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Abstract

A method for determining the resistivity of earth formations penetrated by a wellbore. Electrical current is imparted to the wellbore and the earth formations from a first electrode located on a mandrel disposed in the wellbore. The electrical current is returned at a second electrode on the mandrel at a spaced apart location from the first electrode. Voltage differences are measured between a first pair and a second pair of electrodes located on the mandrel between the first and the second electrodes. Circuits used to measure the voltage differences are adjusted until the measured voltage difference between the second pair is substantially the same as the measured voltage difference between the first pair. Then the electrical current is returned to an electrode at the earth's surface or other location distal from the first electrode, and measuring the voltage differences is repeated. A difference of the voltage differences is then calculated. The difference of voltage difference represents current leakage into the earth formation substantially unaffected by irregularities in the wall of the wellbore or drift in the circuits. The current leakage is inversely related to the resistivity of the earth formations.

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First Claim