Seismic method - Geophysics for the Mineral Exploration Geoscientist - page 387

6000

Velocity (m/s)

Sandstone specimens

3 0 00 3500 4000 4500 5000 5500 60 0 0

0.0 0

Highest
differential
pressure

5000

Frozen

17 C

Water saturated (saline)

10

Water saturated (fresh)

4000

0.5

100C Intermediate

differential
pressure

Methane/air-filled

Ice

20

3000

1.0

Granite

Sandstone

30

2000

Lowest
differential
pressure

Saline water

Constant
differential
pressure

Increasing
differential
pressure

1.5 40

Fresh water

1000

Methane

Air 50

0

2.0

0.0 0.5 1.0 1.5 2.0 2.5

Depth
(km)

Confining
pressure
(MPa)

Density (g/cm 3 )

Figure 6.31 Seismic velocity versus density for various pore contents
and a clean sandstone containing the different pore contents.
Fractional porosity of the samples is about 0.2. Data from Timur
( 1968 , 1977 ) and Wyllie et al.( 1962 ). The broken lines are contours
of acoustic impedance with their separation representing the
contrast required to produce a re

Figure 6.32 Effects of absolute and differential pressure on the
seismic velocity of a sandstone and a granite. Based on diagrams in
King ( 1966 ) and Todd and Simmons ( 1972 ) .

ection coef cient of 0.05.

pressure. Equivalent depth, assuming lithostatic loading,
is also shown. Both rock types exhibit signi cant increase
in velocity with depth, at least to the depths of interest
here, although the rate of increase tends to decline signifi- -
cantly at depths of a few kilometres. The rapid change at
shallow depth is due to the closure of crack-related poros-
ity. Equivalent variations in density are shown in Fig. 3.33 .
The primary cause for the increases in velocity and density
is the reduction in porosity as confining pressure increases.
For crystalline rocks, their generally low porosity tends to
be in the form of cracks, which easily collapse under
pressure. In granular sedimentary rocks, porosity is not
only much greater but also tends to be more stable. All
rock types show this general behaviour so their seismic
property contrasts tend to be preserved as the rocks are
buried.
In rocks with signi cant porosity, the dominant control
on velocity is the differential pressure, i.e. the difference
between the con ning pressure and the pore pressure.
When the differential pressure is constant, velocity also
tends to be fairly constant as is illustrated for the sandstone
in Fig. 6.32 . In this case, a minor decrease in velocity
occurs. When differential pressure decreases, i.e. the pore
pressure increases relative to the con ning pressure, vel-
ocity normally decreases. Consequently, uid-lled areas
with anomalous pressure, such as over-pressured zones,
can give rise to seismic responses.

a signi cant acoustic impedance contrast with adjacent
non-gas-bearing units. Above the water table the pore uid
is most likely to be air, although in cold climates ice,
associated with permafrost, is an alternate non- uid possi-
bility. Velocity versus density data for common pore con-
tents, and for a porous sandstone (the Berea Sandstone from
Ohio, USA) in the presence of the various pore contents, are
shown in Fig. 6.31 . Note the very low velocities and densities
of air and methane. Clearly, there are significant contrasts in
seismic properties between water-filled (saturated) and gas-
filled (dry) rock, but very little difference between specimens
containing saline or fresh water. This demonstrates that a
significant subsurface seismic property boundary may be
associated with the water table.

6.6.2 Effects of temperature and pressure

Figure 6.31 shows that the effects of temperature on a
saturated rock are small until the pore fluid freezes, and
then velocity increases signi cantly. This has important
implications for seismic surveys conducted in permafrost
areas, as changes in velocity of geological origin may be
exceeded by those related to the nature of the pore

contents.
Figure 6.32 shows how the seismic velocities of a granite
and a porous sandstone change with increasing confining