Geophysical surveying with audio frequency electromagnetic fields and orthogonal receiver coils

Claim 2

Geophysical survey receiver apparatus for conducting continuously moving airborne geophysical surveys in conjunction with a primary field transmitter comprising: a. a receiver carrier having a longitudinal axis and adapted to be moved in airborne flight along the longitudinal axis; b. a first receiver coil mounted on the carrier; c. a second receiver coil connected to the first receiver coil and mounted on the carrier in a position of minimum coupling relationship to the first coil and with the plane of the windings of the second coil perpendicular to the plane of the windings of the first coil and intersecting the plane of the first coil windings in a first-second planar intersection line; d. said first and Said second coils each being characterized by a capability for generating an electric signal from a first primary field and also characterized by mutually equal capabilities for generating electromotive force when subjected to identical excitation conditions; e. phase-shift means for introducing a 90* shift in the phase relationship of electric signals generated in the two coils when the coils are excited by an electromagnetic field at the frequency of the first primary field; f. means for combining electric signals generated in the first and second coils, and having a 90* phase shift introduced therein by said phase-shift means, to vectorially add the signals and thereby produce a first resultant signal having a magnitude responsive to both the phase and the magnitude of an electromagnetic field exciting said coils at the frequency of the first primary field; g. the aforesaid combination of receiver coils, phase-shift means and signal combining means being characterized by capability for maintaining electric signals from the first and second coils undistorted so as to maintain the forms of the signals and maintain the magnitude relationship of the signals to each other until the signals are combined; and h. a first resultant signal output means for transmitting the vectorially added first resultant signal.

Claim 3

An apparatus as set forth in claim 1 wherein the means for introducing the 90* phase shift comprises two tuning capacitors, one of said capacitors being connected in a parallel combination across the first receiver coil and the second capacitor being connected in a second parallel combination across the second receiver coil, and said phase-shift means is adapted for shifting the phase of one of the receiver coil signals 45* in one direction and shifting the phase of the other receiver coil signal 45* in the other direction with the aforesaid combinations of capacitors and receiver coils being tuned one to the upper half-power point and the other to the lower half-power point at the frequency of said first primary field.

Claim 4

An apparatus as set forth in claim 1 wherein the receiver carrier is adapted to be towed through the air and wherein the apparatus also comprises a powered aircraft, a transmitter coil mounted on the aircraft, a tow cable connecting the receiver carrier to the aircraft, means for receiving the vectorially added resultant signal transmitted from the signal output means of the receiver carrier and means for measuring and recording the magnitude of the resultant signal received from the receiver carrier output.

Claim 5

Geophysical survey receiver apparatus for conducting continuously moving geophysical surveys comprising: a. a receiver carrier having a longitudinal axis and adapted to be moved along the longitudinal axis; b. a first receiver coil mounted on the carrier; c. a second receiver coil connected to the first receiver coil and mounted on the carrier in a position of minimum coupling relationship to the first coil with the plane of the windings of the second coil perpendicular to the plane of the windings of the first coil and intersecting the plane of the first coil windings in a first-second planar intersection line and with the first and section coils disposed to orient the first-second planar intersection line perpendicular to a vertical plane through the longitudinal axis of the carrier; d. said first and said second coils each being characterized by a capability for generating an electric signal from a first primary field and also characterized by mutually equal capabilities for generating electromotive force when subjected to identical excitation conditions; e. phase-shift means for introducing a 90* shift in the phase relationship if electric signals generated in the two coils when the coils are excited by an electromagnetic field at the frequency of the first primary field; f. means for combining electric signalS generated in the first and second coils, and having a 90* phase shift introduced therein by said phase-shift introduce therein by said phase-shift means, to vectorially add the signals and thereby produce a first resultant signal having a magnitude responsive to both the phase and the magnitude of an electromagnetic field exciting said coils at the frequency of the first primary field; g. the aforesaid combination of receiver coils, phase-shift means and signal combining means being characterized by capability for maintaining electric signals from the first and second coils undistorted so as to maintain the forms of the signals and maintain the magnitude relationship of the signals to each other until the signals are combined; h. a first resultant signal output means for transmitting the vectorially added first resultant signal; and additionally comprising i. a third receiver coil mounted on the carrier with the plane of the third coil windings parallel to the longitudinal axis of the carrier, said third coil being characterized by a capability for generating a third coil signal from a second primary field at a second frequency different than the first primary filed frequency to produce a second resultant signal; and j. a second resultant signal output means for transmitting said second resultant signal.

