In structurally simple areas, measurements of telluric currents and associated magnetic fields indicate where measurable changes in rock conductivity occur and are interpretable in terms of depth to bedrock, rock contacts, faults or dikes. The authors evaluate telluric, magnetotelluric, mixed, wave tilt and AFMAG, and magnetovariation methods; they emphasize the magnetotelluric method by describing their experiences and the instruments, procedures, and analytical techniques used. Telluric methods resemble gravitational and magnetic methods but the telluric current frequency range (from cycles per day to cycles per second) adds a dimension of information. Bibliography is comprehensive.

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