Patent ID: 8760167

Claim:
An apparatus for making resistivity measurements of an underground formation surrounding a borehole, comprising: a conductive tool body; a pad having an insulating front face and a back face conductively connected to the tool body, said pad being mounted on the tool body; at least one current injector electrode and at least one current return electrode mounted on the front face of the pad so as to be positionable, in use, from a wall of the borehole at a stand-off distance from the wall, the at least one current injector electrode and the at least one current return electrode being electrically isolated from each other; wherein a portion of the at least one current return electrode is located between a portion of the current injector electrode and a portion of the back face of the pad so as to decrease the electrical impedance (ZLR) between the tool body and the at least one current return electrode, and increase the electrical impedance (ZLI) between the tool body and the at least one current injector electrode; and the tool body, the pad, the at least one current injector electrode, and the at least one current return electrode are arranged such that: (a) the ratio of the electrical impedance (ZLI) between the at least one current injector electrode and the tool body and the electrical impedance (ZLR) between the at least one current return electrode and the tool body, and (b) the ratio of the electrical impedance (ZMI) between the at least one current injector electrode and the formation, and the electrical impedance (ZMR) between the at least one current return electrode and the formation, differ by 25% or less of the larger of the two impedance ratios (ZLI/ZLR, ZMI/ZMR).