Investigation of the audio-frequency magnetotelluric method of determining an apparent resistivity tensor

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Whiteford, P.C. 1981 Investigation of the audio-frequency magnetotelluric method of determining an apparent resistivity tensor. Wellington: DSIR, Geophysics Division. Report / Geophysics Division 156 64 p.

Abstract: Measurements of the horizontal components of the electric field (E1, E2) and the horizontal and vertical components of the magnetic field (B1,B2,B3) were made at 50 Hz and 85 Hz in order to determine the electric and magnetic field vectors, the vector h, and the tensor Yij relating them. Measurements repeated at several sites showed that the electric and magnetic field vectors could be determined accurately. At sites along a traverse crossing the resistivity boundary of the Broadlands geothermal field the 50 Hz electric field vector showed changes in magnitude corresponding to changes in ground resistivity and a change in direction as the resistivity boundary was traversed. The h vector also showed an increase in magnitude and change in direction as the resistivity boundary was traversed. From the tensor Yij an apparent resistivity and an apparent resistivity tensor were derived and showed similar trends to the d.c. resistivities which had been measured in the same areas. The measurements were repeated at several sites and showed that the apparent resistivity and the apparent resistivity tensor and h vector components sometimes varied by more than an order of magnitude. This poor repeatability is probably due to insufficient difference in the signal polarisation at 50 Hz and at 85 Hz due mostly to the incorrect values of phase obtained for the components of the electric and magnetic field