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

摘要

Controlled source electromagnetic (CSEM) geophysics has been used with a fairamount of success in near surface hydrogeological studies. Recently, these investigationshave been conducted frequently in human impacted field sites containing culturalconductors such as metal fences and buried pipes. Cultural noise adds an element ofcomplexity to the geological interpretation of this type of data. This research investigatesthe influence of mutual induction between two buried targets in a CSEM experiment. Inparticular, it looks at the mutual coupling between a buried cultural conductor and ageological heterogeneity. We attempt to isolate the Hz field induced by tertiary currentsin targets caused by mutual coupling. This is achieved with a Texas A&M 3D CSEMfinite element code, which calculates the secondary Hz fields emanating from a targetburied in a halfspace. Buried geological targets and cultural conductors are modeled asvolumetric slabs embedded in a halfspace. A series of models have been simulated tostudy the effect of varying parameters such as target conductivity, transmitter locationand shape of a target on the mutual inductance. In each case, the secondary Hz field iscalculated for a model with two slabs, and two models with individual slabs. The mutualcoupling is calculated by removing the secondary fields from the individual slab modelsfrom the response of a two slab model. The calculations of mutual inductance from avariety of such models suggests a complicated interaction of EM fields between the twotargets. However, we can explain most of these complexities by adapting a simpleapproach to Maxwell?s equations. Although the tertiary Hz field is complicated, it may be useful in thecharacterization and delineation of electrical heterogeneities in the subsurface, which canthen be related to geological features such as fractures or joints. It is seen that the mostimportant factor affecting the mutual coupling is the host conductivity. The results havealso shown that mutual coupling is very sensitive to transmitter (TX) location, especiallywhen the TX is positioned near one of the targets.