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Velocity (m/ns)
a)
b)
c)
0.130
0.067
0.047
0.039
0.034
0.030
Resistivity ( m)
Resistivity ( m)
Resistivity ( m)
1 0 0
10 1
10 2
10 3
10 4
10 0
10 1
10 2
10 3
10 4
10 0
10 1
10 2
10 3
10 4
Dielectric constant (
)
k
0
0
20
40
60
80
100
Regolith
Regolith
Laterite
20
Quartz
K-feldspar
Plagioclase fsp
Regolith
40
Amphibole
Basement
Rock-forming
minerals
Basement
Pyroxene
60
Olivine
Mica
Calcite
80
Dolomite
Basement
Galena
100
Arsenopyrite
Sphalerite
Cassiterite
Chromite
Haematite
Ore
minerals
(metallic)
Figure 5.23 The variation of electrical resistivity vertically through
regolith. (a) Western Australia (based on a diagram in Emerson et al.
( 2000 )), (b) Burundi (based on a diagram in Peric ( 1981 )), (c) Goias,
Brazil (based on a diagram in Palacky and Kadekaru ( 1979 )).
(a) Based on petrophysical measurements, (b) and (c) based on
resistivity soundings.
Magnetite
Coal
Ore
minerals
(sedimentary)
Sylvite
Zircon
Monazite
Barite
Air
Pore
contents
electrical measurements difficult owing to difficulties in
establishing current flow through the highly resistive
ground, but this is not a problem for inductive EM meas-
urements. These conditions occur in areas of permafrost
and ferricrete.
Fresh water
Saline water
Ice
Permafrost
Silt
Near-surface
materials
Clay
Sand (dry)
Sand (saturated)
Shale
Sandstone
Common
rock types
5.3.4.1 Regolith
Much progress has been made in terms of understanding,
classifying and utilising regolith in the exploration process
(Anand and Paine, 2002 ). A detailed discussion of different
types of regolith and their geophysical characteristics is
beyond our scope, but profiles of resistivity/conductivity
versus depth through regolith in different areas do show
some characteristics in common. Figure 5.23 shows the
variation in resistivity with depth through several regolith
profiles, obtained using a variety of geophysical methods
and survey types. The most common feature is the resistive
surface soil and/or laterite, which are often indurated and
ferruginous (forming ferricrete). Underlying these is very
conductive saprolitic material which may be of consider-
able thickness, beneath which is less weathered, more
resistive bedrock that passes, rapidly or gradually, down into
comparatively unweathered higher-resistivity protolith. Clay
minerals in the saprolite contribute signi cantly to its higher
conductivity. The strong resistivity/conductivity contrasts
between neighbouring geological/weathering horizons,
which form distinct electrical layers, is a characteristic of
the regolith.
The pro les in Fig. 5.23 , although typical, are only
representative of a particular area. Factors responsible for
variations in the thickness and conductivity of the various
Limestone
Dolomite
Granite
Basalt
Figure 5.22 The ranges in dielectric constant and electromagnetic
wave velocity for some commonly occurring minerals, rocks and
near-surface materials measured at radar frequencies. Note the high
values for the different forms of water. Data compiled from various
published sources and velocities calculated from Eq. (5.28). Note the
non-linear velocity scale.
resistivity may be just a few ohm-metres, and less than
1
m in some places. Conductive overburden is the norm
where present-day or past tropical weathering conditions
have produced deeply weathered landscapes and created a
thick regolith. Also, saline groundwater has a signi cant
in uence on the conductivity of the near-surface. Both
features are particularly widely spread in Australia and
South America. This conductive zone can completely mask
the electrical responses of features beneath it. Furthermore,
variations in it over small areas produce responses that
superimpose geological noise on the responses of target
conductors, which can be easily mistaken for deeper con-
ductive bodies.
At the other end of the spectrum, highly resistive near-
surface conditions make the acquisition of good-quality
Ω
 
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