Environmental Engineering Reference
In-Depth Information
25
50
75
R
S
P
100
First
arrival
125
150
175
200
225
250
275
300
FIGURE 2.22
Seismograms of waveforms recorded at 25-ft
intervals as caused by 300-lb weight drop.
2.5 ms per division
The Rayleigh wave (R) appears as a large-amplitude, low-frequency signal arriving late
on the train. In the figure, it crosses the spread at an angle larger than the S wave and
leaves a record at about the 200 ft spread. Although easy to recognize, the beginning is
essentially indeterminate and does not provide much information for engineering studies.
Time-Distance Charts and Analysis
A typical time-distance chart of the first arrivals obtained with a single-unit seismograph
is given in
Figure 2.23.
As the distance from the geophone to the shot point is increased,
eventually the shock waves will have sufficient time to reach the interface between media
of lower and higher velocities, to be refracted and travel along the interface at the higher
velocity, and to arrive before the direct and reflected waves traveling through the shal-
lower, lower-velocity material.
The travel times of the first arrivals are plotted against the distance from the geophone
and the velocity of the various media determined from the slopes of the lines connecting
the plotted points as shown in the figure.
