Gradient array profiles over conductive veins

Abstract

Under certain circumstances thin target bodies in geoelectric profiling exhibit a form of equivalence that is more commonly associated with thin horizontal beds in electrical sounding operations. In particular, gradient array responses measured over inclined conductive veins with sufficiently large depth of burial (in relation to the vein thickness) and sufficiently large conductivity (with respect to the surrounding medium) are essentially equivalent to those over conducting ribbons with the same conductivity-thickness product. This equivalence behaviour is exploited in order to investigate the influence of the body parameters on the geoelectric profiles measured with a gradient electrode arrangement over thin conductive veins. The effect of dip, depth of burial, depth extent and conductivity-thickness product are considered individually. It is demonstrated that the form of the apparent resistivity and apparent chargeability profiles over conductive veins display a simple inverse relationship. Further, it is found that the conductivity-thickness product has little influence on the form of the response profiles ? it only affects the anomaly amplitudes. A similar effect is noted with the dip angle of veins having an extended depth extent. Here the form of the profiles is increasingly antisymmetric and largely independent the dip. This behaviour has significance to the interpretation problem. In this context conductive veins are conveniently classified into two basic types depending on their relative depth extent. The greatest problem occurs with veins of extended depth extent. For these bodies it is concluded that the interpretation procedure can only define the location of the vein apex. All other vein parameters influence only the amplitude of the profiles and not their shape, so that the interpretation problem is fundamentally underdetermined in the absence of further information.