Environmental Engineering Reference
In-Depth Information
40
V
3
Velocity
V
= slope of line
V
1
= 20 ft /15 ms = 1333 ft /s
V
2
= 20 ft /25-20 ms = 4000 ft /s
V
3
= 20 ft /29-27 ms = 10,000 ft /s
X
c
= Critical distance from
intersection of lines
V
1
,
V
2
, and
V
3
30
V
2
20
V
1
10
First arrivals
0
10
20
30
40
50
60
70
80
Distance from geophone (ft)
X
c1
= 24 ft
X
c2
= 56 ft
Seismograph
Shot
point
Geophone
Find depths,
d
1
, and
d
2
d
1
Dry sand
GWL
X
c1
√
V
2
V
1
V
2
+
V
1
−
d
f
= 8.5 ft
d
1
=
(2.1)
2
√
X
c2
2
V
3
−
V
2
V
3
+
V
2
d
2
= 25.4 f t
d
2
0.8
d
1
Saturated sand
d
2
=
+
(2.2)
Shown are presumed paths of first waves to geophone
FIGURE 2.23
Time-distance graph and the solution to a three-layer problem.
Various formulas are available for computing the depth of the interfaces of the various
layers, varying from simple to complex depending on the number of layers involved and
the dip of the beds. The formulas for computing the depths of the relatively simple three-
layer problem are given in Figure 2.23 (see equations (2.1) and (2.2)). For interpretation
some information on topography must be available.
Actual seismograms for three shots along the same spread (each end and the middle)
are given in
Figure 2.24,
and the time-distance plots in
Figure 2.25.
The example, a three-
layer problem in a residual soil profile, is from a continuous profiling study for a railroad.
The resulting subsurface section is given in
Figure 2.26.
Limitations
Softer, lower-velocity material will be masked by overlying denser, higher-veloc-
ity material and cannot be directly disclosed.
●
A stratum with a thickness of less than about one fourth the depth from the
ground surface to the top of the stratum cannot be distinguished.
●
Erratic or “average” results are obtained in boulder formations, areas of irregu-
lar bedrock surfaces, or rock with thin, hard layers dipping in softer rock.
●
Well-defined stratum interfaces are not obtained where velocity increases grad-
ually with depth, as in residual soil grading to weathered to sound rock.
●
