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with larger time constants, an example being a discrete
target conductor hosted in a large formational conductor.
It is also possible for a large body having a large time
constant to shield a smaller body with smaller time con-
stant, a common problem when a target conductor is
hosted by a large formational conductor.
10
4
10
3
Channel
5
6
7
8
10
2
1
11
12
10
1
5.7.6.3
Cultural and topographic effects
As described in
Sections 2.4.1
and
5.4.2.1
,
man-made fea-
tures such as fences and powerlines will produce EM
responses. These can be identified using aerial photographs
and topographic maps and should be noted at the time of
data acquisition.
Where survey lines pass over hills and valleys, typical in
AEM surveys, hills usually appear as conductive features.
Responses are likely in early delay times. The response
from hills is stronger when the survey aircraft passes
closely over the peak of the topography, and is usually
stronger in low-level AEM data. Care should also be taken
where responses coincide with a
13
14
10
0
-10
0
0
200
Metres
-10
1
Location
Figure 5.88
Negative amplitudes in TDEM data indicative of IP
effects. Data were collected with 100 m square moving loop and a
near-coincident vertical component coil sensor. Channel delay times
range from 0.25 to 1.45 ms. Redrawn, with permission, from Flis
(
1989
).
5.7.6.5
Superparamagnetism
An external magnetic
change in
ground clearance, such as a scarp, that can cause artefacts
in 1D inversion models (see
Section 5.7.6.7
)
. Topographic
effects can be identi
ed by observing correlations between
apparent conductivity images and topographic data.
There is no practical way of removing topographic effects
other than to include the topography in computer models
and account for its effect in the interpretation of the
survey data.
'
step-like
'
field causes magnetic domains in
ferromagnetic material to align with it (see
Section 3.2.3
)
.
There is a time lag between removal of the
field and the
magnetic dipoles returning or relaxing to their original
state. The effect is known as magnetic viscosity or super-
paramagnetism (SPM). The strong magnetic
field of a
transmitter loop causes very
fine ferromagnetic grains in
the soil to behave in this way. The changing magnetic
eld
associated with the relaxation can be observed during the
measurement period of most TDEM systems. It is a
logarithmic process in the step response; so in the impulse
response it exhibits power-law decay (
Fig. 5.78a
) with
decay constant (k) equal to
5.7.6.4
Induced polarisation
It is generally assumed in EM that the conductivity of the
ground is frequency independent, i.e. its conductivity does
not change with frequency or delay time. If this is not the
case, electrically polarisable conductors and polarisable
ground become charged (see
Section 5.3.2
) at early delay
later times, when the eddy currents are weaker, the polar-
isation discharges through the conductive ground, and the
magnetic
field associated with the decaying discharge cur-
rent is measured by the EM system. The discharge has
opposite polarity to the induced eddy currents and reduces
the amplitude of the measured secondary response. At late
delay times, IP effects can produce anomalous decay rates
or even reverse the polarity of the measured secondary
between IP and EM effects in EM data, and procedures
for recognising, interpreting and correcting IP effects are
currently the subject of ongoing research.
-
1 (actual measurements range
from
anomalous variations in SPM in weathered materials above
a placer gold deposit and a nickel sulphide deposit.
SPM effect, if present, is evident after the faster decaying
thin-layer and half-space responses have disappeared (see
Section 5.7.2
)
. It persists to later delay times and causes the
ground to appear to be more conductive than it actually is,
i.e. apparent resistivity is lower than expected. SPM effect
is stronger with increasing primary
field strength and can
be measured within several metres of a ground transmitter
Fixed-loop mode in
Section 5.7.3.2
.
It is usually not
observed with the commonly used in-loop (with the
receiver at the centre of the loop) and separated-loop
-
0.8 to
-
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