Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 390

data for sulphide mineralisation occurring in the area
between the common ore and gangue minerals. In most
cases, massive sulphide mineralisation has higher velocities
and higher densities than its host rocks; but velocities may
be lower if sphalerite, chalcopyrite or pyrrhotite constitute
a signi cant proportion of the mineralisation. The con-
tours of acoustic impedance shown in Fig. 6.35b indicate
that in some cases there may be no signi cant difference in
acoustic impedance between mineralization and host
rocks. For example, most types of sulphide mineralisation
can be detected within a felsic host, but not necessarily all
of those hosted in mafic and ultramafic rocks, although
serpentinisation would greatly enhance the physical-
property contrast between mineralisation and host rocks
(see Section 6.6.3.1 ) . Importantly, the relative pyrite con-
tent of a sulphide-bearing mineral deposit is a key factor
controlling the physical-property contrast between the
deposit and its host rocks, owing to the high velocity of
pyrite. For the same reason, layers rich in chromite within
large ma c intrusions have been targeted using seismic
surveys, e.g. Gibson ( 2011 ) .
Coal has very low velocity and low density compared
with the sedimentary rocks within which it occurs, produ-
cing a large contrast in acoustic impedance with the sur-
rounding succession. There is a continuum of variation
from lignite, through bituminous coals to anthracite,
mostly due to an increase in velocity.
Evaporite minerals such as halite and the potassium-
bearing minerals sylvite and carnallite have high velocities
compared with their densities. As a result, evaporite units
often have a significant acoustic impedance contrast with
their host rocks. However, polyhalite and anhydrite have
seismic properties similar to other common sedimentary
minerals, so units containing these minerals may not be
seismically anomalous.

a)

9000

Chromite

Olivine

Garnet

Pyrite

8000

Dolomite

Pyroxene

Magnetite

7000 Amphibole

Calcite

Plagioclase feldspar

Haematite

Quartz

6000

Alkali feldspar

Anhydrite

Sphalerite

Mica

Chalcopyrite

Polyhalite

5000

Pyrrhotite

Barite Pentlandite

Halite

Bornite

Sylvite

4000

Galena

Carnallite

Ice

Graphite

3000

Anthracite
Bituminous
coal

Coal
Evaporite
Metallic mineralisation
Other rock types
Rock-forming minerals
Economic minerals
Pore contents

2000

Lignite

Saline water

Fresh water

1000

Air

0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Density (g/cm 3 )

b)

9000

Chromite

Ultramafic

Sulphide
mineralisation
Economic
minerals

Pyrite

8000

Magnetite

Mafic

7000

Haematite

Felsic

6000

Sphalerite

Chalcopyrite

5000

Pyrrhotite

Pentlandite

Sedimentary

Bornite

4000

Galena

6.6.5 Seismic properties of near-surface
environments

3000

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Density (g/cm 3 )

Weathering usually produces a signi cant increase in por-
osity, so seismic velocity and density are progressively
reduced in more weathered rocks. This is illustrated in
Fig. 6.36 with data from Ishikawa et al. ( 1981 ) . Here

Figure 6.35 Seismic velocity versus density for a wide variety
of differnt types of mineralisation. (a) Integrated with the data
from Fig. 6.28 to show the variation in the parameters for
mineralisation compared with those of common rocks, and (b)
the data for sulphide mineralisation integrated with data for
various rock types. Sulphide data from Adam et al.( 1997 ) , Duff
et al.( 2012 ), Fullagar et al.( 1996 ), Greenhalgh and Mason ( 1997 )
and Heinonen et al.( 2012 ) and unpublished data supplied to the
authors. Coal data from Greenhalgh and Emerson ( 1986 ).
Evaporite data from Wheildon et al.( 1974 ) and Kern ( 1982 ).

The red line is the average crustal density of 2.67 g/cm 3 . The broken
lines are contours of acoustic impedance with their separation
representing the contrast required to produce a re ection

coef cient of 0.05.