Determining resistivity of a formation adjacent to a borehole having casing using multiple electrodes and with resistances being defined between the electrodes

A method for determining the resistivity of a geological formation from within a cased well comprising:

• providing an apparatus having a first electrode that electrically engages a particular section of casing for receiving first voltage related signals at a specific depth within the well;
• said apparatus having a second electrode that electrically engages the particular section of casing for receiving second voltage related signals located a first distance above said first electrode wherein the magnitude of the resistance of the portion of casing between said first and second electrodes is the first resistance;
• and
• said apparatus having a third electrode that electrically engages the particular section of casing for receiving third voltage related signals located a second distance below said first electrode wherein the magnitude of the resistance of the portion of casing between said first and third electrodes is the second resistance;
• causing a first current to flow into the geological formation from the particular section of casing;
• causing a second current to flow along the particular section of casing;
• obtaining said first, second, and third voltage related signals while conducting said first current into formation during the measurement step;
• determining the magnitude of said first resistance using at least a selected one of said first current and said second current;
• determining the magnitude of said second resistance using at least a selected one of said first current and said second current;
• and
• processing the voltage related signals from each of said first, second and third electrodes obtained during the measurement step for use in determining the resistivity of the geological formation of interest, said processing taking into account the determined magnitudes of said first resistance and said second resistance to reduce the inaccuracy associated with the determination of the resistivity of the geological formation of interest.