for layers which are many times thicker than the overburden.