Environmental Engineering Reference
In-Depth Information
Hammer blow
energy source
Detector
Direct wave path
Reflected
wave path
d
V
1
V
2
Refracted wave path
(a)
Reflective distance
(b)
V
1
Second reflection path
V
2
FIGURE 2.20
Transmission paths of (a) direct,
(b) reflected, and (c) refracted seismic
waves through shallow subsurface.
V
3
Second refraction path
(c)
materials to 0.45
V
p
for poorly consolidated soils. They are not transmitted through water
or across air gaps.
Rayleigh (R) waves
propagate only near the surface as a disturbance whose amplitude
attenuates rapidly with depth, traveling at a velocity approximately 0.9
V
p
. The recorded
velocity may be less because R waves travel near the surface where lower-velocity mate-
rial normally occurs, and usually consist of a trail of low-frequency waves spread out over
a long time interval.
Transmission Characteristics
In a given material, the arrival time of each wave at the recording instrument depends on
the travel distance between the energy source and the detector, which is in turn a function
of the depth of the stratum. In a sequence of strata with successively higher velocities,
there is a distance between the energy source and the detector at which the refracted wave
is transmitted through a higher velocity material and arrives at the detector before the
direct or reflected wave. Even though the direct and reflected waves travel shorter dis-
tances, they are transmitted at lower velocities.
For land explorations to depths of less than about 1000 ft (300 m), seismic refraction
techniques traditionally have been used rather than reflection because the direct and
refracted waves arrive first and tend to mask the reflected waves. Reflection seismology is
