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 part 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 an intermediate region; c) measuring voltages between the first part of the casing and a point 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 spaced 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 enable the formation resistivity to be derived from the measured voltages.