1. n. [Formation Evaluation]
A measurement of the high frequency (about 1 GHz) dielectric properties of the formation. In a typical tool, a microwave transmitter is placed a few inches below two receivers separated by 4 cm [1.6 in.]. At this frequency, the response is best explained as the propagation of a wave. Thus the phase shift and attenuation of the wave between the receivers are measured and transformed to give the log measurements of propagation time and attenuation. Because of the short spacings, the measurement has excellent vertical resolution and reads within inches of the borehole wall except at high resistivity. Different transmitter and receiver spacings and orientations are used, leading to different arrays, such as the endfire array and the broadside array. An ideal measurement would give the plane wave properties of the formation. However, the geometry of the measurement precludes this, so that a correction, known as the spreading-loss correction, is needed for the attenuation and to a much smaller extent for the propagation time. The measurement is also affected by the dielectric properties and thickness of the mudcake. Borehole compensation is used to correct for sonde tilt or a rough borehole wall.