Joint inversion of TEM and distorted MT soundings: Some effective practical considerations

Accurate layered-earth interpretation of magnetotelluric (MT) apparent resistivity sounding curves distorted by the presence of near-surface, 3-D bodies in an otherwise horizontally stratified ground requires that the curves be shifted to the level they would have assumed in the absence of these anomalous small-size bodies. The most popular preinterpretation processing involves shifting the biased MT apparent resistivity curves to the levels determined by some additional data. This paper examines the nature of TEM field data, their relationship with MT data, and their usefulness in constraining the interpretation of MT data at locations affected by near-surface 3-D effects. The MT apparent resistivity curves are neglected in a joint inversion scheme involving the (undistorted equivalent) phase information and TEM data, thus eliminating the need for preinterpretation data processing or correction of statically shifted curves. This approach is efficient and effective, and since an overlapping TEM observational bandwidth is not always required for a successful joint interpretation, simple inexpensive TEM equipment can be deployed as a logical compliment to MT in deep basin exploration campaigns.