Detection of Perfectly-Conducting Targets with Airborne Electromagnetic Systems.

摘要

A significant problem with exploring for electrically conductive mineral deposits with airborne electromagnetic (AEM) methods is that many of the most valuable sulphide deposits are too conductive to be detected with conventional systems. High-grade sulphide deposits with bulk electrical conductivities on the order of 100,000 S/m can appear as “perfect conductors” to most EM systems because the decay of secondary fields (the “time constant” of the deposit) generated in the target by the system transmitter takes much longer than the short measuring time of EM systems. Their EM response is essentially undetectable with off-time measurements.;One solution is to make measurements during the transmitter on-time when the secondary field of the target produced by magnetic flux exclusion is large. The difficulty is that the secondary field must be measured in the presence of a primary field which is orders of magnitude larger. The goal of this thesis is to advance the methodology of making AEM measurements during transmitter on-time by analysing experimental data from three different AEM systems.;The first system analysed is a very large separation, two helicopter system where geometry is measured using GPS sensors. In order to calculate the primary field at the receiver with sufficient accuracy, the very large (nominally 400 m) separation requires geometry to be known to better than 1 m. Using the measured geometry to estimate and remove the primary field, I show that a very conductive target can be detected at depths of 200m using the total secondary field. I then used fluxgate magnetometers to correct for receiver rotation which allowed the component of the secondary field to be determined.;The second system I examined was a large separation fixed-wing AEM system. Using a towed receiver bird with a smaller (≈ 135m) separation, the geometry must be known much more accurately. In the absence of direct measurement of this geometry, I used a least-squares prediction approach using measurements of aircraft manoeuvres which allowed primary field contamination to be estimated. Subtracting this estimate from the recorded signal increased the maximum time constant observed in a field survey for conductive targets by a factor of seven.;Finally, a study of a nominally rigid helicopter EM system employing a bucking coil to cancel primary field showed that system geometry (specifically, the position of the receiver coil relative to the transmitter and bucking coils) must be known to better than 0.01 mm to detect deep targets. Again, direct measurements of system geometry were not available. A least-squares prediction filter using helicopter manoeuvre and system pitch and roll measurements was applied, but was not able to estimate primary field well enough to provide an accurate secondary on-time response. Direct measurements of relative motion of the system components might solve this problem.