Electromagnetic borehole logging apparatus and method.

摘要

An apparatus and method for determining the dielectric constant and/or resistivity of an earth formation which is subject to invasion of fluid, for example drilling fluid from a borehole. A transmitter (17) transmits electromagnetic energy at about 20 Mhz. from a first location in the borehole. Three receivers (22, 23, 24) are spaced at locations successively further away from the transmitter. Secondary field ratio means (140, 154, 154A, 168, 170) derives the ratio of the magnitudes of the secondary electromagnetic field at the two nearest of the three receivers, and a phase difference means (130) derives the phase difference between the received energies at the two furthest of the three receivers. The dielectric constant and the resistivity are determined from the secondary field amplitude ratio and the phase difference. The use of the secondary field ratio improves the depth of investigation.