Geology Reference
In-Depth Information
Figure 11.4 Impact source. A half-ton weight being dropped from a portable
crane during a survey of the low-velocity layer.
Relatively small (70-kg) weights falling in evacuated tubes have some-
times been used. The upper surface of the weight is exposed to the air, and
effectively several hundred extra kilograms of atmosphere are also dropped.
The idea is elegant but the source is difficult to transport, because the tube
must be strong and therefore heavy, and must be mounted on a trailer,
together with a motor-driven compressor to pump out the air.
Vibration sources are widely used in large-scale reflection surveys but
produce data that need extensive and complex processing. Advances in
computing power have only recently made such processing feasible in small-
scale reflection surveys. With the source shown in Figure 11.5, the vibrator
is coupled to the ground and sweeps through a computer-selectable range
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