Apparatus and method for wellbore resistivity imaging using capacitive coupling

1. Field of the Invention An apparatus for obtaining resistivity images of a borehole includes an array of measure electrodes separated from a pad or the body of the instrument by a focusing electrode. The focusing electrode is maintained at a slightly higher potential than the measure electrodes. The apparatus is primarily intended for use with conducting fluids but may also be used with non-conducting fluids. A modulated electrical current with a carrier frequency of 1 MHz is injected into the formation. When used with a non-conducting fluid, capacitive coupling between the electrode and the conductive formation is provided by the dielectric of fluid. When used with a conducting borehole fluid, an additional capacitor may be incorporated into the circuit. The current in the measure electrode is indicative of the conductivity of the formation. To avoid cross-talk between conductors in an isolator section between the current source and the measure electrode, signals indicative of the measure currents are demodulated to a frequency of 1.1 kHz. This makes it possible to use the configuration of prior art devices designed for conductive coupling between the logging tool and the formation.