The effects of cultural noise on controlled source electromagnetic resonses of subsurface fractures in resistive terrain

Abstract

Controlled source electromagnetic (CSEM) geophysics has been used with a fair amount of success in near surface hydrogeological studies. Recently, these investigations have been conducted frequently in human impacted field sites containing cultural conductors such as metal fences and buried pipes. Cultural noise adds an element of complexity to the geological interpretation of this type of data. This research investigates the influence of mutual induction between two buried targets in a CSEM experiment. In particular, it looks at the mutual coupling between a buried cultural conductor and a geological heterogeneity. We attempt to isolate the Hz field induced by tertiary currents in targets caused by mutual coupling. This is achieved with a Texas A&M 3D CSEM finite element code, which calculates the secondary Hz fields emanating from a target buried in a halfspace. Buried geological targets and cultural conductors are modeled as volumetric slabs embedded in a halfspace. A series of models have been simulated to study the effect of varying parameters such as target conductivity, transmitter location and shape of a target on the mutual inductance. In each case, the secondary Hz field is calculated for a model with two slabs, and two models with individual slabs. The mutual coupling is calculated by removing the secondary fields from the individual slab models from the response of a two slab model. The calculations of mutual inductance from a variety of such models suggests a complicated interaction of EM fields between the two targets. However, we can explain most of these complexities by adapting a simple approach to Maxwell’s equations. Although the tertiary Hz field is complicated, it may be useful in the characterization and delineation of electrical heterogeneities in the subsurface, which can then be related to geological features such as fractures or joints. It is seen that the most important factor affecting the mutual coupling is the host conductivity. The results have also shown that mutual coupling is very sensitive to transmitter (TX) location, especially when the TX is positioned near one of the targets.