Detection of Perfectly-Conducting Targets with Airborne Electromagnetic Systems by Adam ...

228 Pages·2011·31.26 MB·English

Detection of Perfectly-Conducting Targets with Airborne
Electromagnetic Systems
by
Adam Smiarowski
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Graduate Department of Physics
University of Toronto
Copyright ⃝c 2012 by Adam Smiarowski
Abstract
Detection of Perfectly-Conducting Targets with Airborne Electromagnetic Systems
Adam Smiarowski
Doctor of Philosophy
Graduate Department of Physics
University of Toronto
2012
A significant problem with exploring for electrically conductive mineral deposits with
airborne electromagnetic (AEM) methods is that many of the most valuable sulphide de-
posits are too conductive to be detected with conventional systems. High-grade sulphide
deposits with electrical conductivities on the order of 100,000 S/m can appear as “per-
fect 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 sec-
ondary 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
ii
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.
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Acknowledgements
I would like to thank my supervisor, Dr Richard Bailey, for his continuous academic
support and encouragement throughout my research. I would also like to thank my
Dr James Macnae for providing me with the opportunity to work with him and for
supporting me and my work from afar. I would also like to thank the companies who
provided me with data and assistance, as well as NSERC, MITACS and the Walter C.
Sumner Foundation for financial support.
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Table of Contents
1 Introduction and Background 1
1.1 Electrical Properties of Earth Materials . . . . . . . . . . . . . . . . . . . 2
1.1.1 Electrical Properties of Ore Deposits . . . . . . . . . . . . . . . . 3
1.2 Electromagnetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2.1 Biot-Savart Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.3 AEM Exploration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.3.1 Simulation of the Response from a Target . . . . . . . . . . . . . 12
1.3.2 Frequency-domain EM . . . . . . . . . . . . . . . . . . . . . . . . 16
1.3.3 Transient EM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.4 Difficulties Associated with Detection of Very Conductive Targets . . . . 27
1.5 Previous Work on Detecting Very Conductive Targets . . . . . . . . . . . 31
1.6 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2 Measuring Transmitter-Receiver Offset: The Gemini Experiment 36
2.1 Project Aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.2 Gemini Field Test in Sudbury, Ontario . . . . . . . . . . . . . . . . . . . 41
2.2.1 GPS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.2.2 Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.3 Response Over a Small Target . . . . . . . . . . . . . . . . . . . . . . . . 48
2.3.1 Modelling Results using VTEM Data . . . . . . . . . . . . . . . . 51
2.4 Secondary Target . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
2.5 Graphitic Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3 Correcting Coil Orientation with Fluxgate Readings 59
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.2 Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.3 Effect of a Magnetic Body . . . . . . . . . . . . . . . . . . . . . . . . . . 65
v
3.4 Results of Orientation Correction . . . . . . . . . . . . . . . . . . . . . . 69
3.5 Modelling with Component Data . . . . . . . . . . . . . . . . . . . . . . 73
3.5.1 Graphitic Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
