Patent ID: 7388382

Claim:
An instrument for measuring resistivity of Earth formations from within a conductive pipe inside a wellbore drilled through the formations, comprising: a plurality of housings connected end to end, the housings adapted to traverse the interior of the pipe; at least one electrode on each housing, each electrode adapted to be placed in electrical contact with the interior of the pipe; a source of electrical measuring current; at least one digital voltage measuring circuit; at least a first switch arranged to selectably connect the source of electrical measuring current between one of the electrodes and a current return at a selectable one of the top of the pipe and a location near the Earth's surface at a selected distance from the top of the pipe; at least a second switch arranged to connect selectable pairs of the electrodes to the digital voltage measuring circuit, the pairs selectable to make voltage measurements corresponding to selectable, variable axial distances between the electrodes and corresponding lateral depths in the Earth formations; means for extending at least one of the electrodes laterally outward from the respective housing associated therewith, a resistance measuring circuit operatively coupled between the at least one electrode and the pipe; an electromagnetic transmitter antenna and an electromagnetic receiver antenna disposed proximate a contact end of the at least one electrode; a source of alternating current electrically coupled to the transmitter antenna; and a receiver circuit electrically coupled to the receiver antenna, whereby a quality of contact between the electrode and the conductive pipe is determinable by comparison of a resistance measured by the resistance measuring circuit to a voltage detected by the receiver circuit.