Technical Paper: Statics in Magnetotelluric - Shift or Model?

Society: EAGE
Paper Number: D020
Presentation Date: 2008

Magnetotelluric data sets often show signs of significant local galvanic distortion - static shifts in its simplest form - complicating their quantitative analysis. Our accounting for this problem is two-fold. Colocated magnetic TDEM soundings provide representative resistivity estimates of an approximate 1 D near-surface structure, allowing for calibration of the MT apparent resistivity at the highest frequency limit. Remaining distortion that is unaccounted for can be addressed directly and automatically via 3D inversion: as galvanic effects occur equally in the synthetic simulation of the induction process whenever currents cross resistivity interfaces, the inversion algorithm generates shallow near-surface structure that can reproduce the observed statics. Today's increasing computer resources allow for the detailed meshes and small near surface cell sizes necessary for this approach. Constraints on model roughness prevent local statics solutions from affecting the deeper resistivity image in the minimum structure inversion. For some data sets and geological environments, direct inversion for static shift parameters in addition to the above structural approach may be preferred. Applicability of this two-fold approach is demonstrated on a combined MT data set from the geothermal prospect of Glass Mountain, California.

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