Geoscience Reference
In-Depth Information
regolith pro
le) determines the way in which the data are
acquired. It is impossible to design a single EM survey that
optimally ful
ls all possible requirements
Reducing the base frequency creates a primary
field with
more low-frequency energy. It also increases the transmit-
ter off-time so the secondary
field can be measured to later
delay times, important for resolving good-quality
conductors (see Conductor quality in
Section 5.7.2.3
)
.
Increasing the base frequency has the opposite effect of
increasing the high-frequency energy. It also produces a
faster pulse turn-off allowing the secondary field to be
measured closer in time to the pulse turn-off, important
for resolving poor-quality conductors and near-surface
features. For mobile, i.e. airborne, systems, the shorter
transmit-receive cycle means there is greater lateral reso-
lution because the system travels less during each cycle.
The base frequency of most EM systems can be adjusted
to allow the system to be
one reason why
there are many types of EM survey systems available.
As described in
Section 5.7.2.5
, there are advantages to
the step and impulse responses in different circumstances,
and different targets will require the decay of secondary
fields to be measured over different time intervals. Add-
itional survey parameters that need to be considered are the
base frequency, the system geometry and how the data are
normalised. Together
-
these parameters
influence the
system
s ability to detect and resolve the geometry and
attitude of a conductor. They also strongly determine the
system
'
s depth of investigation in a particular environment.
Sometimes EM data are acquired primarily to determine
vertical variations in conductivity, i.e. EM soundings, the
equivalent of the electrical soundings described in
Section
collected along traverses, albeit with each dataset treated
in isolation.
'
for a particular survey
objective, and to minimise sensitivity to powerline
interference. Setting the system base frequency to that
providing the best resolution of the target sought is funda-
mental to the success of the survey. A series of repeat line-
surveys over known target areas using different base fre-
quencies can help in this regard. Otherwise, modelling
results can be used (if suf
cient information is available
about the actual electrical parameters of the target and the
environment).
'
tuned
'
5.7.3.1
Transmitter waveform and base frequency
The primary
field waveform is a fundamental control on
the response recorded. The waveform can be described by
its frequency spectrum (see
Appendix 2
), with differently
shaped waveforms having different frequency content.
It is impossible to create the ideal step and square pulse
variations in the primary
field because these would require
instantaneous changes in the transmitter loop current,
which is prevented by the inductance of the loop (see
Section 5.7.1.2
)
. Instead a less-desirable slower rise and fall
is produced (
Fig. 5.73a
). The rise may be linear with time
or of some other mathematical relationship; the turn-off is
usually a relatively fast linear ramp. There is a correspond-
ing variation in the responses measured. Systems based on
the transmission of a triangular waveform are less affected
by loop inductance.
Figure 5.97
shows the actual system
waveforms for a variety of AEM systems. Compare these
In general, high-powered systems have slower turn-off
ramp times and lower-powered systems have faster turn-
off. The ramp time of the pulse turn-off determines the
delay time of the earliest measurement and signi
cantly
in
uences the resolution of fast-decaying conductors,
whilst the width of the pulse affects the resolution of
slower-decaying conductors.
A key acquisition variable is the repetition rate of the
primary
5.7.3.2
System geometry
The system geometry is the arrangement, spacing, sizes
and orientations of the transmitter and receiver. Whether
the survey is for reconnaissance work or for prospect-scale
surveying will determine the transmitter
receiver config-
uration used, which also determines the systems lateral
resolution.
A coil or loop which lies in the horizontal plane, such
that its axis is vertical, is described as horizontal. For large
loops placed on the ground and for AEM surveys this is the
only practical possibility. A horizontal loop couples well
with host rocks, horizontal layers and conductors with a
wide range of dips. Transmitter loops for ground surveys
are rectangular and generally consist of a single turn of
insulated wire laid on the surface. The situation for air-
borne surveys is discussed in
Section 5.9.2
. Loop size is
discussed in Transmitter loop size in
Section 5.7.3.2
,
Most EM systems simultaneously measure both the ver-
tical (Z) and the along-line (X) components of the second-
(Y) component is also measured. The perpendicularly
oriented components allow better characterisation of con-
ductors than a single component. When the transmitter
-
field pulse, called the system base frequency.
Search WWH ::
Custom Search