Environmental Engineering Reference
In-Depth Information
FIGURE 2.21
Single-channel refraction seismograph. Man in upper right with hammer striking metal plate causes seismic
waves.
shot point and the farthest geophone), which should be, in general, 3 to 4 times the desired
penetration depth. A normal spread would be about 300 ft (100 m) to investigate depths to
100 ft (30 m) with geophones spaced at 30 ft (10 m) intervals to define the velocity curves.
The geophysicist determines the spread length and the geophone spacing to suit the antic-
ipated geological conditions. In practice, a shot is usually set off at one end of the spread,
and then another at the opposite end (reverse profiling) to detect stratum changes and slop-
ing rock surfaces. At times, charges are set off in the middle of the spread or at other loca-
tions. Multi-channel units are used for deep exploration and all geologic conditions.
Seismograms
Seismic waveforms are usually recorded on photographic paper as seismograms. In
Figure 2.22,
the P wave, traveling at the highest velocity, is the first arrival to be recorded
and is easily recognized. It is used to determine the depths to the various strata on the
basis of their characteristic transmission velocities.
The S wave appears later in the wave train as a large pulse and is often difficult to recog-
nize. In the figure, it is observed crossing the spread at an intermediate angle from the first
arrivals, indicating a lower velocity. S wave velocities are used in conjunction with P wave
velocities to compute the dynamic properties of the transmitting media (
Section 3.5.3).
