The Determination Of Electrical Anisotropy Using Surface Electric And Electromagnetic Methods

The concepts of homogeneity and isotropy play an important role in electromagnetic<br>modeling. Usually we consider models to be composed of elements that are homogeneous and<br>isotropic, whether the models be one-, two-, or three-dimensional. However, real geological<br>formations may exhibit anisotropy in two ways. Firstly, the formation may be intrinsically<br>anisotropic because of the micro-structure of the formation. In this category we find clays that<br>because of the elongated shape of the individual mineral grains and the processes of deposition<br>can have a better conductivity in the direction parallel to the grain planes. Secondly, surface<br>electric and electromagnetic methods have a limited resolution of the conductivity structure of<br>the subsurface, and in one-dimensional modeling we shall often have to consider the collection<br>of many thin layers as one composite layer, which will then be macro-anisotropic. In both cases it<br>is most often assumed in one-dimensional modeling that the conductivity is the same in all<br>horizontal directions, but different from the vertical conductivity.<br>Neither galvanic nor inductive methods alone can resolve the anisotropy of the ground.<br>However, a joint inversion of galvanic and inductive data requires that anisotropy be taken into<br>account and can also resolve the coefficient of anisotropy and thereby contribute to a more<br>detailed description of the subsurface resistivity structure (Jupp and Vozoff 1977).<br>The determination of electrical anisotropy is desirable as it may serve as an indicative<br>parameter for the presence of otherwise unresolved thin layers. From a hydrogeological point of<br>view these may severely influence the hydraulic flow pattern in the ground. Thin clay layers in an<br>otherwise sandy formation will lower the vertical hydraulic conductivity considerably, and the<br>presence of thin sand and gravel layers in an otherwise clayey formation may serve as fast<br>hydraulic conduction channels for polluted water. In connection with mapping of raw materials<br>anisotropy indicates that the material under investigation is not homogeneous throughout and<br>may thus be of inferior quality.<br>In the following an analysis of the importance of taking anisotropy into account in inverse<br>modeling will be presented, and it is shown how the combined use of geoelectrical and transient<br>soundings can resolve the coefficient of anisotropy of a subsurface layer. It is found that the<br>coefficient of anisotropy is only well resolved for layers which are many times thicker than the<br>overburden.