Telluric currents are electrical currents in the earth induced by ionospheric disturbances. They are always present and contain all frequencies, from cycles per day to cycles per second.

The currents and their associated magnetic fields (micropulsations) provide sensitive indications of changes in electrical conductivity in rocks, such as

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commonly occur at basement, at shale-limestone interfaces, at faults, etc. Provided that the geometry of the structure is simple enough and that adequate conductivity contrasts exist, measurements can be interpreted in terms of structure. Hence, one can outline basement depth in a sedimentary basin, measure depth to weathering, or delineate a fault by telluric current methods.

The measurements can be made in a number of different ways, among which are the telluric, magneto-telluric, and wave-tilt techniques. Several of these are being actively studied by various research groups. Instrumentation depends on the depth of investigation desired, as well as on the technique used, but seems to present no fundamental problems for exploration applications.

The major problems at the present time are analyzing and interpreting the measurements. Analysis (filtering) is now being done in a routine fashion by digital computers, but could be done electronically. Interpretation techniques at present allow at least semiquantitative depth estimates (±10%) and can possibly be refined to enable greater accuracy.

Results of our magneto-telluric measurements in areas of well-known geology show that reasonable information can be obtained by rather brute-force interpretation techniques.

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