Transient electromagnetic modeling of shallow A‐type sections with 3-D inhomogeneities

Abstract

A hard rock area underlain by granitic/gneissic or basaltic rocks often has an A‐type three‐layer geoelectric section in which resistivity increases with depth. The middle layer of moderate resistivity caused by fracturing/fissuring that lies between the surface‐weathered layer and the substratum of unfractured rock is not a good target for a direct current (DC) resistivity sounding since it is generally suppressed in the observations. Moreover, its definition requires expanding the electrode spacing to a length several times the depth of the target layer, and this may be a drawback if the target layer is either laterally variable or limited in its horizontal extent. We first studied the transient electric field of a horizontal electric dipole (HED) source excited by a step turn‐off current for a 1-D model of an A‐type geoelectric section. The results of this theoretical study are presented as graphs of normalized apparent resistivity versus a time‐related dimensionless parameter. Irrespective of the separation between the transmitter and receiver dipoles, these transient sounding curves become similar to the corresponding Schlumberger sounding curves at late time. Hence the transient electric field measurement offers the possibility of sounding at a fixed transmitter‐receiver spacing that may be shorter than the target depth. Also, at early times, for a certain ratio of the dipole separation to the target depth, there is a dramatic increase in the resolution of the response. Thus, it is possible to resolve suppressed layers of an A‐type section in this type of sounding. A study of the effects of transmitter ramp time and receiver bandwidth on the transient apparent resistivity curves shows that a very fast current shut‐off and wideband measurement are required to realize all the possibilities suggested by this modeling. Some 3-D transient electromagnetic (TEM) modeling was also done to simulate (1) a lateral variation in the resistivity of the middle layer of an A‐type section and (2) a weak zone of limited horizontal extent in the substratum of a two‐layer section. We observed that the 3-D inclusion has less effect at late time but is more pronounced at early time. In view of the above results, we conclude that the transient E‐field sounding with a grounded wire source can be used in place of a conventional DC resistivity sounding to overcome the problem of poor resolution due to the suppression of the intermediate layer in a geoelectric section where the resistivity increases with depth. As such, it has a potential application in groundwater as well as geotechnical surveys, because together with the overlying weathered layer, the fractured rock constitutes the aquifer in hard rocks.