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the step response, have been computed for the target
conductor set in a high-resistivity background, and with
and without the presence of conductive overburden. Eddy
current
flow is restricted to just the plane of the plate
(see
Section 5.7.1.4
). The overburden is simulated with a
conductive thin plate 20 m below the surface.
Drillholes C and D do not intersect the conductor and
produce off-hole responses. The A-component has a broad
peak response with polarity reversal (change of field
direction) producing cross-overs defining its distant flanks.
The U-component is a single cross-over response. The
responses are symmetrical and coincident, and coincide
with the plane of the horizontal conductor. They are
weaker and broader in the more distant drillhole
D. Drillholes A and B intersect the conductor and produce
in-hole responses, indicated by the opposite polarity of
their A-component responses (at early times in drillhole B)
compared with those of the off-hole responses in
drillholes C and D. Drillhole B intersects the conductor
near its outer edge, the zone of eddy current
flow in the
earlier stage of its inward migration. The A-component
also shows a narrow peak of reversed polarity at mid to
late delay times indicating that the main path of the eddy
current is migrating inward through the drillhole inter-
section at these times. The peak is absent from the
profile of the inner drillhole A indicating that the eddy
current does not pass through this intersection during
the period of the computed decays. As for the off-target
drillholes, the U-component in drillholes A and B show
a cross-over coincident with the A-component peak and
the plane of the conductor. A dipping conductor
produces asymmetry in all the component responses
analogous to those of a
response. The diminishing overburden response with
depth means it produces less distortion of the signal from
deeper conductors. Identifying shallow targets in the pres-
ence of conductive overburden may be dif
cult.
Surveying drillholes on the opposite edge of the body
reverses the polarity of the horizontal component, and
for drillholes located on the adjacent edges the U and
V-components interchange their responses. Relocating
the transmitter loop can change the direction of the eddy
current, and in thick conductors the orientation of the
current system. This changes the strength of the responses
in the three components and can cause them to change
from peaks to cross-overs, and vice versa.
5.8.2.2
Modelling responses
DHEM forward modelling software allows the user to
interactively create and manipulate conductors in 3D
space, as shown in
Fig. 5.94
, and is essential for working
with the complexity of data from multiple loops and mul-
tiple drillholes with different orientations. For example,
when there are several transmitter loops located so that
their primary
fields intersect the conductor in opposite
directions, the induced currents will
flow in opposite dir-
ections so the observed anomalies will have opposite
polarity.
plate-like conductor is restricted to a single plane, so
changing the loop location and coupling will only change
the strength and rotational direction of the induced cur-
rents and not the path or shape of the current flow. The
DHEM response for the various loops will, therefore, only
change in amplitude and sign and not shape. In contrast,
currents induced in a non-tabular or 3D conductor initially
flow perpendicular to the energising field at the conductor,
but with time they are free to flow in any orientation in the
body. For this class of conductors the DHEM anomalies
vary in shape, amplitude and sign with change in transmit-
ter loop location.
The
fixed-loop surface survey as
The response of a conductive surface layer decreases
with depth with a polarity reversal occurring in the
U-component as the probe passes out of the conductive
layer. Similar to the surface
fixed-loop response in
early-time target response in both components in the dis-
tant drillholes C and D, and broadens the responses with
cross-overs occurring at different depths at different delay
times. In contrast, the in-hole target responses in drillholes
A and B are less affected because they are much stronger in
the vicinity of the conductor. At shallower depths the
overburden responses overwhelm the target responses,
causing a reversal
current-
lament model (see Modelling EM
data in
Section 5.7.5.3
)
is useful for locating the centre of
the eddy current system in the conductor and can provide
an indication of dip. When the receiver is close to the
conductor (i.e. the anomaly is an in-hole response or a
near-miss target), tabular body or plate-like models, or
sphere model where appropriate, often provide better
de
nition of the target than do simple current-
lament
approximations. Inverse modelling methods (see
Section
'
simple
'
in polarity of
the U-component
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