Static shift of magnetotelluric data and its removal in a sedimentary basin environment

Previous modeling investigations of the static shift of magnetotelluric (MT) apparent resistivity curves have limited appeal in that the electric fields used were point measurements, whereas field observations are of voltage differences. Thus, inhomogeneities of dimension of the order of the electrode line length could not be investi­ gated. In this paper, by using a modeling algorithm that derives point voltages rather than point electric fields, I consider the effect on the MT responses of local near­ surface distorting structures, which are both outside of, and inside, the telluric electrode array. I show that static-shift effects are of larger spatial size but of less magnitude than would be expected from conventional modeling. Also, the field observation that static shift affects only the apparent resistivity curve but not the phase response can be replicated by the voltage differ­ ence modeling. If there exists within the earth a layer whose variation in electrical resistivity along the profile can be treated in a parametric fashion, then static shift of the apparent resistivity curves can be corrected. Deriving the modal value from a sufficient number of observations for the layer resistivity is the most useful approach.

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