Geoscience Reference
In-Depth Information
superior performance in areas of conductive overburden
(see Sections 5.3.4 and 5.7.6.1 ) compared with FD-AEM
systems; they are capable of detecting good conductors
located at great depth below conductive overburden. FD-
AEM systems now find application mainly to shallower
groundwater and environmental studies. There are also
FD-AEM systems that make use of natural EM fields which
can explore to signi cant depth (see Appendix 4 ) .
Applications of AEM in mineral exploration include
detection of massive sulphides, unconformity-style uran-
ium mineralisation (associated with conductive shear
zones), kimberlites and mapping palaeochannels as poten-
tial hosts for placer deposits and sandstone- and calcrete-
hosted uranium deposits. The use of AEM for geological
mapping is becoming more common to complement
magnetic and gravity surveys, although AEM remains
signi
AEM systems operate in the same way as ground EM
systems, but with some obvious distinctions. The transmit-
ter loop is located at a height above the ground so the eddy
current induced at the instant of the pulse turn-off (see
Section 5.7.2.1 ) is laterally more expansive and locally
weaker than for the same loop on the ground surface. In
addition, the loop is necessarily smaller than that typically
used in ground surveys, the transmitter and receiver are
further from the target and the measurements are made
from a continuously moving platform. Consequently, the
measurements are inherently noisier so more attention is
required to maximise the signal-to-noise ratio in both the
instrumentation and the data reduction. AEM systems
generally have a more powerful transmitter than ground
systems and some also have a multi-turn loop, which
increases its inductance (see Section 5.7.1.2 ) . The lower-
cantly more costly than aeromagnetics (see Section
1.2.3 ) . AEM has a signi
flying helicopter systems place the transmitter loop closer
to the ground counteracting geometrical attenuation of the
loop
cant role in mapping groundwater
and soil salinity. These are important applications in
mine development; moreover, soil moisture and salinity
are usually the source of the EM responses measured
when mapping palaeochannels and the weathered upper
portions of
s field; so some helicopter systems produce a stronger
primary signal at the target than is achievable with some of
the higher-powered, higher- ying, fixed-wing systems. The
continuously moving AEM system
'
an elongate
zone of the subsurface parallel to the flight line to produce
a single reading, which is exacerbated by longer measure-
ment periods, i.e. lower base frequency. There is the added
complication of continuously changing coupling between
the transmitter and any conductors present, and between
the secondary magnetic field and the receiver. These effects
reduce lateral resolution in the survey line direction. The
final along-line data interval is set by the system timing
and the speed of the aircraft.
Applications of AEM in mineral exploration range from
conductor detection to conductivity mapping at a range of
depths, and in both conductive and resistive environments.
It is dif
'
averages
'
some types of mineral
targets,
such as
kimberlite pipes.
We describe the principles of TD-AEM systems and
their applications in mineral exploration with an emphasis
on the types of systems in current use. The reader is
referred to Klein and Lajoie ( 1992 ) for descriptions of some
older AEM systems, including frequency domain systems,
survey procedures and interpretation techniques.
5.9.1 Acquisition of AEM data
Fixed-wing and helicopter AEM systems are in use with
both configurations comprising a large horizontal trans-
mitter loop and small receiver coils (dB/dt sensors) meas-
uring both the along-line horizontal (X) component and
the vertical (Z) component of the
cult to build an AEM system that is fully multi-
purpose because of engineering considerations. For this
reason there exist a variety of TD-AEM systems with indi-
vidual systems tending to be optimised for a selected range
of applications. Characteristics that vary between systems
include the transmitter waveform, signal frequency,
receiver sampling rate, type of response measured by the
receiver, primary field strength, and signal enhancement
and noise suppression applied to the measured signal.
field (see Section 5.7.1.5 ).
A few systems also measure the across-line horizontal (Y)
component. The horizontal orientation of the transmitter
loop is dictated by the practicalities of fitting it to an
aircraft. A consequence is that coupling ( Fig. 5.70 ) is opti-
mum for conductors with large horizontal dimensions
located immediately below the loop, and ideal for measur-
ing the background conductivity (see Section 5.7.2.1 ).
Many potential mineral targets do not ful l this criterion,
but nevertheless, good coupling can be achieved to steeply
dipping targets located in the side regions of the loop.
5.9.1.1 System waveform
A wide variety of transmitted pulses and system waveforms
are used by the various TD-AEM systems ( Fig. 5.97 ) . Like
ground TDEM systems, the actual width of the pulse and
 
Search WWH ::




Custom Search