Magnetotelluric static shift and direct current sensitivity

Details Magnetotelluric static shift and direct current sensitivity Magnetotelluric static shift and direct current sensitivity

Feb 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001geoji.144..289s&link_type=abstract

Geophysical Journal International, Volume 144, Issue 2, pp. 289-299.

Astronomy and Astrophysics

Astronomy

9

Direct Current, Magnetotellurics, Sensitivity, Static Shift

Scientific paper

Magnetotelluric (MT) static shift is a non-inductive change of the MT apparent resistivity response that severely impairs the interpretation of data. Since MT static shift is a pure direct current (DC) phenomenon, we can apply the DC sensitivity distribution to predict its behaviour. We consider a four-point DC arrangement in Schlumberger configuration with current electrode spacing AB. Then, negative sensitivities form two semi-spherical or bowl-shaped structures between current and potential electrodes. With the limit of AB towards infinity, the semi-spheroids grow infinitely large, leaving positive sensitivities only within a vertical dyke structure between the potential electrodes. This arrangement represents the DC analogue to the MT static-shift case, yielding horizontal current systems at the position of the potential electrodes or, in MT terms, the telluric probes. Thus, the sign of static shift is due not only to the resistivity of the perturbing structure but also to its location with respect to the probes, which are separated by a non-infinitesimal distance. This is of crucial importance when investigating MT static-shift phenomena. For example, a conductive body between the telluric probes will decrease the MT apparent resistivity (positive sensitivity), whereas the same body located away from both the probes yields an increase in apparent resistivity (negative sensitivity). 3-D modelling clearly verifies this behaviour. A quantitative relationship between the static-shift factors for MT and DC is derived. In practice, however, a sufficiently large layout has to be chosen to approximate the behaviour at infinity or, if this is not possible, models derived from DC have to be incorporated into MT modelling. Thus, DC investigations are apt to quantify MT static shifts. A field survey comprising MT and large-scale DC resistivity soundings was carried out in the Grabfeld area in Unterfranken, Germany, and serves as a practical example. The MT response pattern in the target area is remarkably homogeneous. However, one site is severely disturbed by static shift. Although serious near-surface resistivity contrasts do exist at the other stations, it is shown that only those at the disturbed station can cause static shift.

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