Calculation of a depth correction factor for the S-layer differential transform

Abstract

The VTEM system developed and operated by Geotech Limited and Geotech Airborne Limited is a central loop configuration system lending itself perfectly to many traditional ground interpretation strategies. One of these is the S-layer (thin, conductive layer) differential transform which is used to generate resistivity-depth sections. An empirical study indicated that delineating conductors in a conductive half space necessitates the implementation of a scale factor in order to obtain the correct depths and conductivity values when applying the S-layer differential transform. Based on an empirical approach, there was found to be an infinite number of depth correction factors that will still yield acceptable conductivity values and the need arose to explain the origin of this discrepancy and to find the correct depth correction factor. Three possible correction strategies were investigated based on comparison with synthetic data from models which have all shown that depths are overestimated by the S-layer differential transform. The most likely conclusion was that the physical assumptions regarding current distributions made in the S-layer transform lead to poor approximations of the conductors in a conductive half space. Assuming that the equivalent filament for the Slayer behaviour, as with the equivalent filament for the half space behaviour, does not coincide with the electric field maxima in the subsurface led to a plausible depth correction factor which was validated on various synthetic models.