High frequency induction log for locating formation interfaces

Abstract

A method and apparatus for logging rock formations penetrated by a borehole to determine both the resistivity and dielectric constant of the formations. The apparatus comprises an induction logging tool having at least a transmitting coil and two receiver coils and operating in the radio frequency range from 20 to 60 megahertz. The method utilizes both the in-phase and out-of-phase, with respect to the transmitter current, voltage components which are measured at each receiver. The in-phase and out-of-phase voltage components are used to compute the phase shift between the voltage signals and the relative attenuation of the voltage amplitudes between the receivers. The phase shifts and relative attenuations are relatively insensitive to the borehole (e.g., borehole radius and drilling mud resistivity) and can be used as inputs into theoretically constructed nomograms to determine both the resistivity and dielectric constant of the formation. If the invention of the drilling mud into the formation is not neglible then a tool with four spatially separated receiver coils is employed and the phase shifts and relative attenuations between the three adjacent pairs of receiver coils are computed from the in-phase and out-of-phase voltage components measured at each receiver. These data are used to determine the resistivity and dielectric constant of the invaded zone, the radius of the invaded zone, and the resistivity and dielectric constant of the non-invaded formation.

Dependencies are inferred from the claim text.