Geoscience Reference
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with their host rocks, with the pyrite content of metallic
mineralisation being an important factor.
10
3
Increasing
attenuation
Igneous & metamorphic
Faults and fault rocks may be associated with seismic-
property contrasts.
10
2
Complex variations in seismic properties are to be
expected in the near-surface owing to the effects of
varying amounts of clay minerals and,
Sedimentary
in particular,
10
1
variations in porosity.
0.001
0.01
0.1
1.0
Attenuation is less in crystalline rocks.
Fractional porosity (
f
)
It is essential to interpret seismic data in the context of
these observations, and when making comparisons with
geological data to acknowledge that lithology has only a
partial control on seismic responses.
Figure 6.38
Quality factor versus fractional porosity for crystalline
and sedimentary rocks. Redrawn, with permission, from Bradley and
due to different methods of measurement and state of the
samples, e.g. saturated versus unsaturated. As is to be
expected, given the role of friction (see
Section 6.3.3.2
),
absorption is greatest in materials where adjacent surfaces
can mechanically move in relation to each other. Conse-
quently, absorption is greatest in porous, especially frac-
tured, and poorly cemented rocks and near-surface
materials where con
ning pressure has not closed voids.
Cracks eventually close as pressure increases with depth
(see
Section 6.6.2
)
, so they are then unable to contribute to
frictional loss, leading to a reduction in attenuation. The data
also show that absorption is much less in crystalline rocks.
This is due to their lower porosity and also probably due to
the greater rigidity of their interlocked mineral structure.
6.6.9
Measuring seismic properties
The seismic velocity of a rock sample can be accurately
measured in the laboratory. However, the results may not
be representative of the rock in situ because factors such as
pore
fluid and its pressure strongly in
uence velocity; and
these factors are hard to reproduce accurately in the
laboratory. Also, measurements on small samples require
the use of much higher-frequency waves than those used
that are too small to affect actual seismic survey data can
significantly influence the laboratory measurements.
Downhole measurement of velocity in situ is known as
sonic logging and produces a continuous record of velocity
as a function of depth. Sonic logs provide more reliable
information than do laboratory measurements on samples,
although the frequencies used are still significantly higher
possibility that the physical properties of the drillhole wall
rocks may have been altered by the drilling, so the rock
volume in
6.6.8
Summary of geological controls
on seismic properties
The important conclusions to be drawn from the above
descriptions of the geological controls on seismic velocity
and density are:
uencing the measurements may no longer be
entirely representative of the rock formation.
The simpler form of sonic logging involves measuring
the travel time of P-waves from one or more sources to
pairs of detectors, all mounted on the logging tool
(
Fig. 6.39
). The distance between the detectors is known
as the span and represents the limit of vertical resolution of
the data. The sources
When porosity is small, seismic properties are mainly
controlled by mineralogy/lithology.
When porosity exceeds about 10% it becomes the dom-
inant control on seismic properties.
In porous rocks, the type and state of the pore contents
are important controls on seismic properties. Important
factors include differential pressure and whether the
pore contents are liquid, gaseous or solid.
alternately as the tool is raised
up the drillhole and the differences in arrival times of the
waves at the detectors are determined. Data may be dis-
played as either a velocity or slowness (1/velocity) with the
result assigned to the depth of the centre of the span.
Velocities are displayed as metres per second or kilometres
'
re
'
Serpentinisation signi
cantly reduces the velocity and
density of ma
c and ultrama
c rocks.
Most kinds of metallic and sedimentary mineralisation
(coal, evaporites) have seismic properties that contrast
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