Monitoring the dipole moment vector of an airborne electromagnetic survey system

1. A method for monitoring a primary field dipole moment in a TDEM geological surveying system that comprises (i) a transmitter loop that lies substantially in a plane during flight and has regions that can have different relative positions to the plane in response to external forces; and (ii) a receiver having three mutually non-parallel sensitive axes, the receiver being positioned at a substantially constant location relative to the transmitter loop, the method comprising:

measuring a sign and amplitude of a primary field generated by the transmitter loop and sensed by the receiver in each of the sensitive axes and in dependence thereon calculating a vector which represents orientation of the primary field at the receiver location in a receiver frame of reference; and

determining, in dependence on the vector an orientation of the primary field dipole moment in a receiver frame of reference.

2. A method of claim 1 including sensing an attitude of the receiver in a geographical frame of reference and rotating the primary field dipole moment to a geographical frame of reference in dependence on the sensed attitude.

3. A method of claim 2 including sensing a heading of the receiver in a geographical frame of reference and rotating the primary field dipole moment to a geographical frame of reference in dependence on both the sensed attitude and heading.

4. A method of claim 1 including sensing an attitude of the transmitter loop in a geographical frame of reference, estimating the primary field dipole moment in a geographical frame of reference in dependence on the attitude of the transmitter loop, and then estimating the receiver attitude in a geographical frame of reference in dependence on the difference between the estimated primary field dipole moment in the geographical frame of reference and the determined primary field dipole moment in a receiver frame of reference.

5. The method of claim 1 wherein the transmitter loop lies in a substantially horizontal plane during flight, the receiver is located substantially at a center of the transmitter loop, and the three sensitive axes are mutually perpendicular.

6. The method of claim 1 wherein the system further includes a bucking loop that lies substantially in a common plane with the transmitter loop and having regions that can have different relative positions to the plane in response to external forces, the receiver being located substantially at a center of the transmitter loop and the bucking loop, the transmitter loop and bucking loop having respective coils in series electrical connection with a current waveform in the coil of the transmitter loop and the coil of the bucking loop having differences, and

wherein the method comprises:

separating the measured sign and amplitude of the primary field sensed by the receiver in each of the sensitive axis into a transmitter loop contributed sign and amplitude component and a bucking loop contributed sign and amplitude component;

calculating the vector that represents orientation of the primary field at the receiver location in dependence on the transmitter loop contributed sign and amplitude components;

calculating a further vector that represents orientation of the field due to the bucking loop at the receiver location in dependence on the bucking loop contributed sign and amplitude components; and

determining the orientation of the primary field dipole moment in dependence on both the vector and the further vector.

7. The method of claim 1 comprising monitoring a transmitter loop current, the vector which represents orientation of the primary field at the receiver location in the receiver frame of reference being determined in dependence on the transmitter loop current.

8. An airborne time domain electromagnetic (TDEM) geological surveying system comprising:

a transmitter loop having regions that can have different relative positions in response to external forces;

a bucking loop having regions that can have different relative positions in response to external forces, the transmitter loop and bucking loop having respective coils in series electrical connection;

a current monitor located to sense current in either the transmitter loop or the bucking loop;

a receiver having at least one sensitive axes;

the transmitter loop, bucking loop and receiver being positioned such that the bucking loop cancels out, at the receiver, at least a portion of a primary field generated by the transmitter loop; and

a processing system connected to receive signals from the receiver and the current monitor, the processing system being configured to:

measure a waveform of a field sensed by the receiver in response to current waveforms in the coil of transmitter loop and the coil of the bucking loop;

measure a waveform of the current sensed by the current monitor; and

isolate, in dependence on the waveform of the field sensed by the receiver and the waveform of the current sensed by the current monitor, components of the field sensed by the receiver that are caused by the current waveform in the transmitter loop from components of the field sensed by the receiver that are caused current waveform in the bucking coil.

9. The system of claim 8 wherein the processing system is configured to determine, in dependence on the isolated components, a vector that represents orientation of the field sensed by the receiver that is caused by the current waveform in the transmitter loop.

10. The system of claim 8 wherein the current sensor measures current in the bucking coil.