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response), dominate the whole pro
le (E). Note how this
response is depressed at early times in the central part of
the pro
le (F) and most likely related to the more resistive
silici
ed zone occupying the conductive weathered zone.
The Eloise SEDEX deposit comprises massive and
stockwork copper
receiver located within a drillhole. Modern DHEM systems
are time domain (TD-DHEM) and use a large transmitter
loop located on the ground surface with measurements
made with the downhole receiver, referred to as the
probe, at various distances along the drillhole trajectory.
The purpose of DHEM surveying is to detect and delineate
electrically conductive targets in the rocks surrounding the
drillhole. This form of downhole electromagnetics is dif-
ferent to downhole electromagnetic logging, known as
induction logging (see
Section 5.8.4
)
, where both the trans-
mitter and receiver coils are located in the logging tool, the
sonde. Instead, the intention here is to measure the elec-
trical conductivity of the drillhole wall rocks in situ and
produce a downhole log of conductivity.
In DHEM the receiver is below the Earth
-
gold mineralisation in the form of a
dipping tabular body. It occurs in Proterozoic rocks of
the Eastern Fold Belt in the Mount Isa Inlier near Clon-
curry, Queensland, Australia, and lies beneath 50
70 m of
conductive cover sediments. Despite the conductive over-
burden, the mineralisation is detectable with surface EM
surveys, of which moving in-loop surveys were instrumen-
tal in its discovery (Brescianini et al.,
1992
). The dB/dt data
from a subsequent fixed-loop survey (
Fig. 5.89c
) show the
late-time cross-over response (A) of the steeply dipping
mineralisation. The response of the conductive overburden
(B) is prominent at earlier delay times along the whole
pro
-
is surface so it
is partially shielded from EM noise (see
Section 5.4.2
), so
the signal-to-noise ratio is usually greater than that obtain-
able with surface and airborne measurements. Also, the
interfering effects of near-surface conductors, such as con-
ductive overburden (see
Section 5.3.4
, and see Late-time
measurements in
Section 5.7.2.3
), are reduced. These
advantages, combined with the fact that the receiver is in
closer proximity to deep target conductors than is possible
for surface and airborne EM systems, mean that DHEM
surveys have greater detection capability. Furthermore,
they provide greater spatial resolution of closely spaced
conductors which may appear as a single feature in surface
and airborne data. Both the impulse and step responses can
be measured in TD-DHEM (see
Section 5.7.1.7
)
, with
dB/dt (coil) and B-field (magnetometer) probes, respect-
ively; the latter is a comparatively recent development.
DHEM is able to determine the location and orientation
of a conductor, whether a conductive intersection is part of
a large conductive body, and the off-hole extent of the
conductor. These are very important bene
'
le and offsets the amplitudes of the target response at
those times. Note how the overburden (and half-space)
response changes polarity (sign) when the expanding sub-
surface current system (see
Section 5.7.2.2
) passes below
the survey station, occurring at later delay times at stations
more distant from the loop. Negative values, less than
1,
are not plotted in
Fig. 5.89c
. The geological section has
been projected by 300 m to the geophysical pro
le shown.
dB/dt (coil sensor) data from the Tripod massive nickel
sulphide deposit located in the Proterozoic Cape Smith
volcano-sedimentary sequence near Raglan, Quebec,
Canada. This is a highly conductive target resulting in slow
decay of the induced eddy currents and requiring the
measurements to be made at very low base frequencies of
5 and 1.67 Hz (see
Section 5.7.3.1
). The data are from a
moving in-loop survey and, as expected, the anomalous B-
-
field (step) response (A) persists to the latest delay time
whilst the associated dB/dt (impulse) response has dimin-
dip of the source. This is a resistive terrain without highly
conductive overburden so there are no conductive-
overburden and half-space responses present to distort
the target response, these distortions being typical of con-
ductive terrains like those shown in
Figs. 5.89a
,
b
and
c
.
ts of DHEM
that aid the targeting of subsequent drilling.
DHEM is one of the most important geophysical tools in
the exploration for, and mining of, conductive massive
sulphide mineralisation, especially deep nickel sulphide
prehensive description of DHEM.
5.8.1
Acquisition of DHEM data
In most DHEM systems the transmitter loop is located
obtain a more favourable orientation for coupling to a
particular target, the loop can be located or on the slopes
5.8
Downhole electromagnetic surveying
The term downhole (or drillhole) electromagnetics
(DHEM) is used here to describe EM systems with a
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