Claim 6

Geophysical survey receiver apparatus for conducting continuously moving geophysical surveys comprising: a. a receiver carrier having a longitudinal axis and adapted to be moved along the longitudinal axis; b. a first receiver coil mounted on the carrier; c. a second receiver coil connected to the first receiver coil and mounted on the carrier in a position of minimum coupling relationship to the first coil with the plane of the windings of the second coil perpendicular to the plane of the windings of the first coil and intersecting the plane of the first coil windings in a first-second planar intersection line and with the first and section coils disposed to orient the first-second planar intersection line perpendicular to a vertical plane through the longitudinal axis of the carrier; d. said first and said second coils each being characterized by a capability for generating an electric signal from a first primary field and also characterized by mutually equal capabilities for generating electromotive force when subjected to identical excitation conditions; e. phase-shift means for introducing a 90* shift in the phase relationship if electric signals generated in the two coils when the coils are excited by an electromagnetic field at the frequency of the first primary field; f. means for combining electric signalS generated in the first and second coils, and having a 90* phase shift introduced therein by said phase-shift introduce therein by said phase-shift means, to vectorially add the signals and thereby produce a first resultant signal having a magnitude responsive to both the phase and the magnitude of an electromagnetic field exciting said coils at the frequency of the first primary field; g. the aforesaid combination of receiver coils, phase-shift means and signal combining means being characterized by capability for maintaining electric signals from the first and second coils undistorted so as to maintain the forms of the signals and maintain the magnitude relationship of the signals to each other until the signals are combined; h. a first resultant signal output means for transmitting the vectorially added first resultant signal; and additionally comprising i. a third receiver coil mounted on the carrier with the plane of the third coil windings parallel to the longitudinal axis of the carrier, said third coil being characterized by a capability for generating a third coil signal from a second primary field at a second frequency different than the first primary filed frequency to produce a second resultant signal; and j. a second resultant signal output means for transmitting said second resultant signal.

Claim 7

An apparatus as set forth in claim 4 wherein the means for introducing the 90* phase shift comprises two tuning capacitors, one of said capacitors being connected in a parallel combination across the first receiver coil and the second capacitor being connected in a second parallel combination across the second receiver coil, and said phase shift means is adapted for shifting the phase of one of the receiver coil signals 45* in one direction nd shifting the phase of the other receiver coil signal 45* in the other direction with the aforesaid combinations of capacitors and receiver coils being tuned one to the upper half-power point and the other to the lower half-power point at the frequency of said first primary field.

Claim 8

An apparatus as set forth in claim 4 which additionally comprises a fourth receiver coil connected to the third coil and mounted on the carrier in a position of minimum coupling to the third coil and with the plane of the fourth coil windings perpendicular to the plane of the third coil windings, said fourth coil being characterized by a capability for generating from the second primary field a fourth coil signal at the frequency of the second primary field and characterized by a capability for generating an electromotive force equal to the electromotive force generated by the third coil when the third and the fourth coils are subjected to identical excitation conditions, means for introducing a 90* shift in the phase relationship of electric signals generated in the third coil and the forth coil when the third and fourth coils are excited by an electromagnetic field at the frequency of the second primary field, and distortion-free means for combining the fourth coil signal into the second resultant signal after introducing the 90* phase shift.

Claim 9

Geophysical surveying apparatus comprising, in combination with receiver apparatus in accordance with claim 4, a signal analysis circuit for comparing the magnitude of a first resultant signal from the first resultant signal output means with the magnitude of a second resultant signal from the second resultant signal output means and for producing a resultant difference signal responsive in magnitude to the difference between the magnitudes of the first resultant signal and the second resultant signal and a voltage recorder for recording the magnitude of the resultant difference signal.

Claim 10

Geophysical surveying apparatus for conducting airborne geophysical surveys comprising, in combination with apparatus in accordance with claim 7, a powered aircraft having attached thereto a first audiofrequency transmiTter with a first transmitter coil for radiating a first audiofrequency primary field, a second audiofrequency transmitter with a second transmitter coil for radiating a second audiofrequency primary field perpendicular to said first field and a tow cable connected to the receiver carrier.

