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a)
Helicopter
time-domain AEM
Fixed-wing
time-domain AEM
Gravity
Ground
magnetics
3.6 km
2
IP/resistivity
10 km
2
20 km
2
20 km
2
4 km
2
Fixed-wing
aeromagnetics/radiometrics
Drillhole
(too small to show to scale)
Airborne
gravity gradiometry
50 km
2
160 km
2
160 km
2
of fixed-wing aeromagnetics with radiometrics (100 m line spacing).
50 km
2
of airborne gravity gradiometry (100 m line spacing).
20 km
2
of fixed-wing TDEM with magnetics and radiometrics (100 m line spacing).
20 km
2
of helicopter TDEM with magnetics and radiometrics (100 m line spacing).
10 km
2
of differential GPS-controlled ground magnetics (50 m line spacing, 1 m stn spacing).
4 km
2
of gradient array IP/resistivity (100 m line spacing, 50 m dipoles).
3.6 km
2
ground gravity stations (differential GPS-controlled, 100 m grid).
b)
Drillhole (too small to show to scale)
Fixed-loop TDEM
CSAMT
IP/resistivity
TDEM soundings
Shallow seismic
25 km
Figure 1.2
Approximate relative costs of different
kinds of geophysical surveys in terms of (a) area and
(b) line-length surveyed for the cost of a single
drillhole. AEM
10 km
8-10 km
6 km
2 km
-
airborne electromagnetics, CSAMT
-
controlled source audio-frequency magnetotellurics,
EM
25 line km of fixed-loop TDEM profiles.
10 line km of 50 m dipole 12-frequency CSAMT sections.
8-10 line km of dipole-dipole IP/resistivity (50 to 100 m dipoles).
6 km coincident-loop TDEM soundings (100 m stn spacing).
2 line km of detailed shallow seismic data.
-
electromagnetics, IP
-
favourable economies-of-scale. Additional costs can be
incurred through
the approximate relative cost of different geophysical
methods. Of course the
figures on which these diagrams
are based can be highly variable owing to such factors as
the prevailing economic conditions and whether the
surveys are in remote and rugged areas. They should
be treated as indicative only. The seismic method is by
far the most expensive, which is one reason why it is
little used by the mining industry, the least expensive
methods being airborne magnetics and radiometrics. The
areasoverwhichinforma ionisgatheredforeach
method are compared in
Fig. 1.3
, noting that cost
related to factors such as
adverse weather and access restrictions to the survey area,
all preventing progress of the survey. Local conditions are
widely variable, so it is impossible to state here the costs of
different kinds of geophysical surveys. Nevertheless, it is
useful to have an appreciation for the approximate relative
costs of various geophysical methods compared with the
cost of drilling. Drilling is not only a major, and often the
largest, cost in most exploration and mining programmes,
it is often the only alternative to geophysics for investi-
gating the subsurface.
'
lost time
'
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