Earth currents. The term usually refers to natural earth currents originating as a result of variations in the earth's magnetic field. Sometimes the term is applied also to earth currents resulting from artificial electric or magnetic fields.

As applied to well logging, a component of telluric current along the borehole may produce an undesired variable voltage between measure and ground electrodes which is superimposed on the [[SP]]. Such interference is easily detected by holding the [[sonde]] stationary in the hole and observing whether the SP voltage continues to vary. The greatest interference is likely to occur in regions of high magnetic activity and for large [[formation resistivity factor|formation-resistivity]] to mud-[[resistivity]] ratios. In some instances, moving the ground electrode improves the situation. When interference is severe, it may be necessary to forego recording the usual SP [[curve]] and record, instead, a [[differential SP]] between two electrodes on the downhole configuration.

Telluric currents may create problems on resistivity measurements with large spacings and at very low frequencies (e.g., the [[ultra long spaced electric log|ultra-long-spaced electrical log]]).