Preserving for the future...

1. The TEMPEST Airborne
Electromagnetic System and the
Walford Creek Prospect
Richard Lane*, Andy Green†
, Chris Golding*,Matt Owers*,
Caleb Plunkett*, Phil Pik†
, Daniel Sattel*, Bob Thorn†
* World Geoscience Corporation †
CSIRO

2. Tempest Design Philosophy
Cost effective AEM for geological mapping & deep targets
Small aircraft Wide Bandwidth
Low Moment Single Turn
Square Wave Transmitter
Very low noise levels
• Rapid sampling
• Record everything
• Sophisticated Filtering
• Monitor & Compensate
• Calibrate & Deconvolve

3. 25 Hz base frequency
50% Duty Cycle
40 µs Ramp
Tx Area 186 m2
Max. Current 300 A
Single Turn Loop
Current Measurement
Specifications
Continuous 75 kHz sampling and recording
Moment 55,800 Am2
(QUESTEM > 500,000 Am2
)
Continuous monitoring of transmitter and receiver orientation
Flying height 120m (subject to safety considerations)
EM sensor Towed bird with 3 component dB/dt coils
Tx-Rx separation 100m horizontal 55 m vertical (nominal)
40 ms
1500 samples

4. AEM Noise
N.W. Cape -VLF
50Hz
• Classical EM
Electronic, Sferics, Cultural (e.g...
Powerline, VLF & aircraft)
• Motion in the earth’s field
Coil rotation ( f ~ base frequency )
Translation through magnetic
anomalies
• Variable system geometry
Changing secondary coupling in
conductive areas (f < base
frequency)
Variable aircraft transient (system
self response)

5. Sophisticated Sferics
Rejection
Transmitter
Pulse

6. Common
structure relating
to the system
transfer function
Original transient
corrupted by
sferics
Next equivalent
transient
Sophisticated Sferics
Rejection

7. Data Processing
Fourier Deconvolution
V I
V I
g
g
R
ga a a a
( ) ( )
( ) ( )
( )ω ω
ω ω
ω
≈ +
Waveform at
the sensor at
high altitude*
Transmitter
current when Va
was measured*
Transmitter
current when V
was measured*
Waveform at
the sensor on
survey*
Primary field
coupling at high
altitude†
Primary field
coupling on
survey
FREQUENCY
RESPONSE OF
THE GROUND
* Measured †
Estimated from high altitude data

8. All AEM systems are
on-time systems
Aircraft
Transient
Response
Transmitter
B-field
• To estimate g we must
make assumptions the
ground response at late
time or low frequency
• Over conductive ground
the assumptions are much
more important than in
resistive areas.
V I
V I
g
g
R
ga a a a
( ) ( )
( ) ( )
( )ω ω
ω ω
ω
≈ +

9. Processed Data
R(ω) F -1
{S(ω)R(ω)}

10. Late-time X component data can be
dangerously corrupted by pitching motions.
X-coil pitching, uniform host 100S target at 150m
Late
Time
Early Time
Late
Time
Early Time
Normalized
to the host
response

11. Compensation using the
measured system geometry
Measuring M, r and za allows a
solution for zd which can then be
used to get a corrected B.
Before AfterM
I
r
ri
za
(za+ zd)
S = σd
n
Rx
Tx
Late Time
(16.2 ms) Data
corrected for
Transmitter
Pitch

12. Walford Creek
Base metal mineralization in pyritic sediments
Across this fault, the mineralization increases in depth to over 300 m, then
gradually shallows to the west, decreasing in sulphide content and
conductivity. The mineralization dips at a shallow angle to the south,
Shallow (20-150 m) mineralization east of the Dividing Fault

13. EM Flow and C_in_3D
outputs over Walford East

14. EM Flow and C_in_3D
outputs over Walford West

15. Slices of average conductivity
derived from the CDI’s
20 - 40 m 120 - 140 m 220 - 240 m
320 - 340 m 420 - 440 m
log(conductivity) mS/m
2.2
1.4
0.8
5 km

16. Sections from the 3-D Volume
Section 8030100 N
Section 211650 E
(~ line 10281)

17. Iso-surfaces bounding
conductivity > 30 mS/m.

18. Conclusions
System hardware, data quality, calibrations including
knowledge of system geometry, data processing methods
and visualisation methods have improved substantially.
TEMPEST has mapped the Walford West conductive
body at around 300 m (not done before with AEM)
It defined a similar (previously unknown) target several
km to the south at approximately 400 m depth.
This gives cause for adjustment of the role of airborne
EM in exploration in this environment.