Method and apparatus for determining the resistivity and conductivity of geological formations surrounding a borehole

Apparatus for determining selected parameters of an earth formation surrounding a borehole, comprising:

• (a) an instrument having a single source electrode for injecting an electrical current of a predetermined value into the formation surrounding the borehole,
• (b) a plurality of measurement electrodes vertically spaced from said source electrode along the body of said instrument, wherein successive groups of at least three selected adjacent measurement electrodes are adapted to derive first and second difference potentials between said at least three selected adjacent measurement electrodes within each said successive group of measurement electrodes, and wherein the locations of said successive groups of at least three selected adjacent measurement electrodes are uniformly and vertically spaced at increasing distances from said source electrode,
• (c) wherein said first and second difference potentials for each of said successive groups of measurement electrodes spaced vertically an increased distance from said source electrode are derived in response to current from said source electrode traveling generally vertically in an orientation generally parallel to the axis of the borehole in the geologic formation and spaced horizontally an increased radial distance surrounding the borehole and adjacent to the instrument, and
• (d) wherein a value representative of the selected formation parameter for said horizontal radial distance associated with each of said vertically spaced groups of at least three measurement electrodes may be derived from said first and second difference potentials for each of said groups of at least three measurement electrodes.