Apparatus and Method for Balancing Impedance of a Resistivity Measuring Tool

An apparatus for making resistivity measurements of an underground formation surrounding a borehole, comprising:

• a conductive tool body;
• a pad mounted on the conductive tool body, wherein the pad comprises:
• an insulating front face;
• at least one current injector electrode mounted on the front face;
• at least one current return electrode mounted on the front face,
• wherein the at least one current return electrode is electrically isolated from the at least one current injector electrode;
• and
• a back face conductively connected to the conductive tool body;
• wherein 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).