Azimuthally segmented resistivity measuring apparatus and method

Abstract

An instrument for measuring resistivity of earth formations within azimuthal segments. The instrument includes azimuthally separated measure electrodes placed in contact with the wall of a wellbore penetrating the earth formations. The electrodes are placed in contact with the wall by springs, hydraulics or similar mechanisms. A guard electrode surrounds each measure electrode. The instrument includes a booster current electrode axially spaced apart from the guard electrodes. An insulator axially separates the guard electrodes from the booster electrode. Monitor electrodes are disposed on the exterior of the insulator. A first bucking current circuit is connected to the guard electrodes. The first bucking current constrains the flow of measuring currents from each measure electrode to a path substantially perpendicular to the axis of the instrument. A booster current circuit is connected to the booster electrode. The booster current circuit is controllably operated to maintain substantially zero voltage drop across the monitor electrodes. The instrument includes first measuring current sources each connected to one of the measure electrodes, and circuits for measuring voltage and current emitted through each of the measure electrodes. The resistivity of the segment in contact with each measure electrode can be determined from the voltage and current. In a preferred embodiment, the lengths of the booster electrode, insulator and guard electrodes are selected to enable determination of resistivity without having symmetric flow of bucking currents about the axis of the instrument with respect to the measure electrodes.