Thermally stable coil assembly for magnetic susceptibility logging

Claim 2

Apparatus for logging the magnetic susceptibility of the earth formation adjacent a borehole comprising: a. a solenoidally wound transmitter search coil including a glass core which has a coefficient of thermal expansion no greater than 1.6 X 10 5 inches per inch per *F; b. an electrical power supply for providing alternating current power for said transmitter search coil whereby an alternating magnetic field is generated; c. means including a logging tool case for supporting said transmitter search coil in said borehole in a position such that the earth formation adjacent said borehole is subjected at least in part to said magnetic field; d. a solenoidally wound receiver search coil including a glass core which has a coefficient of thermal expansion no greater than 1.6 X 10 5 inches per inch per *F. said receiver search coil being mounted in said case (c) in a position axially spaced apart from said transmitter coil (a) a predetermined distance to obtain magnetic coupling with said transmitter coil at least in part through the earth formation adjacent said borehole whereby said alternating magnetic field induces in said receiver coil a first alternating voltage signal dependent in part on the magnetic susceptibility of said earth formation and which is in quadrature phase relationship with the alternating current in said transmitter coil; e. a nulling coil having a primary winding connected in series with said transmitter coil and a secondary winding connected in series opposition with said receiver coil said nulling coil being mounted in said case (c) in a position such that said alternating magnetic field has substantially no affect on said primary and secondary winding thereof whereby the induced voltage in said secondary winding tends to be suBtracted from said receiver coil voltage rendering detectable the portion of said receiver coil voltage representative of the magnetic susceptibility of the earth formation; and, f. phase sensitive detecting means coupled to said receiver search coil for deriving a magnetic susceptibility signal voltage corresponding to the portion of said receiver search coil voltage representative of the magnetic susceptibility of the earth formation in quadrature phase relationship with the alternating current in said transmitter coil.

Claim 3

The apparatus of claim 1 wherein the core of said transmitter search coil (a) and said receiver search coil (d) comprises a mandrel core common to both of said coils wherein each of said coils is a plurality of electrical windings on said mandrel and wherein the axial distance of the nearest winding of said receiver search coil to the nearest winding of said transmitter search coil is in the range of about 200 to 400 percent of the axial length of said transmitter search coil on said mandrel.

Claim 4

The apparatus of claim 1 wherein each of said transmitter and receiver search coils includes a pair of end caps of boron nitride material each having an opening adapted to fit closely over said mandrel for providing axial support for the windings of said transmitter and receiver coils, said end caps being cemented to said mandrel.

Claim 5

The apparatus of claim 1 wherein said transmitter coil (a) includes a sufficient number of windings to provide said magnetic field as a total magnetic field the strength of which is about 20 to 60 Oersteds.

Claim 6

Apparatus for logging the magnetic susceptibility of the earth formation adjacent a borehole comprising: a. a glass mandrel having a coefficient of thermal expansion no greater than 1.6 X 10 5 inches per inch per *F; b. a transmitter search coil including a multiplicity of electrical windings solenoidally wound over said mandrel; c. means including a logging tool case for supporting said mandrel in said borehole in a position such that the magnetic axis of said transmitter search coil is approximately parallel with the axis of said borehole; d. an electrical power supply for providing alternating current power for said transmitter search coil whereby an alternating magnetic field is generated which passes at least in part through said earth formation; e. a receiver search coil solenoidally wound over said mandrel axially spaced apart from said transmitter search coil a predetermined distance to obtain magnetic coupling with said transmitter coil at least in part through said earth formation by a desired penetration, whereby said alternating magnetic field induces in said receiver coil a first alternating voltage signal dependent in part on the magnetic susceptibility of said earth formation and which is in quadrature phase relationship with the alternating current in said transmitter coil; f. at least one end cap of boron nitride material for each of said search coils for providing axial support for the electrical windings thereof, each of said end caps including an opening adapted to fit closely over said mandrel; g. a nulling coil having a primary winding connected in series with said transmitter coil and a secondary winding connected in series opposition with said receiver coil said nulling coil being mounted in said case (c) in a position such that said alternating magnetic field has substantially no affect on said primary and secondary windings thereof, whereby the induced voltage in said secondary winding tends to be subtracted from said receiver coil voltage rendering detectable the portion of said receiver coil voltage representative of the magnetic susceptibility of the earth formation; and, h. phase sensitive detecting means coupled to said receiver search coil for deriving a magnetic susceptibility signal voltage corresponding to the portion of said receiver search coil voltage representative of the magnetic susCeptibility of the earth formation in quadrature phase relationship with the alternating current in said transmitter coil.

Claim 7

Apparatus for logging the magnetic susceptibility of the earth formation adjacent a borehole comprising: a. a glass mandrel having a coefficient of thermal expansion no greater than 1.6 X 10 5 inches per inch per *F; b. a transmitter search coil including a multiplicity of electrical windings solenoidally wound over said mandrel; c. means including a logging tool case for supporting said mandrel in said borehole in a position such that the magnetic axis of said transmitter search coil is approximately parallel with the axis of said borehole; d. an electrical power supply for providing alternating current power for said transmitter search coil whereby an alternating magnetic field is generated which passes at least in part through said earth formation; e. a receiver search coil solenoidally wound over said mandrel axially spaced apart from said transmitter search coil a predetermined distance to obtain magnetic coupling with said transmitter coil at least in part through said earth formation by a desired penetration, whereby said alternating magnetic field induces in said receiver coil a first alternating voltage signal dependent in part on the magnetic susceptibility of said earth formation and which is in quadrature phase relationship with the alternating current in said transmitter coil; f. at least one end cap of boron nitride material for each of said search coils for providing axial support for the electrical windings thereof, each of said end caps including an opening adapted to fit closely over said mandrel; g. a nulling coil having a primary winding connected in series with said transmitter coil and a secondary winding connected in series opposition with said receiver coil said nulling coil being mounted in said case (c) in a position such that said alternating magnetic field has substantially no affect on said primary and secondary windings thereof, whereby the induced voltage in said secondary winding tends to be subtracted from said receiver coil voltage rendering detectable the portion of said receiver coil voltage representative of the magnetic susceptibility of the earth formation; and, h. phase sensitive detecting means coupled to said receiver search coil for deriving a magnetic susceptibility signal voltage corresponding to the portion of said receiver search coil voltage representative of the magnetic susCeptibility of the earth formation in quadrature phase relationship with the alternating current in said transmitter coil.