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Airborne ElectroMagnetic systems background
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With reference to the above diagram, the magnetic field strength B at a distance
Z from an n turn coil is given by the following expression:
One observation is that the field strrength decreases with the third power of
distance between the coil on the aircraft and the region where the eddy current
is induced.
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The traditional manned aircraft approach
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- from Fugro document called AEMdev.pdf
Up to six windings are in the large coil running along the outside of a Fugro
Airborne Survey aircraft, from Fugro documentation, FTEM2.pdf.
The latest Fugro MegaTEM II achieved a dipole moment of 2.2 M Am
2
. Both items from Fugro documentation. The optimal shape for an Unmanned Air
Vehicle would be a large diameter (say 24 m
diameter 452 m
2
area) flying saucer, with the coil cables running around the perimeter of the
craft. With a six turn (6T) coil, one would need a peak current of 811 A, to
generate a dipole moment (= NumTurns x PeakCurrent x Area) of 2.2 M Am
2
.
The sheer size of the coil and the weight of the power supply suggests that
Airborne Electromagnetic probing will remain the domain of large, manned,
aircraft for some time to come...
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An approach using Unmanned Aircraft
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From a paper presented by Dr Mark Patterson at the 23rd Bristol International
UAV Systems Conference in April 2008:
A 2 amp current is passed through the coil wrapped around the relatively small
Silver Fox Unmanned Aircraft shown above. The current in the coil is modulated
at around 88 kHz. A sensing coil is towed behind the Unmanned Aircraft and the
signals detected by the towed sensor, shown below, enable the Unmanned Aircraft
to detect underground tunnels and buried wires.
Above: the Unmanned Aircraft with the towed sensor behind it.
We compare the magnetic field strength 50 m below the surface of the earth, for
example.
|
manned
|
unmanned
|
units
|
|
4
|
50
|
|
|
406
|
5
|
m
2
|
|
665
|
2
|
amps
|
|
1,080,000
|
500
|
amps.m
2
|
|
170
Aircraft flies 120 m AGL
|
60
UA flies 10 m AGL
|
m
|
|
1.0
|
0.01
|
|
By flying close to the surface of the earth, the Unmanned Aircraft can be used
to perform moderate AEM survey activities using only a small fraction of the
current used in manned aircraft AEM work.
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Weight of the exciter coil on the Unmanned Aircraft
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Further to the above example, let us consider a 5 m
2
square coil of side = 2.236 m, with 50 turns.
|
AWG gauge
|
Conductor
Dia mm
|
Ohms
per km
|
Maximum amps
for power transmission
|
Maximum freqency
for 100% skin depth
for solid conductor copper
|
Weight
g / m
|
|
18
|
1.02362
|
20.9428
|
2.3
|
17 kHz
|
7.333
|
|
19
|
0.91186
|
26.40728
|
1.8
|
21 kHz
|
5.815
|
|
20
|
0.8128
|
33.292
|
1.5
|
27 kHz
|
4.611
|
|
21
|
0.7239
|
41.984
|
1.2
|
33 kHz
|
3.657
|
|
22
|
0.64516
|
52.9392
|
0.92
|
42 kHz
|
2.901
|
|
23
|
0.57404
|
66.7808
|
0.729
|
53 kHz
|
2.300
|
|
24
|
0.51054
|
84.1976
|
0.577
|
68 kHz
|
1.839
|
|
25
|
0.45466
|
106.1736
|
0.457
|
85 kHz
|
1.446
|
|
26
|
0.40386
|
133.8568
|
0.361
|
107 kH
|
1.147
|
|
27
|
0.36068
|
168.8216
|
0.288
|
130 kHz
|
0.910
|
|
28
|
0.32004
|
212.872
|
0.226
|
170 kHz
|
0.721
|
|
29
|
0.28702
|
268.4024
|
0.182
|
210 kHz
|
0.572
|
|
30
|
0.254
|
338.496
|
0.142
|
270 kHz
|
0.454
|
|
31
|
0.22606
|
426.728
|
0.113
|
340 kHz
|
0.360
|
In order to carry a 2 Amp RMS current, we select AWG 20 wire, since we need to
minimise the coil weight. For a 50 turn coil, we will need 50 x 4 x 2.236 m =
447.2 m. The weight of the coil, if made from AWG 20 wire, would be 447.2 x
4.611 x 0.001 kg = 2 kg.
We may be able to reduce the weight of the coil through the use of thin copper
foil, where the foil, as used in the PCB industry, is "0.5 ounce per sq foot",
ie. around 17 um thick, or "1 ounce per square foot", ie. 34 um thick.
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