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.
| Identifer | oai:union.ndltd.org:TORONTO/oai:tspace.library.utoronto.ca:1807/32892 |
| Date | 31 August 2012 |
| Creators | Smiarowski, Adam |
| Contributors | Bailey, Richard |
| Source Sets | University of Toronto |
| Language | en_ca |
| Detected Language | English |
| Type | Thesis |
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