1. n. [Formation Evaluation]
The change in position of the peaks of a sinusoidal electromagnetic wave as it passes through the formation. If the sinusoidal wave picked up by two receivers a certain distance apart in a formation are compared, it is found that the wave has been attenuated and shifted in time. The shift is known as a phase shift. The term is used in particular with reference to the propagation resistivity log and the electromagnetic propagation log.