Abstract
Galvanic distortion of magnetotelluric (MT) data due to small-scale surficial bodies or due to topography is one of the major factors that prevents accurate imaging of the subsurface. We present a 3-D algorithm for joint inversion of MT impedance tensor data and a frequency-independent full distortion matrix that circumvents this problem. We perform several tests of our algorithm on synthetic data affected by different amounts of distortion. These tests show that joint inversion leads to a better conductivity model compared to the inversion of the MT impedance tensor without any correction for distortion effects. For highly distorted data, inversion without any distortion correction results in strong artefacts and we cannot fit the data to the specified noise level. When the distortion is reduced, we can fit the data to an RMS of one, but still observe artefacts in the shallow part of the model. In contrast, in both cases our joint inversion can fit the data within the assumed noise level and the resulting models are comparable to the inversion of undistorted data. In addition, we show that the elements of the full distortion matrix can be well resolved by our algorithm. Finally, when inverting undistorted data, including the distortion matrix in the inversion only results in a minor loss of resolution. We therefore consider our new approach a promising tool for the general analysis of field MT data.
Highlights
One problem that has prevented the wider use of magnetotelluric (MT) data in the geophysical community is the influence of galvanic distortion
We propose a similar approach to Sasaki & Meju (2006), but we include the frequency-independent elements of the full distortion matrix C, rather than static shifts, as additional parameters into our 3-D inversion
We show that the joint inversion of MT impedance tensor data and full distortion matrix can very well recover the elements of the distortion matrix and obtain a resistivity model similar to the
Summary
One problem that has prevented the wider use of magnetotelluric (MT) data in the geophysical community is the influence of galvanic distortion. Groom & Bailey 1989; Bibby et al 2005) In both cases these scale factors shift the apparent resistivities at all frequencies by an unknown amount and they are termed static shifts. Sasaki & Meju (2006) jointly invert for resistivity structure and static shift They test their inversion on various synthetic data and obtain encouraging results. Arnason et al (2010) describe a procedure to incorporate static shifts determined from coincident MT and TEM into 3-D MT inversion They assume a diagonal form of C and the comment of Jones (2011) applies in a similar way as it does for Sasaki & Meju (2006). When the distortion is relatively small we can fit the data within the assumed noise, but still observe significant artefacts in the near surface layers
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