Geology Reference
In-Depth Information
Figure 11.1
Variation of V
p
/
V
s
ratios with Poisson ratio.
the rocks can be ripped apart by a spike mounted on the back of a bulldozer.
S-waves are now being used in some shallow reflection surveys, where their
slower velocities provide improved resolution (see Section 12.2.7).
11.1.3 Velocities and the time-average equation
Within quite broad limits, the seismic velocity in a mixture of different
materials can be calculated by averaging the transit times (the reciprocals of
velocities) through the pure constituents, weighted according to the relative
amounts present. The principle can be used even when, as in Example 11.1,
one of the constituents is a liquid.
Example 11.1 Seismic velocities in mixtures
The velocity of the P-wave in a sandstone that is 80% quartz and 20%
water-filled porosity can be calculated from:
V
p
(quartz)
=
5200 m s
-1
V
p
(water)
=
1500 m s
-1
/
V
p
=
/
+
/
=
1
0.000 287
i.e.
V
p
(sandstone) is approximately 3500 m s
-1
.
0.8
5200
0.2
1500
