EXTENDED DECOMPOSITION OF MT DATA

Xavier Garcia, Alan G. Jones
Geological Survey of Canada
615 Booth St., Ottawa, Ontario, K1A 0E9, Canada
garcia@cg.NRCan.gc.ca    

Decomposition of magnetotelluric data into local galvanic 3D 
distortion matrix and a regional 2D Earth has 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 this 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 either to 
inappropriately small error estimates for the data, or to the model 
being invalid.  In this work we describe and demonstrate our attempts 
to extend MT tensor decomposition to 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.