4 Predicting Primary Field in a Fixed-wing AEM system 79
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2 Least-squares Prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
4.3 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.3.1 Prediction Filter Performance . . . . . . . . . . . . . . . . . . . . 84
4.3.2 Spectral Performance . . . . . . . . . . . . . . . . . . . . . . . . . 89
4.4 Explaining Primary-Field Residual . . . . . . . . . . . . . . . . . . . . . 91
4.4.1 Residual due to Conductive Overburden . . . . . . . . . . . . . . 94
4.5 Computing Primary-Stripped Time Constants . . . . . . . . . . . . . . . 96
4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5 Primary Field in a Nominally-Rigid System 101
5.1 Introduction and Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.1.1 Residual Primary Field from Simple Translation and Rotation of
System Components . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.1.2 Noise from Coil Rotations . . . . . . . . . . . . . . . . . . . . . . 114
5.2 AeroTEM Deformation from FEM Analysis and How Oscillation Modes
Cause EM Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
5.3 Calculating Response for a Simulated Flight Line . . . . . . . . . . . . . 122
5.3.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.3.2 Theoretical Approach to Calculating Secondary Response . . . . . 122
5.3.3 Comparing Amplitude of Response with Maxwell Software . . . . 125
5.3.4 Current Generated in the Target . . . . . . . . . . . . . . . . . . 128
5.3.5 Residual Primary Field Along a Synthetic Line . . . . . . . . . . 130
5.4 Response Along a Synthetic Line . . . . . . . . . . . . . . . . . . . . . . 134
5.4.1 Comparing On-time and Off-time Response . . . . . . . . . . . . 137
5.4.2 AeroQuest On-Time Processing Procedure . . . . . . . . . . . . . 144
5.4.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
5.5 Designing a Prediction Filter to Remove Residual Primary Field . . . . . 149
5.5.1 Prediction Filters - Theory . . . . . . . . . . . . . . . . . . . . . . 150
5.5.2 Selecting Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . 151
5.5.3 High Altitude On-Time Noise Power Spectrum . . . . . . . . . . . 154
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5.5.4 Prediction Filter Results for AeroTEM IV . . . . . . . . . . . . . 158
5.6 Utilising Receiver GPS Tilt Meter Flight Information - USGS Survey . . 163
5.6.1 Assessing Significance of Predictor Variables . . . . . . . . . . . . 163
5.6.2 Non-linear Effects: Including Product Terms . . . . . . . . . . . . 175
5.7 Possible Causes of Inadequate Filter Performance . . . . . . . . . . . . . 181
5.7.1 On-time Noise due to a Conductive Earth . . . . . . . . . . . . . 181
5.7.2 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
5.8 Conclusions and Recommendations . . . . . . . . . . . . . . . . . . . . . 191
6 Conclusions 195
6.1 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
Bibliography 198
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List of Tables
1.1 Range of conductivity values for some common soil types. From Bevan
(1998), Telford et al. (1990) and Angenheister (1982). . . . . . . . . . . . 4
1.2 Upper range estimate of time constant for massive ore bodies. L and W
are the length and width of the body. . . . . . . . . . . . . . . . . . . . . 5
4.1 Computed F-test statistic for possible input parameters. Parameters with
larger F-values are more likely to cause a statistically significant reduction
in the misfit (between primary field and predicted field). The vertical
stroke ′ signifies the time derivative. . . . . . . . . . . . . . . . . . . . . . 86
