Environmental Engineering Reference
In-Depth Information
Irregularly distributed points reflect
changes in topography or irregularity in depth
100
V 2 = 6000
t 2
75
V 2 = 5500
V 2 = 4500
V 2 = 4500
75
t 2
V 1 = 1500
V 1 = 800
58
V 1 = 1250
10
54
V 1 = 800
Depth
(m)
1.5
3.2
28.3
Velocity
(m/s)
V 0 = 300/400
V 1 = 900
V 2 = 5000
Depth
(m)
1.5
3.3
30.8
Depth
(m)
1.5
3.2
22.1
Velocity
(m/s)
V 0 = 350/400
V 1 = 1000
V 2 = 5000
Depth
(m)
1.5
3.5
24.9
t 1
t 1
20
14
12
15
V 0 = 400
V 0 = 200
V 0 = 400
V 0 = 350
0
12
Left end shot
3 4 5 6 7 8 9 101112131415161718192021 222324
Geophones, 10 - m stationing Center shot
Geophone numbers
Right end shot
FIGURE 2.25
Time-distance graphs for the seismograms of Figure 2.24. (Courtesy of Technosolo.)
In frozen ground, the shot point and geophones must be below the frozen zone
because the shock waves travel much faster through frost than through the
underlying layers.
Application in urban areas is limited because of utility lines, pavements, foun-
dations, and extraneous noise sources.
Applications
The method is most suitable as an exploration tool where there are media with den-
sities that increase distinctly with depth, and fairly planar interface surfaces. In such
instances, it can economically and efficiently provide a general profile of geologic condi-
tions ( Figure 2.26) .
Material types are estimated from computed P wave velocities. Typical velocities for
many types of materials are given in Table 2.9, and for weathered and fractured igneous
and metamorphic rocks in Table 2.10.
Velocity data are also used to estimate rock rippability (see Table 3.7).
Seismic Direct Methods
Applications
Seismic direct methods are used to obtain data on the dynamic properties of rocks and
soils (Chapter 3), and to evaluate rock-mass quality (Section 5.2.7). See also Auld (1977),
Ballard Jr. (1976), and Dobecki (1979).
Techniques (Figure 2.27)
Uphole survey: The geophones are laid out on the surface in an array, and the energy source
is set off in an uncased mud-filled borehole at successively decreasing depths starting at
the bottom of the hole. The energy source is usually either an explosive or a mechanical
pulse instrument composed of a stationary part and a hammer.
 
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