Geology Reference
In-Depth Information
(a) P-wave
Compressions
Undisturbed medium
Dilatations
(b) S-wave
Fig. 3.3
Elastic deformations and ground
particle motions associated with the passage
of body waves. (a) P-wave. (b) S-wave.
(From Bolt 1982.)
independent of density and can be used to derive Pois-
son's ratio, which is a much more diagnostic lithological
indicator. If this information is required, then both
v
p
and
v
s
must be determined in the seismic survey.
These fundamental relationships between the veloc-
ity of the wave propagation and the physical properties
of the materials through which the waves pass are inde-
pendent of the frequency of the waves. Body waves are
non-dispersive; that is, all frequency components in a
wave train or pulse travel through any material at the
same velocity, determined only by the elastic moduli and
density of the material.
Historically, most seismic surveying has used only
compressional waves, since this simplifies the survey
technique in two ways. Firstly, seismic detectors which
record only the vertical ground motion can be used, and
these are insensitive to the horizontal motion of S-
waves. Secondly, the higher velocity of P-waves ensures
that they always reach a detector before any related
S-waves, and hence are easier to recognize. Recording
S-waves, and to a lesser extent surface waves, gives
greater information about the subsurface, but at a cost of
greater data acquisition (three-component recording)
and consequent processing effort. As technology ad-
vances multicomponent surveys are becoming more
commonplace.
One application of shear wave seismology is in engi-
neering site investigation where the separate measure-
ment of
v
p
and
v
s
for near-surface layers allows direct
calculation of Poisson's ratio and estimation of the elas-
tic moduli, which provide valuable information on the
in situ
geotechnical properties of the ground.These may
be of great practical importance, such as the value of
rip-
pability
(see Section 5.11.1).
3.3.2 Surface waves
In a bounded elastic solid, seismic waves known as sur-
face waves can propagate along the boundary of the
solid.
Rayleigh waves
propagate along a free surface, or
along the boundary between two dissimilar solid media,
the associated particle motions being elliptical in a plane
perpendicular to the surface and containing the direc-
tion of propagation (Fig. 3.4(a)). The orbital particle
motion is in the opposite sense to the circular particle
motion associated with an oscillatory water wave, and is
therefore sometimes described as
retrograde
. A further
major difference between Rayleigh waves and oscilla-
tory water waves is that the former involve a shear strain
and are thus restricted to solid media. The amplitude of
Rayleigh waves decreases exponentially with distance
below the surface. They have a propagation velocity
lower than that of shear body waves and in a homoge-
neous half-space they would be non-dispersive. In prac-
tice, Rayleigh waves travelling round the surface of the
Earth are observed to be dispersive, their waveform un-
dergoing progressive change during propagation as a re-
sult of the different frequency components travelling at
different velocities. This dispersion is directly attribut-
able to velocity variation with depth in the Earth's
