AC resistivity sounding

Abstract

DC resistivity sounding is a galvanic method, and the type of information obtained from application of th at method is determined by the behaviour of galvanic currents in the ground. This behaviour imposes inherent limitations on the scope for determination of earth parameters from the measurements. Among these limitations the equivalences of high-resistivity layers and low-resistivity layers are weil known. The high-resistivity equivalence, where neither the thickness nor the resistivity of a highly resistive layer embedded in better conducting surroundings may be determined but only the product of the two, is annoying in the context of many practical applications. In many cases highly resistive layers of dry sand and gravel are underlain by either wet and better conducting layers of sand and gravel or by low-resistivity c1ays. In these cases it becomes impossible to determine the amount of dry sand and gravel from DC resistivity soundings alone when prospecting for raw materiaIs, and in hydrogeological applications the depth to the water table is of ten undetermined. Furthermore, DC resistivity measurements do not all ow a determination of the anisotropy of the ground. Besides the galvanic method of DC resistivity sounding, there are a number of electromagnetic or inductive methods such as SLINGRAM and AMT (audiomagnetotelluric) with or without controlled source. The information gained from these methods is determined by the behaviour of induced currents in the ground and differs from the gal van ie information. Measurements with the inductive methods are strongly influenced by the presence of good conductors, while poor conductors are more or Iess invisible. The depth to a good conductor is usually accurately determined from inductive methods.