Geology Reference
In-Depth Information
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Mafic volcanic rocks
Mesozoic sediments
Greywacke
Granite
Amphibolite
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Fig. 9.17
INPUT
®
profile across part of
the Itapicuru Greenstone Belt, Brazil.
(After Palacky 1981.)
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INPUT
®
is more expensive than other airborne EM
methods but provides greater depth penetration, possi-
bly in excess of 100 m, because the secondary signal can
be monitored more accurately in the absence of the
primary field. It also provides a direct indication of
the type of conductor present from the duration of the
induced secondary field.
As well as being employed in the location of conduct-
ing ore bodies, airborne EM surveys can also be used as
an aid to geological mapping. In humid and subtropical
areas a weathered surface layer develops whose thickness
and conductivity depend upon the local rock type.
Figure 9.17 shows an INPUT
®
profile across part of the
Itapicuru Greenstone Belt in Brazil, with sampling times
increasing from 0.3 ms at channel 1 to 2.1 ms at channel
6. The transient response over mafic volcanic rocks and
Mesozoic sediments is developed in all six channels, in-
dicating that their weathered layer is highly conductive,
while the response over greywacke is only apparent in
channels 1-4, indicating a comparatively less conductive
layer.
EM methods are being used increasingly in hydro-
geological studies as they are more efficient than the re-
sistivity methods classically used for this purpose.A series
of case histories of the use of EM methods in ground-
water studies is given in McNeill (1991).
nominally-horizontal receiver is towed behind the
aircraft on a cable some 150 m long.
In quadrature systems the orientation and height of
the receiver cannot be rigorously controlled as the
receiver 'bird' oscillates in the slipstream. Consequently,
the measurement of real and imaginary components is
not possible as the strength of the field varies irregularly
with movement of the receiver coil. However, the phase
difference between the primary field and the resultant
field caused by a conductor is independent of variation
in the receiver orientation. A disadvantage of the
method is that a given phase shift
f
¢ may be caused by
either a good or a poor conductor (Fig. 9.18).This prob-
lem is overcome by measuring the phase shift at two
different primary frequencies, usually of the order of
400 and 2300 Hz. It can be shown that, if the ratio of
low-frequency to high-frequency response exceeds
unity, a good conductor is present.
Figure 9.19 shows a contour map of real component
anomalies (in ppm of the primary field) over the
Skellefteå orefield, northern Sweden. A fixed separation
system was used, with vertical, coplanar coils mounted
perpendicular to the flight direction on the wingtips of a
small aircraft. Only contours above the noise level of
some 100 ppm are presented. The pair of continuous
anomaly belts in the southwest, with amplitudes exceed-
ing 1000 ppm, corresponds to graphitic shales, which
serve as guiding horizons in this orefield.The belt to the
north of these is not continuous, and although in part re-
lated to sulphide ores, also results from a power cable. In
the northern part of the area the three distinct anomaly
centres all correspond to strong sulphide mineralization.
9.8.2 Quadrature systems
Quadrature systems were the first airborne EM methods
devised.The transmitter is usually a large aerial slung be-
tween the tail and wingtips of a fixed-wing aircraft and a
