Magnetotellurics with a remote magnetic reference

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.

Resistivity distribution from mid-crustal conductor to near-surface across the 1200 km long Liquiñe-Ofqui Fault System, southern Chile

“…Time series data were processed to determine the impedance tensor using mutual remote reference processing in order to reduce local electromagnetic noise (Gamble et al 1979). The improvements of different processing steps and further optimization are shown for a representative example (site 5) in Fig.…”

A magnetotelluric investigation of the Scandinavian Caledonides in western Jämtland, Sweden, using the COSC borehole logs as prior information

“…Three different sampling rates were applied: 20 Hz for about 21.5 hr, 1000 Hz for 2 hr starting from midnight and 3000 Hz for about half an hour during daytime. The MT transfer functions Z and T were calculated using Smirnov's (2003) algorithm MTU2000, which applies amongst other processing, levelling and notch filtering (removing power line signals) to the segmented time series, uses a robust estimation procedure with a high break-down point and has an option for remote reference processing (Gamble et al 1979). MT data from sites in northern Sweden and northern Norway (Cherevatova et al 2015) recorded at the same time as our data were used for remote referencing.…”

Hydrothermal system beneath the crater of Tarumai volcano, Japan: 3-D resistivity structure revealed using audio-magnetotellurics and induction vector

“…Time-series data were collected for a few hours at each site and then transformed to power spectra using a Fourier transformation. The remote reference technique (Gamble et al, 1979) was applied for eight AMT sites for which time-series data had been simultaneously measured at the site 290, locating about 2 km north from the study area (Fig. 1b).…”