Electromagnetic well logging system having receiver and nulling coils mounted on separate magnetic cores

Claim 1

Apparatus for electromagnetically logging the earth formation surrounding a borehole, including: a. a pair of coaxially supported magnetic cores, b. a transmitter coil mounted partly on each of said magnetic cores, c. means for providing alternating current to said transmitter coil to thereby cause an alternating magnetic field in the earth formation surrounding the borehole, d. a receiver coil mounted on one of said magnetic cores in coaxial-spaced relationship with said transmitter coil, e. a nulling coil connected in series opposition to said receiver coil and mounted on the other of said magnetic cores in such a position that the total induced voltage across the nulling and receiver coils caused by said alternating magnetic field is indicative of the magnetic susceptibility and electrical conductivity of the earth formation, and f. said magnetic cores being of similar low-reluctance material whereby said induced voltage across the nulling and receiver coils is not affected by temperature variations throughout the length of the borehole.

Claim 8

A system for logging the magnetic susceptibility of an earth formation adjacent a borehole, having: a. a pair of cores comprised of a lowreluctance magnetic material located within the borehole, b. a transmitter coil mounted on said cores, c. a receiver coil mounted on one of said cores such that is is electromagnetically coupled with the transmitter coil in part through the earth formation adjacent the borehole, d. a nulling coil connected in series opposition with the receiver coil and mounted on the other of said cores such that the electromagnetic coupling between it and the transmitter through the earth formation adjacent the borehole is minimized, and e. a phase-sensitive detector which distinguishes between an in-phase component and a quadrature component of the total voltage induced across both the receiver and the nulling coils when the transmitter coil is energized with alternating current, such quadrature component being a measure of the magnetic susceptibility of the earth formation adjacent the borehole.

Claim 9

A system for logging the magnetic susceptibility of an earth formation adjacent a borehole, having: a. a pair of cores comprised of a lowreluctance magnetic material located within the borehole, b. a transmitter coil mounted on said cores, c. a receiver coil mounted on one of said cores such that is is electromagnetically coupled with the transmitter coil in part through the earth formation adjacent the borehole, d. a nulling coil connected in series opposition with the receiver coil and mounted on the other of said cores such that the electromagnetic coupling between it and the transmitter through the earth formation adjacent the borehole is minimized, and e. a phase-sensitive detector which distinguishes between an in-phase component and a quadrature component of the total voltage induced across both the receiver and the nulling coils when the transmitter coil is energized with alternating current, such quadrature component being a measure of the magnetic susceptibility of the earth formation adjacent the borehole.