Claim 11

In a continuously moving airborne geophysical surveying process wherein a transmitter coil on a transmitter aircraft and a search coil on an airborne receiver carrier are flown over the terrain with the receiver carrier remote at a substantially constant distance from the transmitter aircraft and flexibly attached to the transmitter aircraft while the transmitter coil radiates an audiofrequency electromagnetic primary field and the search coil picks up an electric signal from the electromagnetic flux of the primary field and the magnitude of the search coil signal is measured, the improvement for suppressing orientation error noise that undesirably tends to arise in the search coil signal and result in undesired fluctuations in the measured magnitude of the signal when maneuver variations in the relative angular orientation of the receiver carrier to the transmitter aircraft cause variations in the angle at which the search coil intersects the flux of the primary field comprising: a. picking up a second electric signal from said primary field with a second coil characterized by a signal pickup capability equal to the signal pickup capability of the search coil and disposed perpendicular to the search coil with the planes of the two coils intersecting in a line transverse to the primary field flux at the search coil and also disposed in zero mutual coupling relationship with said search coil; b. maintaining the signal from the search coil and the signal from the second coil undistorted,whereby the forms of the signals and the magnitude relationship of the search coil signal to the second coil signal are maintained; c. introducing a 90* shift in the phase relationship of the search coil signal and the second coil signal while maintaining the signals undistorted; and d. then combining the undistorted signals, with the 90* phase shift introduced therein, to vectorially add the two signals to each other and produce therefrom a resultant signal; e. thereby obtaining from said primary field a resultant signal characterized by a magnitude that is representative of the magnitude of the primary field and remains essentially constant when the search coil and the second coil are rotated together in the primary field on an axis defined by the intersection of the planes of said two coils and thereby suppressing rotational orientation error noise that would otherwise arise if the search coil were rotated alone on said axis in the primary field.

Claim 12

In a continuously moving airborne geophysical surveying process wherein a transmitter coil on a transmitter aircraft and a search coil on an airborne receiver carrier are flown over the terrain with the receiver carrier remote at a substantially constant distance from the transmitter aircraft and flexibly attached to the transmitter aircraft while the transmitter coil radiates an audiofrequency electromagnetic primary field and the search coil picks up an electric signal from the electromagnetic flux of the primary field and the magnitude of the search coil signal is measured, the improvement for suppressing orientation error noise that undesirably tends to arise in the search coil signal and result in undesired fluctuations in the measured magnitude of the signal when maneuver variations in the relative angular orientation of the receiver carrier to the transmitter aircraft cause variations in the angle at which the search coil intersects the flux of the primary field comprising: a. picking up a second electric signal from said primary field with a second coil characterized by a signal pickup capability equal to the signal pickup capability of the search coil and disposed perpendicular to the search coil with the planes of the two coils intersecting in a line transverse to the primary field flux at the search coil and also disposed in zero mutual coupling relationship with said search coil; b. maintaining the signal from the search coil and the signal from the second coil undistorted,whereby the forms of the signals and the magnitude relationship of the search coil signal to the second coil signal are maintained; c. introducing a 90* shift in the phase relationship of the search coil signal and the second coil signal while maintaining the signals undistorted; and d. then combining the undistorted signals, with the 90* phase shift introduced therein, to vectorially add the two signals to each other and produce therefrom a resultant signal; e. thereby obtaining from said primary field a resultant signal characterized by a magnitude that is representative of the magnitude of the primary field and remains essentially constant when the search coil and the second coil are rotated together in the primary field on an axis defined by the intersection of the planes of said two coils and thereby suppressing rotational orientation error noise that would otherwise arise if the search coil were rotated alone on said axis in the primary field.

Claim 13

A process as set forth in claim 9 wherein a secondary field, that is generated by the primary field in the terrain and radiated therefrom, is received along with the primary field at the search coil and at the second coil and introduces secondary field components into the search coil signal and into the second coil signal, and wherein the phase-shifting and signal combining operations recited respective to the primary field signals in claim 17 are also performed on the secondary field components to vectorially add the secondary field signal components together with the primary field signals and thereby obtain a resultant signal indicative, by comparison with signals received in the presence of the primary field alone, of the generation of a secondary field in the terrain under survey.

Claim 14

An electromagnetic geophysical survey process comprising: a. radiating a first audiofrequency primary field over the terrain from a first transmitter coil mounted on a moving transmitter carrier and oriented with the coil axis parallel to a vertical plane through the direction of transmitter movement; b. radiating a second primary field at a second audiofrequency different than the first audiofrequency from A second transmitter coil mounted on said moving transmitter carrier and oriented with the plane of the second transmitter coil windings parallel eo the direction of transmitter movement; c. carrying a pair of mutually perpendicular first and second receiver coils on a receiver carrier moving at a substantially constant distance from the transmitter carrier and proximate to a vertical plane through the direction of transmitter carrier movement and also carrying a third receiver coil on said receiver carrier and oriented with the plane of the coil windings parallel to the vertical plane through the direction of transmitter carrier movement, said first and second receiver coils being disposed in a position of minimum mutual coupling and being characterized by mutually equal capability for generating electromotive forces when subjected to identical excitation excitations; d. intercepting with said first and second receiver coils a first resultant electromagnetic field resulting from the first primary field and any copresent secondary field generated from the first primary field; e. generating from the first resultant field electric signals in each of the first and second receiver coils at the same frequency as the first primary field frequency and responsive in phase and magnitude to the phase and magnitude of the first resultant field; f. maintaining the two receiver coil signals undistorted so as to maintain the forms of the signals and the magnitude relationship of the signals to each other; g. introducing a 90* shift in the phase relationship of the two receiver coil signals while maintaining the signals undistorted; h. then combining the undistorted signals from the first and second receiver coils to vectorially add the signals and thereby produce a first resultant signal having a magnitude responsive to both the phase and the magnitude of the first resultant field; i. intercepting with said third receiver coil a second resultant electromagnetic field resulting essentially from the second primary field; j. generating from the second resultant field a second resultant electric signal in the third receiver coil at the same frequency as the second primary field frequency and responsive in phase and magnitude to the phase and magnitude of the second resultant field; k. comparing the magnitude of the first resultant signal with the magnitude of the second resultant signal; and l. producing a resultant difference signal responsive in magnitude to the difference between the magnitudes of the first resultant signal and the second resultant signal.

