Geoscience Reference
In-Depth Information
7000
Velocity
(km/s)
Density
(g/cm
3
)
Fractional
porosity (
)
Depth
(m)
f
Plagioclase feldspar
0.0
4.0
8.0
1.5 2.5 3.5
0.1
0.3
0.5
Least
weathered
0
Colluvial
hardpan
Lateritic
residuum
Ferruginous
saprolite
(mottled zone)
Quartz
6000
Alkali feldspar
20
5000
Fresh
granite
40
Most
weathered
Rock-forming
minerals
4000
Saprolite
60
3000
Saprock
80
Bedrock
2000
100
Kaolinite
Unconsolidated
siliceous sediments
Figure 6.37
Variations in seismic velocity, density and fractional
porosity through the regolith in a greenstone terrain in Western
Australia. The horizontal bars represent the range in values and the
lines are the mean values. Based on diagrams in Emerson et al
(
2000
).
1000
1.0
1.5
2.0
2.5
3.0
Density (g/cm
3
)
Figure 6.36
Seismic velocity versus density from progressively more
weathered granite. Weathered materials data from Ishikawa et al.
broken lines are contours of acoustic impedance with their
separation representing the contrast required to produce a reflection
coefficient of 0.05.
is a substantial reduction in water saturation, the velocity of
saprolitic layers may decrease by as much as 40 to 50%. The
depth pro
les demonstrate that the seismic properties of
the regolith are complex, with both increases and decreases
in properties to be expected. This will be further compli-
cated by lateral changes in the bedrock, variations in the
thickness of the various intra-regolith layers, perched water
tables and multiple weathering fronts. Importantly, there is
likely to be a signi
cant increase in seismic velocity at, or
near, the base of the regolith.
weathered granite is assigned to one of eight categories
representing the degree of weathering, based upon several
factors including hardness, cohesion and condition of con-
stituent minerals. The data form a continuum from fresh
granite through to siliceous unconsolidated materials. The
degree of weathering correlates very well with position
within the continuum, defining a progression from values
close to those of the relevant rock-forming minerals
towards those of common pore contents.
The high porosity of weathered materials means that the
degree of saturation is an important control on seismic
properties. The formation of low-velocity clay minerals
may also be important.
The laterites and thick clay-rich regolith that form under
tropical weathering conditions are another weathering-
related phenomenon of importance. Emerson et al.(
2000
)
provide a detailed description of the physical characteristics
of the regolith in greenstone terrain near Lawlers, Western
Australia.
Figure 6.37
shows the variation in velocity, dens-
ity and porosity through the regolith and into underlying
bedrock. A surprising result is the high porosities, even in
clay-rich saprolite materials. As expected, velocity and
density vary approximately in sympathy, both being
inversely correlated with porosity. Again, the high porosity
suggests that the degree of water saturation will be a key
6.6.6
Anisotropy
Most rock formations are seismically anisotropic, exhibit-
ing the lowest seismic velocity in the direction perpen-
dicular to any planar fabric and the maximum velocity
usually parallel to any linear fabric. Velocity anisotropy
may arise from small-scale interbedding of different litho-
types, mineralogical layering or preferred orientation/
alignment of mineral grains with intrinsic single-crystal
seismic anisotropy. There is evidence that variations in the
intensity and orientation of planar fabrics can create suffi-
cient changes in seismic properties to affect the seismic
response.
6.6.7
Absorption
Figure 6.38
shows absorption as a function of fractional
porosity (
ϕ
) for crystalline and sedimentary rocks. The
data are for a range of frequencies, and the scatter is partly
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