Electrode array for measurement of borehole resistivity

Claim 1

Apparatus for measuring the apparent resistivity of a borehole, including: a drill string segment; a layer of insulation surrounding said segment along the length thereof; an array of five electrode means positioned at predetermined locations in said layer of insulation; a current source connected to a first of said electrode means; first return circuit means connected from a second of said electrode means to said current source for returning measuring current flowing through the formation between said first and second electrode means whereby said first and second electrode means are in an electrical circuit including said first return circuit means and said current source, said second electrode means being spaced from said first electrode means in a first direction; third and fourth electrode means between said first and second electrode means; voltage measuring means connected to said third and fourth electrode means for determining the voltage between said third and fourth electrode means; current measuring means connected in one of said first or second return circuit means for determining the current in said first return circuit means; fifth electrode means spaced from said first electrode means in a second direction; and second return circuit means connected from said fifth electrode means to said current source for returning any leakage current flowing through the formation between said first and fifth electrode means whereby said first and fifth electrode means are in an electrical circuit including said second return circuit means and said current source.