US Patent 7397250 High resolution resistivity earth imager

Phase-sensitive measurements are made by a resistivity imaging tool in a borehole having non-conductive mud in a conductive earth formation at a plurality of frequencies. From the phase sensitive measurements, the formation resistivity can be determined with higher sensitivity than is possible with the single frequency measurements. Tool standoff can also be determined from a knowledge of the mud resistivity and/or dielectric constant. Formation resistivity may also be determined when the effect of formation capacitance cannot be ignored.