METHOD AND APPARATUS FOR MEASURING FORMATION RESISTIVITY IN CASED HOLES

A method of determining the resistivity of an underground formation surrounding a cased borehole including the steps of: a) passing a current along a first pa rt of the casing and measuring the voltage; b) passing a current along a second part of the casing and measuring the voltage, the second part being spaced from the first part by a n intermediate region; c) measuring voltages between the first part of the casing and a poi nt in the intermediate region and between the second part of the casing and the same point; and d) using the measured voltages to derive the resistivity of the formation. A logging tool for use in determining the resistivity of an underground formation surrounding a cased borehole includes a body having first and second parts separated by an intermediate region, wherein: a) the first body part includes a first current source having space d electrodes arranged so as to enable a current to be passed through a corresponding part of the casing and a voltage monitor having spaced electrodes for measuring the voltage across the part of the casing; b) the second body part includes second current source having spaced electrodes arranged so as to enable a current to be passed through a corresponding part of the casing and a second voltage monitor having spaced electrodes for measuring the voltage across the part of the casing; and c) the intermediate region includes intermediate voltage monitors including electrodes for measuring a voltage between the first part of the casing and a point in a corresponding intermediate region of the casing, and between the second part of the casing and the point in the intermediate region of the casing so as to enabl e the formation resistivity to be derived from the measured voltages.