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 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 electrical log).