Magnetotellurics with a remote magnetic reference

TLDR

The authors performed simultaneous magnetotelluric measurements at two sites 4.8 km apart using SQUID magnetometers, then used the magnetic field from the remote site as a reference, multiplying the Fourier equations and averaging cross‑products to obtain an impedance tensor that is unbiased by noise when the remote and local noises are uncorrelated. Using this remote‑reference technique, apparent resistivities were recovered with far less bias—up to two orders of magnitude lower than conventional methods—and achieved standard deviations below 5 % for periods under 3 s (and below 2 % for 87 % of the data), with only a 1.8 % average discrepancy in overlapping 0.33–1 s bands.

Abstract

Magnetotelluric measurements were performed simultaneously at two sites 4.8 km apart near Hollister, California. SQUID magnetometers were used to measure fluctuations in two orthogonal horizontal components of the magnetic field. The data obtained at each site were analyzed using the magnetic fields at the other site as a remote reference. In this technique, one multiplies the equations relating the Fourier components of the electric and magnetic fields by a component of magnetic field from the remote reference. By averaging the various crossproducts, estimates of the impedance tensor not biased by noise are obtained, provided there are no correlations between the noises in the remote channels and noises in the local channels. For some data, conventional methods of analysis yielded estimates of apparent resistivities that were biased by noise by as much as two orders of magnitude. Nevertheless, estimates of the apparent resistivity obtained from these same data, using the remote reference technique, were consistent with apparent resistivities calculated from relatively noise‐free data at adjacent periods. The estimated standard deviation for periods shorter than 3 sec was less than 5 percent, and for 87 percent of the data, was less than 2 percent. Where data bands overlapped between periods of 0.33 sec and 1 sec, the average discrepancy between the apparent resistivities was 1.8 percent.