Lateral effects in controlled source audiomagnetotellurics.

Abstract

A popular electrical exploration method is controlled source audiomagnetotellurics (CSAMT). Although the CSAMT method has had practical sucess, the theory used in CSAMT interpretation remains limited. The controlled source in CSAMT is a grounded electric dipole placed as far as is practical from the survey area. When the source-receiver separation is large enough, source fields can be adequately modeled by a single plane wave and conventional magnetotelluric interpretation methods can be used. Quite often however, data collected at lower frequencies can not be interpreted with magnetotelluric algorithms. If the electrical distance between source and receiver is too small, mathematical models must explicitly include the dipolar nature of the source fields to accurately model the data. Models which include a dipole source are limited. Most interpretation of CSAMT data is based upon horizontally layered models. In this dissertation, the model of plane layers excited by an arbitrary source is extended to include the effects of lateral changes in layer thickness. The method developed for modeling undulating interfaces mimics nature more accurately than plane-layered models, while preserving the utility of quick evaluation and insight lost to more general numerical methods. While the main emphasis here is on CSAMT applications, the theoretical results are valid for any source type. Topographic effects are modeled for plane wave and magnetic dipole sources. The effects of relief in basement topography on the response of a horizontal loop-loop system are computed and compared with scale-model measurements. CSAMT field data which can not be adequately explained by plane-layered models are analyzed using models with variable layer thickness. The correctness of the theoretical development is verified by comparison with physical measurements. The utility of the theory is illustrated by application to problems which can not be explained by plane-layered models.