NULLING COIL APPARATUS FOR MAGNETIC SUSCEPTIBILITY LOGGING

• US 3,644,823 A
• Filed: 01/02/1970
• Issued: 02/22/1972
• Est. Priority Date: 01/02/1970
• Status: Expired due to Term

• Current Assignee: Donald J. Dowling, George R. Atwood
• Original Assignee: Donald J. Dowling, George R. Atwood
• Inventors: Dowling, Donald J., Atwood, George R.
• Primary Examiner(s): Strecker, Gerard R.
• Application Number:
  US05/000,329
• Time in Patent Office: 781 Days
• Field of Search: 324/1,3,4,6 336/229
• US Class Current: 324/340
• CPC Class Codes: G01V 3/28 using induction coils

Abstract

Disclosed is an improved logging apparatus having sufficient sensitivity and environmental stability suitable for magnetic susceptibility logging which includes a transmitter search coil, a receiver search coil, a power supply for providing alternating current power to the transmitter coil and an improved nulling coil including a primary winding wired in series opposition with the receiver coil. The nulling coil includes construction and mounting features uniquely suitable to render detectable the small quadrature phase signal representing the magnetic susceptibility of borehole formations.

Claims

1. In an apparatus for logging the magnetic susceptibility of the earth formation adjacent a borehole including a solenoidally wound transmitter search coil, a receiver search coil, a logging tool case for supporting said transmitter and receiver coils in saiD borehole in a position such that said receiver coil is magnetically coupled with said transmitter coil at least in part through said earth formation, and a power supply for providing alternating current power to said transmitter coil for inducting in said receiver coil an alternating voltage dependent in part on the magnetic susceptibility of said earth formation, said induced alternating voltage including an in phase component being in phase with the alternating current in said transmitter coil and related to the conductivity of an adjacent earth formation, and first and second quadrature components being in quadrature phase relationship with said current in said transmitter coil, said first quadrature component being a magnetically induced voltage resulting from magnetic coupling between said transmitter and receiver coils, and said second quadrature component being a quadrature component resulting from the magnetic coupling from the transmitter to the receiver through earth formations surrounding the borehole whereby it is dependent upon the magnetic susceptibility of said formation, the improvement which comprises;

a nulling coil supported by said logging tool case, said nulling coil having a primary winding and a secondary winding both being toroidally wound over a toroidal core, said primary winding being connected in series with said transmitter search coil, and said secondary winding being connected in series opposition with said receiver search coil;

said nulling coil primary and secondary windings being sufficient in number to provide an induced voltage in said secondary winding substantially equal and opposite to the quadrature voltage induced in said receiver search coil by said transmitter search coil when said logging tool case is subjected to a reference medium such that said magnetic coupling between said transmitter coil and said receiver coil is at least in part through said reference medium; and

, phase sensitive detecting means coupled to said receiver coil for selectively separating said quadrature voltage component thereby providing a signal indicative of the magnetic susceptibility of said adjacent earth formations.

2. The improvement as claimed in claim 1 wherein said nulling coil is mounted in said logging tool case in a position such that its toroidal axis is substantially orthogonal to the magnetic axis of said transmitter search coil.

3. The improvement as claimed in claim 1 wherein said toroidal core of said nulling coil is a nonmetallic material having a coefficient of thermal expansion no greater than 1.9 X 10 5 inches per inch per *F.

4. The improvement as claimed in claim 1 wherein the material of said toroidal core of said nulling coil is boron nitride.

5. In an apparatus for logging the magnetic susceptibility of the earth formation adjacent a borehole including a solenoidally wound transmitter search coil, a solenoidally wound receiver search coil, a logging tool case for supporting said transmitter and receiver coils in said borehole in a position such that their axes are substantially parallel to the axis of said borehole and in a position such that said receiver coil is magnetically coupled with said transmitter coil at least in part through said earth formation, and a power supply for providing alternating current power to said transmitter coil for inducing in said receiver coil an alternating voltage dependent in part on the magnetic susceptibility of said earth formation, and wherein the electrical windings of said transmitter and receiver search coils are sufficient in number to induce in said receiver coil an alternating voltage having a value in the range of about 25 to 28 mv. RMS when said logging apparatus is in a medium of air, the improvement which comprises;

a nulling coil having toroidally wound primary and secondary windings over a toroidal core of boron nitride, said primary and secondary windings being sufficient in number to induce in said secondary winding an alternating voltage havIng a value of about 25 mv. to within a tolerance of plus and minus 3 mv., said primary winding being connected in series with said transmitter search coil and said secondary winding being connected in series opposition with said receiver search coil whereby said two induced alternating voltages are in phase opposition to one another, said nulling coil being mounted in said logging tool case with its toroidal axis substantially orthogonal with said axis of said transmitter search coil.