Patent ID: 6987386
Filing Date: 2006-01-17
Classification: G01V

Abstract:
1. A method for obtaining information useful to determine resistivity of a geological formation involving a borehole casing having an upper end that is adjacent to the earth's surface, comprising: providing a plurality of current conducting electrodes including a first current conducting electrode and a second current conducting electrode; locating circuitry including at least one amplifier within said borehole casing and below said borehole casing upper end; reducing a common mode signal input to said at least one amplifier, wherein a signal output by said at least one amplifier is more responsive to current flowing into the geological formation than to a relatively larger current flowing along at least a first portion, defined between first and second voltage measurement electrodes, of said borehole casing; measuring at least a first voltage across at least said first portion of said borehole casing using said circuitry, said circuitry being located closer to said first portion of said borehole casing than to said borehole casing upper end; measuring at least a second voltage across at least a second portion, defined between third and fourth voltage measurement electrodes at least one of which is different from said first and second voltage measurement electrodes, of said borehole casing using said circuitry, and using at least said first and second voltages to obtain information useful to determine the resistivity of the geological formation.