Geology Reference
In-Depth Information
and 5. If boreholes exist in the vicinity of a seismic
survey, it may be possible to correlate velocity values so
derived with individual rock units encountered within
borehole sequences. As discussed in Chapter 11, veloc-
ity may also be measured directly in boreholes using a
sonic probe, which emits high-frequency pulses and
measures the travel time of the pulses through a small
vertical interval of wall rock. Drawing the probe up
through the borehole yields a
sonic log
, or continuous
velocity log (CVL), which is a record of velocity varia-
tion through the borehole section (see Section 11.8, Fig.
11.14).
In the laboratory, velocities are determined by meas-
uring the travel-time of high-frequency (about 1 MHz)
acoustic pulses transmitted through cylindrical rock
specimens. By this means, the effect on velocity of vary-
ing temperature, confining pressure, pore fluid pressure
or composition may be quantitatively assessed. It is im-
portant to note that laboratory measurements at low
confining pressures are of doubtful validity.The intrinsic
velocity of a rock is not normally attained in the labora-
tory below a confining pressure of about 100 MPa
(megapascals), or 1 kbar, at which pressure the original
solid contact between grains characteristic of the pristine
rock is re-established.
The following empirical findings of velocity studies
are noteworthy:
1.
Compressional wave velocity increases with confin-
ing pressure (very rapidly over the first 100 MPa).
2.
Sandstone and shale velocities show a systematic
increase with depth of burial and with age, due to
the combined effects of progressive compaction and
cementation.
3.
For a wide range of sedimentary rocks the compres-
sional wave velocity is related to density, and well-
established velocity-density curves have been published
(Sheriff & Geldart 1983; see Section 6.9, Fig. 6.16).
Hence, the densities of inaccessible subsurface layers may
be predicted if their velocity is known from seismic
surveys.
4.
The presence of gas in sedimentary rocks reduces the
elastic moduli, Poisson's ratio and the
v
p
/
v
s
ratio.
v
p
/
v
s
ra-
tios greater than 2.0 are characteristic of unconsolidated
sand, whilst values less than 2.0 may indicate either a
consolidated sandstone or a gas-filled unconsolidated
sand. The potential value of
v
s
in detecting gas-filled
sediments accounts for the current interest in shear wave
seismic surveying.
Typical compressional wave velocity values and ranges
for a wide variety of Earth materials are given inTable 3.1.
Table 3.1
Compressional wave velocities in Earth materials.
v
p
(km s
-
1
)
Unconsolidated materials
Sand (dry)
0.2-1.0
Sand (water-saturated)
1.5-2.0
Clay
1.0-2.5
Glacial till (water-saturated)
1.5-2.5
Permafrost
3.5-4.0
Sedimentary rocks
Sandstones
2.0-6.0
Tertiary sandstone
2.0-2.5
Pennant sandstone (Carboniferous)
4.0-4.5
Cambrian quartzite
5.5-6.0
Limestones
2.0-6.0
Cretaceous chalk
2.0-2.5
Jurassic oolites and bioclastic limestones
3.0-4.0
Carboniferous limestone
5.0-5.5
Dolomites
2.5-6.5
Salt
4.5-5.0
Anhydrite
4.5-6.5
Gypsum
2.0-3.5
Igneous/Metamorphic rocks
Granite
5.5-6.0
Gabbro
6.5-7.0
Ultramafic rocks
7.5-8.5
Serpentinite
5.5-6.5
Pore fluids
Air
0.3
Water
1.4-1.5
Ice
3.4
Petroleum
1.3-1.4
Other materials
Steel
6.1
Iron
5.8
Aluminium
6.6
Concrete
3.6
3.5 Attenuation of seismic energy
along ray paths
As a seismic pulse propagates in a homogeneous ma-
terial, the original energy
E
transmitted outwards from
the source becomes distributed over a spherical shell, the
wavefront, of expanding radius. If the radius of the wave-
front is
r
, the amount of energy contained within a unit
area of the shell is
E
/4
p
r
2
.With increasing distance along
a ray path, the energy contained in the ray falls off as
r
-2
due to the effect of the
geometrical spreading
of the energy.
