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a)
7000
9000
V
P
-
calcite
V
P
Olivine
-
quartz
6000
8000
Argillaceous rocks
Carbonate rocks
Siliceous rocks
Unconsolidated (argillaceous)
Unconsolidated (siliceous)
5000
Pyroxene
7000
Amphibole
Plagioclase feldspar
4000
Quartz
6000
Alkali feldspar
3000
5000
Felsic
Intermediate
Mafic
Ultramafic
Rock-forming minerals
2000
V
P
-
water
4000
1000
0.0
0.2
0.4
0.6
0.8
1.0
Fractional porosity (
f
)
3000
2.0
2.5
3.0
3.5
4.0
Figure 6.30
Seismic velocity versus fractional porosity for various
sedimentary rocks and unconsolidated sediments.
Density (g/cm
3
)
b)
7000
Calcite
Plagioclase feldspar
Figure 6.30
shows velocity versus porosity data for sedi-
mentary rocks and unconsolidated materials. The relation-
ship is non-linear and there is much scatter in the data,
shows a straightforward linear dependence with porosity.
Various equations have been empirically derived to rep-
resent the relationship between velocity and porosity, with
some of these accounting for the effects of grain size,
carbonate content of siliciclastics, clay content, pressure
and pore contents. See for example De Hua et al.(
1986
)
relating velocity and porosity is unlikely to be universally
applicable. What is important is that seismic responses at
porosity contrasts, which may or may not coincide with
lithological contacts, are to be expected.
The magnitude of the effect of porosity on seismic
properties is dependent on the material occupying the pore
space, which is nearly always some kind of
fluid. This is
most likely to be water (fresh or saline), or (less probably)
liquid or gaseous hydrocarbons. The presence of gas
(methane), for example in coal-bearing strata, will mark-
edly reduce the seismic velocity of a porous rock, creating
Quartz
6000
Argillaceous
Carbonates
Siliceous clastics
Unconsolidated
Rock-forming minerals
Alkali feldspar
5000
4000
3000
2000
Kaolinite
1000
1.0
1.5
2.0
2.5
3.0
Density (g/cm
3
)
c)
7000
f
6000
0.0 - 0.1
0.1 - 0.2
0.2 - 0.3
0.3 - 0.4
0.4 - 0.5
> 0.5
Pore water
5000
4000
3000
2000
selected minerals, (b) for unconsolidated sediments and sedimentary
rocks, and (c) the data in (b) grouped according to fractional
porosity (
Saline water
Fresh water
). Data for selected minerals and pore contents are also
shown. 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 reflection
coefficient of 0.05.
ϕ
1000
1.0
1.5
2.0
2.5
3.0
Density (g/cm
3
)
Figure 6.29
Subsets showing detail of the seismic velocity versus
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