Apparatus and method for formation resistivity measurements inoil-based mud using a floating reference signal

摘要

Disclosed is an apparatus for estimating a property of an earth formation penetrated by a borehole containing an oil-based drilling fluid. The apparatus includes an electrode disposed at a carrier and configured to inject alternating current into the formation. An electrically conductive plate is disposed between the first electrode and a borehole wall. An electrical insulator is disposed between and contacts the first electrode and the conductive plate. The apparatus is configured to prevent the drilling fluid from being disposed between the first electrode and the conductive plate. A first sensor is used to measure an electric field established between the first electrode and the conductive plate. A processor is configured to receive a measurement of the electric field to use as a floating reference signal to determine a phase difference with respect to a measured electrical quantity related to the injected electrical current in order to estimate the property.