1. n. [Formation Evaluation]

The effect of a sharp change in the borehole diameter, such as that caused by a cave or rugose hole, on an induction log. In smooth boreholes of constant diameter, the effect of the borehole is well understood and can be corrected for. However, a sharp increase in diameter over a small depth interval can induce signals on one coil in the array and not in others. This signal is not handled by the normal borehole correction and may result in a spike on the log. The spike usually is significant only when the resistivity is high and the contrast between formation and borehole resistivity is very large. The spike also depends on the design of the array or the processing.

See: array induction