Receiver coil assembly for airborne geophysical surveying with noise mitigation

1. An airborne geophysical surveying system comprising:

a receiver coil assembly for towing by an aircraft, the receiver assembly including at least one receiver coil for sensing changes in a magnetic field component of a magnetic field, and a receiver coil orientation sensing system for sensing orientation changes of the receiver coil; and

a controller receiving signals representing the sensed changes in the magnetic field component from the receiver coil and the sensed orientation changes from the receiver coil orientation sensing system, and correcting the sensed changes in the magnetic field component to provide a signal that is corrected for noise caused by changing orientation of the receiver coil in a static geomagnetic field, wherein the controller provides the corrected signal by:

predicting, in dependence on the signal from the receiver coil orientation system, a resulting signal from the receiver coil, and

subtracting the resulting signal from the signal received from the receiver coil.

2. The system of claim 1 wherein the receiver coil orientation sensing system includes one or more accelerometers or angular rate sensors for sensing rotations of the receiver coil assembly about at least two independent axes.

3. The system of claim 2 wherein the orientation sensing system includes a three-axis fluxgate magnetometer having sensitive axes that correspond to the receiver coil axis for measuring a magnitude and direction of the static geomagnetic field relative to the receiver coil axis, the controller determining an instantaneous angular rotation rate of the receiver coil assembly about the receiver coil axis and combining the rotation rate with the geomagnetic field magnitude and direction measurement to predict the resulting signal from the receiver coil.

4. The system of claim 2 wherein the controller processes signals from the orientation sensing system to determine the orientation of the receiver coil in a geographic coordinate system, determine the static geomagnetic field from a known formula, determine the instantaneous angular rotation rate of the receiver coil assembly about the receiver coil axis and combine the rotation rate with the determined orientation and static geometric field to predict the resulting signal from the receiver coil.

5. The system of claim 2 wherein the controller corrects the magnetic field component for noise caused by changing orientation of the receiver coil in the static geomagnetic field by applying an adaptive noise cancellation algorithm to signals from the accelerometers or angular rate sensors to predict the resulting signal from the receiver coil.

6. The system of claim 1 wherein the receiver coil assembly includes three receiver coils each having a coil axis that is substantially maintained at a known angle relative to the axes of the other receiver coils, each of the receiver coils sensing changes in a different magnetic field component of the magnetic field, the receiver coil orientation sensing system sensing orientation changes of all of the receiver coils, the controller receiving signals representing the sensed magnetic field component changes from each of the receiver coils and the sensed orientation changes from the receiver coil orientation sensing system and correcting the sensed magnetic field component changes to provide magnetic field components that are corrected for noise caused by changing orientation of the receiver coils in the static geomagnetic field.

7. The system of claim 6 wherein the axes of the three receiver coils are mutually orthogonal and the three independent axes correspond to the three receiver coil axes.

8. The system of claim 6 wherein the orientation sensing system includes a three-axis fluxgate magnetometer having three sensitive axes that correspond to the three receiver coil axes for measuring a magnitude and direction of the static geomagnetic field relative to the receiver coil axes, the controller combining the signals received from the three receiver coils in dependence on geomagnetic field measurements from the fluxgate magnetometer to obtain a signal corresponding to changes in the magnetic field component which is parallel to the geomagnetic field.

9. The system of claim 6 wherein the three receiver coils are air core coils elastically suspended within a rigid spherical outer shell.

10. The system of claim 6 wherein the three receiver coils are elastically suspended within a semi-rigid skeletal frame having the form of a regular octahedron.

11. The system of claim 6 wherein the receiver coil assembly comprises a semi- rigid outer shell having a nominally vertical extent which is substantially less than a horizontal extent thereof, one of the receiver coils being a multiturn air core coil housed within a tubular loop defined by the outer shell and having a nominally vertical axis and the other receiver coils each comprising multiturn solenoid windings with ferromagnetic cores having a horizontal axis.

12. The system of claim 11 wherein the tubular loop of the semi-rigid outer shell takes the form of a polygonal or circular loop, the semi-rigid outer shell having two perpendicular diameters across the loop in a nominally horizontal plane, the multiturn air core coil being located in the loop and the other receiver coils each being positioned in a respective one of the perpendicular diameters of the outer shell.

13. The system of claim 12 wherein each of the receiver coils are elastically suspended from a semi-rigid inner member which in turn is elastically suspended within the semi-rigid outer shell.

14. The system of claim 13 wherein at least part of the orientation sensing system is attached to the semi-rigid inner member.

15. The system of claim 1 further including a transmitter coil for generating a time varying pulsed signal having successive on and off durations, the receiver coils sensing changes in the magnetic field components during the off-durations of the transmitter coil.

16. The system of claim 1 wherein the changes in magnetic field components sensed by the receiver coils are naturally occurring random fluctuations of the magnetic field.

17. A method of performing an airborne geophysical survey comprising:

towing a receiver coil assembly with an aircraft, the receiver assembly including three receiver coils each having a respective coil axis that is substantially maintained at a known angle relative to the axes of the other receiver coils;

sensing, with each of the receiver coils, a different magnetic field component of a magnetic field;

sensing orientation changes of the receiver coil assembly in a static geomagnetic field; and

correcting the sensed magnetic field component changes to provide magnetic field components that are corrected for noise caused by changing orientation of the receiver coils in the static geomagnetic field, wherein correcting the sensed magnetic field component changes comprises:

predicting, in dependence on the sensed orientation changes, a resulting signal from the receiver coil, and

subtracting the resulting signal from the sensed magnetic field components.