Near-Surface Seismic Sources
Hammers directed vertically on the earth surface are the most common energy sources. Normally the hammer striking a steel plate that has been firmly connected to the earth is used. The photographs below show various sources.
Figure 1. Photographs of a hammer seismic source. The instant of impact is sensed by a sensitive switch.
Figure 2. Two photographs are shown above of an accelerated weight drop source mounted on the back of a truck. This system accelerates a 45 kg mass against a base plate that has been firmly connected to the earth. Repetition rates of 5 strikes per minute permit signal averaging and rapid coverage.
Figure 3. A 'Buffalo Gun' permits the use of shotgun shells as an energy source. Two photographs of this source are shown above. This is a particularly good energy source for near-surface surveys, but needs careful procedures for safe use.
Figure 4. Shear-wave sources are very useful for near-surface reflection and engineering studies. Estimates of the dynamic moduli (Poisson's Ratio, Shear, Young's and Bulk) can be obtained from the ratio of s-wave to p-wave velocity and a knowledge of the bulk density of the materials.