4.2 Rows 1-3 show the L1-norm calculated for measured on-time and residual
on-time response for each line. The ratio expresses the signal to noise
improvement. The mean signal-to-noise improvement for survey lines is
1.36. Row 4 shows the fractio nof primary field remaining in the signal
for all frequencies, while Row 5 shows the fraction of low-frequency (0-0.2
Hz) primary field remaining after predicted field has been removed. The
mean power remaining at low frequencies for all survey lines is 0.28. . . 89
5.1 This table reports ppm on-time residual (defined as ppm of nominal geom-
etry transmitter field) and residual field ratio of secondary field for targets
30 m and 100 m below nominal system position. . . . . . . . . . . . . . . 120
5.2 Label and description for the different plate-targets used to compare sec-
ondary response with residual primary field. . . . . . . . . . . . . . . . . 135
5.3 A summary of the on-time noise caused by various system movements.
The observed on-time noise is from channel 28 of stacked but otherwise
unprocessed data. The calculated on-time noise is compiled from various
parts of this report but scaled to a system with a peak transmitter current
of 150 A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
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List of Figures
1.1 Typical range of resistivity for a variety of common earth materials. After
Palacky (1987). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Calculatedcontourplotsofthenormalcomponentofcurrentdensity(A/m2)
at times (a) t/σ=0.01 s/(S/m), (b) t/σ=0.1 s/(S/m), (c) t/σ=0.4 s/(S/m)
(d) t/σ=1.6 s/(S/m). From Nabighian (1979). . . . . . . . . . . . . . . . 9
1.3 Schematic diagram of a typical fixed-wing EM system. The receiver (Rx)
is offset horizontally by r and vertically by ∆z from the receiver (distance
ρ and angle θ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.4 Calculated total magnetic field for a receiver towed behind and below a
transmitter ranging from coplanar (zero vertical offset) to coaxial (verti-
cal offset equals cable length, zero horizontal offset). The field has been
normalised by the coplanar value. . . . . . . . . . . . . . . . . . . . . . . 10
1.5 Process of EM induction. A time-varying current in the transmitter gen-
erates a time-varying primary magnetic field, which causes a secondary
magnetic field according to Faraday’s law. The time-varying secondary
magnetic field then causes an emf at the receiver. Figure taken from
Grant and West (1965). . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.6 The model used to analyse the response due to a conducting loop. A
current I flows in the transmitter, causing an electromotive force ε (not
0 1
to be confused with dielectric permittivity ϵ) in the target coil and ε in
2
the receiver coil. From Grant and West (1965). . . . . . . . . . . . . . . 13
1.7 Value of the response function ranging from the resistive limit (ω = 0) to
inductive limit (R → 0). From Grant and West (1965). . . . . . . . . . . 15
1.8 Sligram HCP in-phase response for a horizontal plate with strike length
of 2r and width 0.25r at depth 0.2r. The distance between the negative
troughs is ≈ r. From Frischknecht et al. (1970). . . . . . . . . . . . . . . 17
1.9 Sligram HCP in-phase response for a dipping plates with strike length of
4r and width 2r at depth 0.2r. From Frischknecht et al. (1970). . . . . . 18
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1.10 Example of a TEM transmitter waveform and receiver measurement gates.
Figure (a) shows the sinusoidal current waveform. Figure from Palacky
and West (1991). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.11 To illustrate the stacking process, the top figure shows the 30 Hz transmit-
ter waveform (black) and 60 Hz power line (gray). The positive half-cycle
is indicated with a “+” and the negative with a “-”. The bottom figure
shows, when the half-cycles are prepared for stacking, the positive half-
cycle at the top and the negative half-cycle multiplied by -1. Adding these
together will reinforce the transmitter signal and suppress the power line
noise. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
1.12 Typical geometry of a fixed-wing TEM system. Tx denotes transmitter
loop, while Rx is the receiver. . . . . . . . . . . . . . . . . . . . . . . . . 22
1.13 A picture of the VTEM system during a survey test conducted for this
thesis. At alternating corners, the attachments used to mount the GPS
sensors can be seen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
1.14 VTEMtransmitter-currentwaveformrecoveredfromalow-resistance1Ωm
loop compared with a high-resistance 1000 Ωm loop and streamed VTEM
data. From Davis and Macnae (2008). . . . . . . . . . . . . . . . . . . . 24
1.15 A picture of the AeroTEM II system. The bucking coil and receiver coil
can be seen at the centre of the transmitter loop. . . . . . . . . . . . . . 25
1.16 A picture of the AFMAG receiver shell (orange egg-shape at centre of
picture). TheAFMAGinstrumentcontainsthreeinductioncoilsandthree
fluxgatemagnetometers. VisibleonthetopoftheAFMAGisaGPSsensor
attached specifically for the Gemini test described in Chapter 2. . . . . . 26
1.17 SensorsmeasuringdB/dtaremoresensitivetoweaklyconductivematerial.
For a linear-ramp shutdown transmitter current waveform, the thick black
lines show dB/dt for a completely resistive earth, a weak conductor, and
an excellent conductor. The thin black line shows dB/dt measured at
receiver due to the primary field. Figure adapted from Watts (1997). . . 27
1.18 Primary (black) and secondary (gray) response for a weak, moderate and
excellent conductor. Notice the excellent conductor has no off-time signal. 28
1.19 In-phase and quadrature response for the two lowest frequencies of the
DIGHEM system for a line passing directly over the Voisey’s Bay Ovoid
deposit. I denotes in-phase while Q signifies the quadrature component. . 29
x