Claim 15

An electromagnetic geophysical survey process comprising: a. radiating a first audiofrequency primary field over the terrain from a first transmitter coil mounted on a moving transmitter carrier and oriented with the coil axis parallel to a vertical plane through the direction of transmitter movement; b. radiating a second primary field at a second audiofrequency different than the first audiofrequency from A second transmitter coil mounted on said moving transmitter carrier and oriented with the plane of the second transmitter coil windings parallel eo the direction of transmitter movement; c. carrying a pair of mutually perpendicular first and second receiver coils on a receiver carrier moving at a substantially constant distance from the transmitter carrier and proximate to a vertical plane through the direction of transmitter carrier movement and also carrying a third receiver coil on said receiver carrier and oriented with the plane of the coil windings parallel to the vertical plane through the direction of transmitter carrier movement, said first and second receiver coils being disposed in a position of minimum mutual coupling and being characterized by mutually equal capability for generating electromotive forces when subjected to identical excitation excitations; d. intercepting with said first and second receiver coils a first resultant electromagnetic field resulting from the first primary field and any copresent secondary field generated from the first primary field; e. generating from the first resultant field electric signals in each of the first and second receiver coils at the same frequency as the first primary field frequency and responsive in phase and magnitude to the phase and magnitude of the first resultant field; f. maintaining the two receiver coil signals undistorted so as to maintain the forms of the signals and the magnitude relationship of the signals to each other; g. introducing a 90* shift in the phase relationship of the two receiver coil signals while maintaining the signals undistorted; h. then combining the undistorted signals from the first and second receiver coils to vectorially add the signals and thereby produce a first resultant signal having a magnitude responsive to both the phase and the magnitude of the first resultant field; i. intercepting with said third receiver coil a second resultant electromagnetic field resulting essentially from the second primary field; j. generating from the second resultant field a second resultant electric signal in the third receiver coil at the same frequency as the second primary field frequency and responsive in phase and magnitude to the phase and magnitude of the second resultant field; k. comparing the magnitude of the first resultant signal with the magnitude of the second resultant signal; and l. producing a resultant difference signal responsive in magnitude to the difference between the magnitudes of the first resultant signal and the second resultant signal.

Claim 16

A process as set forth in claim 11 wherein the 90* phase shift is introduced by shifting the phase of one of the receiver coil signals 45* in one direction and shifting the phase of the other receiver coil signal 45* in the other direction with the receiver coils tuned one to the upper half-power point and the other to the lower half-power point at the primary field frequency.

Claim 17

A process as set forth in claim 11 wherein a proportionate measure of a direct-current voltage component of the resultant signal is continuously recorded during at least a portion of the survey.

Claim 18

A process as set forth in claim 11 wherein the pair of mutually perpendicular first and second receiver coils is oriented with the planar intersection line, defined by the intersection of the plane of the windings of the first coil with the plane of the windings of the second coil, horizontal and perpendicular to a plane through the line of movement of the coils.

Claim 19

A process as set forth in claim 11 wherein the transmitter carrier is a powered aircraft moving in flight and the receiver carrier is a bird moving in tow behind said aircraft.

Claim 20

A process as set forth in claim 11 which additionally comprises carrying on the receiver carrier a fourth receiver coil essentially identical in size and inductance to the third receiver coil and disposed peRpendicularly to and in zero mutual coupling relationship with the third coil, intercepting the second resultant field with the fourth coil and generating in the fourth coil electric signals at the same frequency as the second primary field frequency and responsive in phase and magnitude to the phase and magnitude of the second resultant field, introducing a 90* shift in the phase relationship of the third receiver coil signal and the fourth receiver coil signal and then combining said signals to vectorially add the fourth receiver coil signal into the second resultant signal before comparing the magnitude of the first resultant signal with the magnitude of the second resultant signal.