Airborne time domain electromagnetic transmitter coil system and appratus

1. A tow assembly for an airborne electromagnetic surveying system, comprising:

a transmitter coil frame supporting a transmitter coil, the transmitter coil frame being formed from a plurality of serially connected frame sections forming a loop, the transmitter coil frame having joints at a plurality of locations about a circumference thereof enabling at least portions of the connected frame sections to at least partially rotate about a long axes thereof such that the transmitter coil frame to at least partially bend about its circumference to adjust during landing contact with a ground surface to reduce stress on the transmitter coil frame; and

a suspension assembly for towing the transmitter coil frame behind an aircraft, the suspension assembly being attached to the circumference of the transmitter coil frame at spaced apart locations.

2. The tow assembly of claim 1 wherein the tow assembly includes a plurality of radial ropes that each extend outward from a central region to a respective location on the circumference of the transmitter coil frame.

3. The tow assembly of claim 2 comprising a receiver coil frame supporting a receiver coil, the receiver coil frame being supported by the plurality of radial ropes with the receiver coil substantially concentric with the transmitter coil.

4. The tow assembly of claim 1 wherein the suspension assembly has a shorter vertical height at a leading side thereof than a trailing side thereof thereby supporting the transmitter coil frame in a substantially horizontal position at a desired flight speed.

5. The tow assembly of claim 1 wherein the suspension assembly comprises a tow rope for attachment to an aircraft and a plurality of suspension ropes, the suspension ropes each having an upper end attached to an end of the tow rope and a lower end connected to a respective one of the spaced apart locations about the circumference of the transmitter coil frame.

6. The tow assembly of claim 5 wherein the lengths of the suspension ropes attached to a trailing portion of the transmitter coil frame are longer than the suspension ropes attached to a leading portion of the transmitter coil frame.

7. The tow assembly of claim 1 wherein the joints of the transmitter coil frame are rotating joints between at least some of the serially connected frame sections.

8. The tow assembly of claim 7 wherein the rotating joints between at least some of the serially connected frame sections are lubricated.

9. The tow assembly of claim 7 wherein the rotating joints are each formed by a male portion of one of the transmitter frame sections received within a corresponding female portion of another of the transmitter frame sections.

10. The tow assembly of claim 9 wherein the frame sections are tubular sections that collectively define a continuous internal coil passage in which the transmitter coil is received.

11. The tow assembly of claim 10 wherein the tubular sections include straight tubular sections and elbow tubular sections.

12. The tow assembly of claim 10 wherein the transmitter coil frame has multiple sides that collectively define a polygonal shape.

13. The tow assembly of claim 12 wherein multiple straight tubular sections are included in each side of sides.