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.