Receiver coil assembly with air and ferromagnetic cored sensors for geophysical surveying

1. A receiver coil assembly for towing by an aircraft for performing geophysical surveys, comprising a semi-rigid outer shell having a nominally vertical extent which is substantially less than an horizontal extent thereof, the outer shell housing a multiturn air core receiver coil and at least one solenoid receiver coil, each receiver coil sensing changes in a different magnetic field component and having an axis at a known angle to the axis of the other receiver coil, wherein the multiturn air core receiver coil has a nominally vertical axis and the solenoid receiver coil comprises a multiturn solenoid winding with a ferromagnetic core and has a horizontal axis.

2. The receiver coil assembly of claim 1 in which the outer shell includes a tubular loop portion defining a polygonal or circular loop in which the multiturn air core receiver coil extends.

3. The receiver coil assembly of claim 2 comprising a further solenoid receiver coil comprising a multiturn solenoid winding with a ferromagnetic core and having a horizontal axis, the further solenoid receiver coil sensing changes in a different magnetic field component than the other receiver coils and having an axis at a known angle to the axes of the other receiver coils,

the outer shell comprising a cross-shaped portion having a first tubular cross member extending across a first diameter of the tubular loop portion and a second tubular cross member extending across a second diameter of the tubular loop portion, the first diameter and second diameter being perpendicular to each other;

one of the solenoid receiver coils being located in the first tubular cross member and the other of the solenoid receiver coils being located in the second tubular cross member.

4. The receiver coil assembly of claim 3 wherein the receiver coils are each elastically suspended from an inner semi-rigid frame which in turn is elastically suspended within the outer shell.

5. The receiver coil assembly of claim 4 wherein the tubular loop portion and cross-shaped portion are substantially in a common plane, and the receiver coils have mutually orthogonal axes.

6. The receiver coil assembly 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.

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

8. The receiver coil assembly of claim 1 wherein the solenoid receiver coil is located adjacent a region of the air core receiver coil within a common passageway in the outer shell.

9. A receiver coil assembly for performing geophysical surveys, comprising:

a hollow outer shell defining a continuous internal passage that forms a loop;

a multiturn receiver air coil extending around the continuous internal passage; and

a first cored coil comprising multiturn solenoid windings about a ferromagnetic core and having a sensing axis in a different direction than a sensing axis of the air coil, wherein the first cored coil is located adjacent a region of the air coil in the internal passage.

10. The receiver coil assembly of claim 9 wherein windings of the air coil in the region extend substantially parallel to the sensing axis of the first cored coil.

11. The receiver coil assembly of claim 10 wherein the windings of the air coil in the region are symmetrically positioned around the core of the first cored coil.

12. The receiver coil assembly of claim 11 wherein common elastic members suspend both the first cored coil and the region of the air coil within the internal passage.

13. The receiver coil assembly of claim 12 wherein the common elastic members are attached to an intermediate member which is suspended by further elastic members within the internal passage.

14. A receiver coil assembly for performing geophysical surveys, comprising:

a hollow outer shell defining a continuous internal passage that forms a loop;

a multiturn receiver air coil extending around the continuous internal passage; and

a first cored coil comprising multiturn solenoid windings about a ferromagnetic core and having a sensing axis in a different direction than a sensing axis of the air coil, wherein the first cored coil comprises multiple core segments positioned end to end with gaps between adjacent core segments.

15. The receiver coil assembly of claim 14 wherein the core segments are connected by flexible connectors.

16. A receiver coil assembly for performing geophysical surveys, comprising:

a hollow outer shell defining a continuous internal passage that forms a loop;

a multiturn receiver air coil extending around the continuous internal passage;

a first cored coil comprising multiturn solenoid windings about a ferromagnetic core and having a sensing axis in a different direction than a sensing axis of the air coil; and

a second cored coil comprising multiturn solenoid windings about a ferromagnetic core, the second cored coil located adjacent a second region of the air coil within the internal passage and having a sensing axis in a different direction than the sensing axis of the air coil and the sensing axis of the first cored coil.

17. A receiver coil assembly for performing geophysical surveys, comprising:

a hollow outer shell defining a continuous internal passage that forms a loop;

a multiturn receiver air coil extending around the continuous internal passage;

a first cored coil comprising multiturn solenoid windings about a ferromagnetic core and having a sensing axis in a different direction than a sensing axis of the air coil;

wherein the loop is a polygonal or circular loop, further comprising a second cored coil comprising multiturn solenoid windings about a ferromagnetic core, the first and second cored coil and the air coil each being maintained at known relative positions, the coils each sensing changes in a different magnetic field component of a magnetic field.

18. The system of claims 17 wherein the outer shell has a nominally vertical extent which is substantially less than a horizontal extent thereof, the air core receiver coil housed within the tubular outer portion having a nominally vertical axis, the first and second solenoid receiver coils each having a horizontal axis.

19. The receiver coil assembly of claims 9 wherein the assembly is part of a tow assembly for towing from an aircraft.

20. A receiver coil assembly for performing geophysical surveys, comprising:

a multiturn air coil receiver defining a loop;

at least one cored coil receiver having a ferromagnetic core and a solenoid winding, supported immediately adjacent a region of the air coil, with a long axis of the core being substantially parallel to turns of the adjacent region of the air coil, wherein the cored coil comprises multiple core segments positioned end to end with gaps between adjacent core segments.

21. The receiver coil assembly of claim 20 wherein the air coil and the cored coil are supported by an elastic suspension which reduces vibration transmitted to the coils.

22. The receiver coil of assembly of claim 20 in which the cored coil is attached to the air coil.

23. The receiver coil assembly of claim 20 wherein the receiver coil assembly is incorporated into a tow assembly adapted to be towed by an aircraft.