Geology Reference
In-Depth Information
Figure 12.9
MicroVib shear-wave (SH) reflection source.
12.2.7 Use of S-waves
As discussed in Section 11.1.2, S-waves (shear waves) travel more slowly
than P-waves. In many materials the velocity ratio is about 1:2. This implies
that the wavelength of an S-wave will be significantly less than that of the
equivalent P-wave, and that higher resolution can be achieved using S-waves.
This can be very useful in shallow reflection surveys. Also, S-waves are not
as easily generated as P-waves, and are present to only a minor extent in
traffic noise, making them ideally suited for surveys in urban environments.
Another advantage of S-waves in some surveys is that they do not propagate
in water. They therefore do not change velocity at the water table (i.e.
V
water
would be zero for S-waves in Example 11.1), and no S-wave reflections are
produced there. This can be useful because the water table may be hard to
penetrate using P-waves.
S-waves can be crudely generated by banging a spike into the ground and
hitting it from the side with a hammer. This is rarely effective. The S-wave
source shown in Figure 12.9 uses a swept frequency from 30 to 350 Hz over
a period of 6 s. On soft ground, spikes are attached to the base of the vibrator
to couple it to the ground. On tarmac and other hard surfaces, the spikes are
removed and the vibrator is coupled through an array of studs protruding
from its base. The coupling is further improved if a member of the survey
team stands on it.
