1. n. [Formation Evaluation]

The measurement of an induction tool in a nonconducting medium before correction. Electronic offsets and coupling within the tool cause a signal in the receivers even in a nonconducting medium such as air. This signal is cancelled either electronically or in software. Sonde errors change with temperature and pressure downhole. This can be allowed for by characterizing the sonde's response to temperature and pressure on the surface. Sonde error is measured by placing the tool far from the ground in air. Ideally measurements are made at two distances, since the ground signal can then be determined from the difference and eliminated. Originally, sonde error referred only to the R-signal, since this was the only signal being used. The term now refers to both R- and X-signals.

See: calibration,  R signal,  X-signal