Monitoring the dipole moment vector of an airborne electromagnetic survey system
An airborne electromagnetic survey system that monitors the dipole moment. A TDEM system includes a flexible transmitter loop and a receiver having three sensors with mutually perpendicular sensitive axes. In flight, mechanical and aerodynamic forces cause the transmitter loop to tilt and flex, which affects the direction and magnitude of the primary field dipole moment. The processing system measures the sign and amplitude of the primary field sensed by the receiver in each of its sensitive axes and calculates a vector which represents the orientation of the primary field in the receiver frame of reference at the receiver location. Using this vector, the processing system determines the orientation of the primary field dipole moment.
1. An airborne time domain electromagnetic (TDEM) geological surveying system comprising:
a transmitter loop that lies substantially in a plane during flight, the transmitter loop having regions that can have different relative positions to the plane in response to external forces;
a receiver having three mutually non-parallel sensitive axes, the receiver being positioned at a substantially constant location relative to the transmitter loop;
a processing system connected to receive signals from the receiver for the three sensitive axes, the processing system:
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
calculating, in dependence on the measured signs and amplitudes, 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. The system of claim 1 comprising an attitude and heading sensor unit sensing an attitude and heading of the receiver, the processing system rotating the primary field dipole moment into a geographical frame of reference in dependence on the sensed attitude and heading.
3. The system of claim 1 including an attitude sensing system for sensing an attitude of the transmitter loop in a geographical frame of reference, wherein the processing system estimates a primary field dipole moment in a geographical frame of reference in dependence on the attitude of the transmitter loop, and then estimates 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.
4. The system of claim 3 wherein the attitude sensing system comprises a plurality of spaced apart GPS sensors.
5. The system of claim 1 wherein the receiver is located substantially at a center of the transmitter loop.
6. The system of claim 1 wherein the three sensitive axes are mutually perpendicular.
7. The system of claim 1 further comprising:
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 processing system:
separates 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;
calculates the vector that represents orientation of the primary field at the receiver location in dependence on the transmitter loop contributed sign and amplitude components;
calculates 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
determines the orientation of the primary field dipole moment in dependence on both the vector and the further vector.
8. The system of claim 1 wherein the transmitter loop comprises a transmitter frame comprised of a plurality of frame segments connected together to form a loop supporting a transmitter coil.
9. The system of claim 8 wherein the receiver is secured in the location by radial cables that extend to the transmitter loop, and the transmitter loop is supported at multiple locations about a perimeter thereof by a suspension assembly.
10. The system of claim 1 wherein the processing system monitors 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.