MAGNETOTELLURICS USING LOCK-IN SIGNAL DETECTION

Young Alumni Association reception at Squatters Pub

MAGNETOTELLURICS WITH A REMOTE MAGNETIC REFERENCE

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, one can obtain estimates of the impedance tensor that are unbiased by noise, provided there are no correlations between the noises in the remote channels and the noises in the local channels. Even for data for which the E-E predicted coherencies were as low as 0.1, the apparent resistivities obtained from this technique were consistent with apparent resistivities calculated from high coherency data at adjacent periods. Apparent resistivities calculated by conventional analysis of the same data were biased by as much as two orders of magnitude. The estimated standard deviation for periods shorter than 3 s was less than 5%, and, for 87% of the data, was less than 2%. Where data bands overlapped between periods of 0.33 s and 1 s,the average discrepancy between the apparent resistivities was 1.8%

Argonne National Laboratory Reports

The abstracts are given of thirteen papers presented at a ''SQUID Symposium'' organized by the Division of Materials Sciences of the U.S. Department of Energy and held March 23-25, 1978, at the University of Virginia. Since SQUID systems have already been utilized in feasibility demonstration in geothermal reservoir exploration, it was recognized that these devices also hold great potential for many other important scientific measurements. Many of these are energy-related, and others include forefront investigations in a diverse group of scientific areas, from biomedical to earthquake monitoring. Research in SQUIDs has advanced so rapidly in recent years that it was felt that a symposium to review the current status and future prospects of the devices would be timely. The abstracts given present an overview of work in this area and hopefully provide an opportunity to increase awareness among basic and applied scientists of the inherent implications of the extreme measurement sensitivity in advanced SQUID systems