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Great volumes of data are available about the properties of the near surface of the Earth from borehole surveys and from direct-current and controlled-source electromagnetic soundings. A significant result derived front these data is a surface conductance map for the continental United States, showing that the conductance in the near-surface section ranges from <1 S/m in exposed shield areas to >5,000 S/m in the deep sedimentary basins. The existence of a surface layer with high conductance significantly interferes with our capability to probe into the mantle with electrical geophysical methods.

Information about the more conductive regions in the lower part of the crust and in the mantle is obtained from magnetotelluric and geomagnetic deep soundings. Perhaps 10,000 magnetotelluric soundings have been carried out in the last decade, but only about 1,200 are available in the public domain. Results are available from six networks of temporary magnetic observatories, using from several tens to 50 recording instruments. These various data sets have been obtained primarily from the western part of the United States, with only very sparse data available from the East, and virtually no data from the Southeast.

A striking result is the observation that, in the Basin and Range province of the western United States, anomalously high values of conductivity are observed at depths as shallow as 10 km in the crust. It has been hypothesized that these conductive zones are caused either by partial melting of crustal rocks in regions of high heat flow, or by the presence of significant amounts of water in fractured rock.

There appears to be a lack of correspondence between some of the main features of the geoelectric section and the general features of the interior of the Earth, which have been developed from seismic and gravity data. The base of the sedimentary and weathered section at the surface of the crust is the only important boundary in the conductivity profile that coincides with a corresponding seismic and density boundary. The other important geoelectric features of the crust and outer mantle seem discordant with the Earth’s structure as we know it from seismic and gravity data.

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