Patent ID: 6373254
Filing Date: 2002-04-16
Classification: G01V

Abstract:
An apparatus, on a drill string conveyed tool, for determining the resistivity of a formation surrounding a borehole, said apparatus being capable of controlling the effect of contact impedance on the formation resistivity measurement, comprising:a) an elongated electrically conductive body that is moveable through the borehole; b) a current source; c) a ring current electrode positioned on said body and adapted for receiving current from said source; d) a plurality of ring monitor electrodes positioned on said body in close proximity to said current electrode; e) a guard electrode positioned on said body and adapted to assist in directing current from said formation into said current electrode, wherein at least one ring monitor electrode is embedded in said guard electrode; f) a means for controlling detected potentials at at least two monitor electrodes of said plurality of ring monitor electrodes; and g) a means for deriving an indication of formation resistivity from said current received at said current electrode.