Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 384

depths of less than about 1 km. At these depths, variations
due to open cracks may mask the in uence of other factors
that control seismic properties (see Section 6.6.2 ) , but this
complication re ects the actual situation with mining-
oriented seismic surveys.

rocks re ecting the importance of porosity as a control
on seismic properties. Since seismic properties are not
controlled only by mineralogy, they are not diagnostic of
lithology. A one-to-one correlation between lithologically
and seismically de ned structure/stratigraphy should not
be expected, as discussed in Section 1.1 .

6.6.1 Seismic properties of common rock types

6.6.1.1 Effects of chemistry and mineralogy
Figure 6.29a shows variations in velocity versus density for
igneous rocks, differentiating felsic, intermediate, mafic and
ultramafic types. There is a rough correlation between how
silicic the rocks are and their seismic properties. Felsic rocks
exhibit velocities and densities that are distinctly lower than
those of ultramafic rocks, etc. The former have properties
similar to those of quartz and feldspar. The latter group
exhibit higher velocities and densities consistent with the
greater abundance of minerals having relatively high vel-
ocity and density in their assemblages, such as olivine,
pyroxene and amphibole. The rocks whose chemistry lies
between these two extremes have intermediate velocities
and densities. This compositional dependency of seismic
properties allows igneous or metamorphic layering to
produce seismic responses, for example in large ma c
intrusions (Adams et al., 1997 ). Note that the ma c and
ultrama c rocks show greater scatter than the other types,
with the data extending into the region beyond the lower
limits of the velocities and densities of their major constitu-
ent minerals. Serpentinisation is most likely to be the cause
for this (see Section 6.6.3.1 ) .
Metamorphic rocks behave much like igneous rocks.
Those of mafic and ultramafic composition have the
highest seismic velocities and densities, whilst metasedi-
mentary rocks, such as marble and quartzite, exhibit the
lower velocities of their constituent quartz/dolomite/calcite
crystals.
Figure 6.29b shows velocity versus density for sediment-
ary rocks and unconsolidated sediments. The distinction
between lithotypes is less obvious than for igneous rocks,
although in general carbonates have higher velocities than
siliceous rocks, followed by mudrocks and then the uncon-
solidated materials.

Figure 6.28 comprises a series of plots of seismic velocity
versus density. Contours showing variations in acoustic
impedance are also shown, their interval representing the
contrast required to produce a reflection coefficient of 0.05
(see Energy partitioning in Section 6.3.4.2 ) . This is roughly
the magnitude required to produce a recognisable reflec-
tion under circumstances. Data for the main rock-

forming minerals and the economically signi

' normal '

cant min-
erals (average values are shown, given the possible ranges
in mineral compositions and the fact that velocity varies
with direction in most crystals) and common pore con-
tents are also included.
From Fig. 6.28a it is immediately clear that P-wave
velocity is approximately proportional to density, but there
is considerable scatter. For example, materials with the
' crustal density of 2.67 g/cm 3 (see Section 3.8 )

may have velocities between roughly 3000 and 6000 m/s,
which is nearly half the overall range.
Referring to Fig. 6.28b to d , the velocity versus density
data for the main rock classes de ne a continuous distri-
bution with an overall trend from unconsolidated mater-
ials, exhibiting the lowest velocities and densities, through
sedimentary rocks to crystalline rocks, which exhibit the
highest values. The lower end of the continuum
approaches the properties of water, the most common pore
fluid. At the high end of the continuum, the data plot in the
vicinity of the values for individual rock-forming minerals.
As demonstrated below, this occurs because when porosity
is small mineralogy is the dominant in

average '

uence on velocity
and density, and as porosity increases the pore uid
becomes an important in uence. Consequently, data from
sedimentary rocks, which normally have signi cant poros-
ity, form a continuum from the properties of water to the
properties of their most common matrix minerals, namely
quartz, feldspar and calcite. The data from igneous and
metamorphic rocks, which normally have much lower
porosity, occupy the region bounded by the properties of
their most common mineral constituents, such as quartz,
feldspar, pyroxenes, amphiboles etc. The range in velocity
and density is much lower than is seen in sedimentary

6.6.1.2 Effects of porosity and pore contents
In Fig. 6.29c the sediments and sedimentary rocks in the
database are grouped according to their fractional porosity
(

ϕ

). Comparison with Fig. 6.29b clearly shows that for
rocks with signi cant porosity the main control on seismic
properties is their porosity and not

their mineralogy.