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Optimizing the Estimation of Distortion Parameters in Magnetotelluric 3D Inversion
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 80th EAGE Conference and Exhibition 2018, Jun 2018, Volume 2018, p.1 - 5
Abstract
Summary
Most magnetotelluric (MT) data sets exhibit signs of galvanic distortion caused by near-surface resistivity structure, measured primarily in the electric field channels. While accurate resolution of detailed near-surface structure is out of scope, three-dimensional inversion typically accounts for this via inclusion of small anomalies around MT sites without affecting deeper structure resolution, as long as distortion is moderate. Direct inversion for distortion, which is mathematically represented by frequency-independent real-valued 2×2 matrices multiplying the undistorted impedances, reduces the structural impact from distorting structures in 3D inversion. The effectiveness of the method is demonstrated with synthetic examples and on a real dataset. These also show that the recovery of the input distortion parameters depends significantly on the chosen model regularization controlling shallow model smoothness. Furthermore, a distortion term that is added to the objective function quantifies the estimates’ deviation from a neutral identity matrix via a multiplicative parameter. This turns out to be a useful means to control the overall absolute level of estimates in real data inversion. We conclude that inversion for galvanic distortion is a valuable tool in addressing observed distortion effects in MT data, but is no golden bullet to fully isolate it, which remains impossible without further information.
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