Adaptation of Zohdy’s Method for Controlled-Source Audio-frequency Magnetotelluric (CSAMT) Data Interpretation with Layered Model

Abstract

Surveys with Controlled-Source Audio-frequency Magnetotellurics (CSAMT) are often performed in scalar mode for practical considerations. In such case, CSAMT data are obtained from electric and magnetic fields oriented along and perpendicular to the traverse line, respectively. With closely spaced sounding stations along a profile, layered or 1D model can be considered as sufficeint to obtain an overall view of the subsurface resistivity distribution in the study area. Zohdy’s method that was used to infer resistivity variation with depth from Vertical Electrical Sounding (VES) data is extended for CSAMT data interpretation. An initial model is determined from apparent resistivity sounding data using skin depth principle or its alternative, e.g. Bostick transform. With the number of layers equals to the number of data (in frequency or period), the discrepancy of calculated from observed data can be used to modify iteratively the layer parameters (resistivity and thickness). Preliminary tests using synthetic data associated with simple synthetic models showed satisfactory results. On-going study is still underway to assess smooth resistivity variation with depth and to correlate models from stations along a profile to obtain a quasi-2D resistivity model. Application for CSAMT data from a geothermal prospect is also sought.