Method and apparatus for determining the resistivity and conductivity of

Abstract

A differential array instrument for determining selected parameters of an earth formation surrounding a borehole, including an instrument mandrel carrying a single source electrode for injecting an electrical current of a predetermined value into the formation surrounding the borehole, and an array of a plurality of measurement electrodes uniformly and vertically spaced from said source electrode along the instrument mandrel. A predetermined group of the uniformly and vertically spaced electrodes are adapted to derive first and second difference potentials between the predetermined group of electrodes, wherein successive ones of a plurality of the predetermined group of selected measuring electrodes uniformly and vertically spaced at increasing distances from the source electrode axially of the borehole are adapted to derive a plurality of the first and second difference potentials between the predetermined group of electrodes. The first and second difference potentials are derived in response to current from the source electrode travelling generally vertically in an orientation generally parallel to the axis of the borehole in the formation to successive ones of the predetermined groups of selected measuring electrodes and which plurality of first and second difference potentials may be correlated to a plurality of values representative of the selected formation parameters. The plurality of values representative of the selected formation parameters may provide a profile of the selected parameters over an increasing radial distance from the borehole.