The ideas of flux quantization and Josephson tunneling are reviewed, and the operation of the dc SQUID as a magnetometer is described. The SQUID currently used for magnetotellurics has a sensitivity of 10/sup -14/ T Hz/sup -1/2/, a dynamic range at 10/sup 7/ in a 1 Hz bandwidth, a frequency response from 0 to 40 kHz, and a slewing rate of 5 x 10/sup -5/T s/sup -1/. Recent improvements in sensitivity are discussed: SQUIDS are rapidly approaching the limit imposed by the uncertainty principle. The essential ideas of magnetotelluric (MT) measurements are outlined, and it is shown how the remote …
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The ideas of flux quantization and Josephson tunneling are reviewed, and the operation of the dc SQUID as a magnetometer is described. The SQUID currently used for magnetotellurics has a sensitivity of 10/sup -14/ T Hz/sup -1/2/, a dynamic range at 10/sup 7/ in a 1 Hz bandwidth, a frequency response from 0 to 40 kHz, and a slewing rate of 5 x 10/sup -5/T s/sup -1/. Recent improvements in sensitivity are discussed: SQUIDS are rapidly approaching the limit imposed by the uncertainty principle. The essential ideas of magnetotelluric (MT) measurements are outlined, and it is shown how the remote reference method can lead to major reductions in bias errors compared to more conventional schemes. The field techniques of the Berkeley group are described. The practical application of MT requires that amplitude and phase spectra of apparent resistivities be transformed into a geologically useful distribution of subsurface resistivities. In many areas where MT is being applied today, the technique may not provide the information needed because stations are too few and widely spaced, or because we are unable to interpret data influenced by complex 3-D resistivity features. The results of two surveys, one detailed, the other regional, over the Klamath Basin, Oregon, are examined. The detailed survey is able to resolve small (1 km wide) structural features that are missed or add a component of spatial aliasing to the regional data. On the other hand, the regional survey avoids truncation effects that may occur when the survey undersamples an area.
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