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The electrical conductivity of the Earth is a property that responds primarily to changes in the water content of a rock or to changes in temperature. At the temperatures present on the Earth’s surface, conductivity depends most strongly on the amount and salinity of the ground water in a rock. At great depths, where the ambient temperature exceeds 500° to 600°C, conductivity depends on the motion of ions or electrons in the solid minerals of a rock, which in turn depends strongly on temperature. At intermediate depths, ranging from tens to hundreds of kilometers, conductivity is a function of both temperature and water content, with the relative importance varying with rock type.

Many methods have been used to study the conductivity of the Earth. Relatively direct measurements are made in wells and boreholes, to depths of about 8 km. Direct-current resistivity sounding has been used to explore the depths of sedimentary basins and the upper part of the crystalline basement. Most information about the crust and mantle has been obtained using the time-varying part of the Earth’s magnetic field. In the magnetotelluric method, simultaneous observations of the magnetic and electric fields at a single site are used to determine a conductivity profile. In the geomagnetic deep-sounding method, the magnetic field is observed simultaneously at many stations, forming a network. The conductivity structure is determined from the spatial variation of the time-varying magnetic field strengths. Relatively simple methods of interpretation used in the past have provided ambiguous results, so that much of the information about the electrical structure of the Earth lacks the desired degree of reliability. More recently, methods of data acquisition and interpretation have been developed to make future use of the magnetotelluric and geomagnetic deep-sounding methods considerably more reliable.

Based on results from all these methods, the conductivity structure of the Earth appears to be grossly zoned. The zones, or shells, in order from the surface inward are: (1) a surface veneer of wet, conductive rocks, to depths ranging from a few hundred meters to a few tens of kilometers: (2) a zone of very low conductivity crystalline rocks that are relatively dry and cool, extending to depths of several tens or hundreds of kilometers; (3) a zone of gradually increasing conductivity, attributed to the development of thermally excited conduction mechanisms; (4) a zone beginning at a depth between 300 and 700 km, in which the conductivity rapidly increases by several orders of magnitude; and (5) a zone of gradually increasing conductivity to the base of the mantle. Sparse evidence indicates that the core is highly conductive, with a conductivity of 100,000 to 1 million Siemans per meter (S/m).

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