Method and apparatus for evaluating formation resistivity using focused annular and azimuthal electrodes

Apparatus for evaluating the resistivity of earth formations having a borehole passing therethrough, the apparatus comprising:

• an elongate body adapted to be moved along the borehole;
• an array of azimuthal current electrodes circumferentially spaced apart from one another on said body;
• two annular guard electrodes disposed longitudinally on said body on opposite sides of said array of azimuthal current electrodes;
• a plurality of azimuthal monitor electrodes each disposed on said body in the vicinity of a respective azimuthal current electrode;
• two annular monitor electrodes disposed on opposite sides of said array of azimuthal current electrodes;
• means for emitting measurement currents via said current electrodes and focusing current via said guard electrodes;
• means responsive to the potentials on said monitor electrodes to control the emission of said currents longitudinally and azimuthally;
• and
• means responsive to the measurement currents for generating output signals evaluating the resistivity of the formations.

Apparatus for evaluating the resistivity of earth formations having a borehole passing therethrough, the apparatus comprising:

• an elongate body adapted to be moved along the borehole;
• an array of azimuthal current electrodes circumferentially spaced apart from one another on said body;
• two annular guard electrodes disposed longitudinally on said body on opposite sides of said array of azimuthal current electrodes;
• a plurality of azimuthal monitor electrodes each disposed on said body in the vicinity of a respective azimuthal current electrode;
• two electrically connected annular monitor electrodes disposed on opposite sides of said array of azimuthal current electrodes;
• means for emitting measurement currents via said current electrodes and focusing current via said guard electrodes;
• means for detecting the potential differences between each of said azimuthal monitor electrodes and said annular monitor electrodes;
• means for controlling the emission of said measurement currents in such a manner as to maintain said detected potential differences at subtantially zero;
• and
• means responsive to the measurement currents for generating output signals evaluating the resistivity of the formations.