Extended decomposition of MT data
Garcia, X. and A.G. Jones
Decomposition of magnetotelluric data into a local galvanic 3D distortion matrix
and a regional 2D Earth caused a quantum leap in our understanding of complex data
and our ability to handle those data. The Groom-Bailey method is the most widely
adopted tensor decomposition approach, and rightly so given its physical basis and
its separation of distortion parameters into determinable and indeterminable parts.
However, on occasion the 3D over 2D (3D/2D) decomposition fails in that the misfit
of the model to the data is far greater than the data errors permit, and this failure
is due to either the distortion model being invalid or to inappropriately small
error estimates for the data.
In this paper we describe and demonstrate our attempts to extend MT tensor decomposition
to local galvanic 3D distortion of regional 3D data (3D/3D). There are insufficient data
to accomplish this uniquely for a single MT site, so some approximations must be made.
The approach we use is to assume that two neighboring sites sense the same regional
structure if they are sufficiently close compared to the skin depth to the structure,
but that the two sites have differing galvanic distortion matrices. We use a decomposition
method similar to the Groom-Bailey one, but with a different parameterization, and we solve
the problem using a Newton method. We demonstrate the method to a synthetic dataset, and
highlight the difficulties that result as a consequence of inherent parameter-resolution
In: Three-Dimensional Electromagnetics, edited by M.S. Zhdanov and P.E. Wannamaker,
publ. by Elsevier, Methods in Geochemistry and Geophysics (ISBN 0 444 50429 X), 35, 235-250, 2002.
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Alan G Jones / 10 June 2004 /