Method and apparatus for measuring formation resistivity in cased holes

A method of determining the resistivity of an underground formation surrounding a borehole which is lined with a casing comprising the steps of:

• a) causing a first current to flow in a first direction along a first part of the casing and measuring voltage across the first part;
• b) causing a second current to flow in a second, opposite direction along a second part of the casing and measuring voltage across the second part, said second part being spaced from the first part by an intermediate region;
• c) measuring voltages between a point in the first part of the casing and a point in the intermediate region to obtain a first voltage measurement, and between a point in the second part of the casing and the point in the intermediate region to obtain a second voltage measurement;
• d) measuring the resistance of the intermediate region of the casing;
• and
• e) using the first and second voltage measurements and the measurement of the resistance of the intermediate region to derive the resistivity of the formation from the voltages.

Apparatus for use in determining the resistivity of an underground formation surrounding a borehole which is lined with a casing, the apparatus comprising a body including first and second parts separated by an intermediate region, wherein:

• a) the first body part comprises first current source means including spaced current electrodes arranged so as to enable a current to flow in a corresponding first part of the casing and first voltage monitoring means including spaced monitor electrodes for sensing a voltage across said first part of the casing;
• b) the second body part comprises second current source means including spaced current electrodes arranged so as to enable a current to flow in a corresponding second part of the casing and second voltage monitoring means including spaced monitor electrodes for sensing a voltage across said second part of the casing;
• c) the intermediate region comprises an intermediate monitor electrode, first intermediate voltage monitoring means connected between the intermediate electrode and an adjacent one of the monitor electrodes of the first voltage monitoring means for sensing a first voltage between a point in the first part of the casing and a point in a corresponding intermediate region of the casing, and second intermediate voltage monitoring means connected between the intermediate electrode and an adjacent one of the monitor electrodes of the second voltage monitoring means for sensing a second voltage between a point in the second part of the casing and the point in the intermediate region of the casing.