Investigation of MT static shift correction methods

Very small scale inhomogeneities (that is, with dimensions much less than a skin depth at the highest recorded frequency) can lead to severe problems in the interpretation of magnetotelluric (MT) data. These small-scale inhomogeneities can produce a shift of the MT apparent resistivity sounding curve such that the shifted curve is parallel to the undistorted curve on a log apparent resistivity versus log frequency plot. This parallel shift is commonly referred to as a static shift. The amount of static shift of the MT curves cannot be determined directly from conventionally recorded MT data. The most practical method for measuring the static shift appears to be a controlled-source measurement of the magnetic field, since magnetic fields, unlike electric field measurements, will be relatively unaffected by small scale inhomogeneities. The relative strengths and weaknesses of a number of magnetic field controlled-source methods are compared for this application, including central-loop frequency-domain sounding, central loop time-domain sounding, and far-field loop-loop sounding. The controlled-source sounding (which may be a relatively shallow sounding and made with lightweight equipment) can be combined in a joint inversion with a deep MT sounding to obtain a complete, undistorted model of the earth.