Geoscience Reference
In-Depth Information
across the pro
le. Its strong response obliterates the early-
time Z-component response of the target conductor, for all
conductor orientations. Note that for the in-loop con
gur-
ation that the X- (and Y-) component response is zero for a
horizontal conductive layer and a conductive half-space, so
the X component of the target conductor
the latter are simpler to analyse in terms of target param-
eters. For all arrays, the relationships between the shape
and polarity of the X- and Z-component responses deter-
mine whether a conductor is horizontal or is steeply dip-
ping and the amount and direction of its dip, and they
locate the conductor. Often the survey data may resolve the
lateral migration, in the down-dip direction, of the anom-
aly peaks with delay time, caused by the eddy current
'
is response is
resolved without interference from the overburden
response (the effects of coupling between the overburden
and the conductor notwithstanding).
The moving geometrically asymmetric separated-loop
configuration produces asymmetric responses in both the
X and Z components (
Figs. 5.85
and
5.86
), and strongly so
for a dipping conductor. Furthermore, whether the trans-
mitter (Tx) or the receiver (Rx) leads the moving array also
determines the nature of the asymmetry, and in particular
the polarity of the X-component response. For both orien-
tations, responses are strongest on the down-dip side.
Asymmetry makes it dif
s
inward migration into the conductor. The fixed-loop array
is strongly affected by a conductive surface layer and the X-
component response of the in-loop array has the greatest
immunity to it.
'
Decay analysis
For measurements defining the peak of a target anomaly
(the strongest signal to late times), plotting the measured
decay on log
log axes (
Fig. 5.78a
) allows the early-time
power-law decay of the overburden response to be identi-
fied and removed as a sloping line from the observed
decay. In the same way, the slower-decaying half-space
response, extending to mid-times, can then be removed.
The residual decay is plotted on log
-
cult to compare responses from
opposite survey directions over a conductor, a fundamental
issue with towed-bird AEM data (see
Section 5.9.2.1
). The
model responses are for Rx
Tx separation equal to twice
the loop dimension; varying the separation has a signifi-
-
cant effect on the amplitude, width and cross-over loca-
tions of the responses. The response of conductive
overburden is strong in both components.
coupling does not vary and depends on the loop
-
linear axes (
Fig. 5.78b
)
to reveal the exponential decay of an underlying con
ned
conductor as a straight line, if present (
Fig. 5.81
). The time
constant can be determined from the slope of the line and,
possibly, estimates made of the
-
'
'
of the conductor,
i.e. its conductivity and thickness (
Eq. (5.26)
)
. Note that
not all discrete bodies exhibit an exponential decay, owing
to inhomogeneity and the effects of current channelling
(see
Section 5.7.2.4
).
quality
s location
with respect to the conductor. When the loop is offset from
a dipping conductor, the X component has a broad asym-
metric peak response located over the conductor with
polarity cross-overs defining its distant flanks. The Z com-
ponent is a single cross-over response located near the top
of the conductor. The responses are symmetrical and coin-
cident over a vertical conductor. For the case of a horizon-
tal conductor located directly below the loop, the responses
exchange their form and both are symmetrical. Conductive
overburden produces strong responses in both compon-
ents with the X-component cross-over occurring at the
centre of the loop. The Z-component cross-overs occur at
increasing distance from the loop with increasing delay
time, as the current system expands outward. The overbur-
den response causes signi
cant distortion and broadening
of the target response in both components. Note the diffi-
-
culty in identifying the cross-overs of the target response in
the presence of the overburden response.
The moving-loop arrays produce complex target
responses over a relatively smaller distance than those of
the fixed-loop configuration, but the broader responses of
'
Modelling EM data
Both forward and inverse modelling is used to interpret
EM data but the complexity of the responses means use of
numerous examples of the modelling of EM data.
One-dimensional inverse modelling of decays is
described in
Section 5.7.4.3
and is useful for more qualita-
tive interpretation. For more accurate analysis of individ-
ual anomalies, modelling techniques based on more
realistic approximations of the geometries of anomalous
conductivity distributions are required. The complexity of
mathematically describing the current
flow in complex
geometrical forms has restricted the development of
computer-based numerical methods of calculating the
response of con
ned bodies and complex 3D conductivity
distributions. An example of a typical graphical display for
Search WWH ::
Custom Search