Geology Reference
In-Depth Information
head-wave
, which leaves the interface at the original angle of incidence
(Figure 11.3b). This is the basis of the refraction methods discussed in
Chapter 13. At greater angles of incidence there can be no refracted ray and
all the energy is reflected.
When drawing ray paths for either reflected or critically refracted waves,
allowance must be made for refraction at all shallower interfaces. Only
the
normal-incidence
ray, which meets all interfaces at right angles, is not
refracted.
11.2 Seismic Sources
The traditional seismic source is a small charge of dynamite. Impact and
vibratory sources are now more popular but explosives are still quite com-
monly used.
11.2.1 Hammers
A 4- or 6-pound sledgehammer provides a versatile source for small-scale
surveys. The useful energy produced depends on ground conditions as well
as on strength and skill. Hammers can nearly always be used in refraction
work on spreads 10 to 20 m long but very seldom if the energy has to travel
more than 50 m.
The hammer is aimed at a flat plate, the purpose of which is not so much to
improve the pulse (hitting the ground directly can sometimes provide more
seismic energy) but to stop the hammer abruptly and so provide a definite
and repeatable shot instant. Inch-thick aluminium or steel plates used to be
favoured, but are now being replaced by thick rubber discs that last longer
and are less painfully noisy. The first few hammer blows are often rather
ineffective, as the plate needs to 'bed down' in the soil. Too much enthusiasm
may later embed it so deeply that it has to be dug out.
11.2.2 Other impact sources
More powerful impact sources may be used in larger surveys. Weights of
hundreds of kilograms can be raised by portable hoists or cranes and then
dropped (Figure 11.4). The minimum release height is about 4 m, even if
a shorter drop would provide ample energy, since the support rebounds
when the weight is released and creates its own seismic wave-train. A long
drop allows these vibrations to die away before the impact occurs. Tractor-
mounted post-hole drivers, easily found in farming areas, are also convenient
sources. The weight drops down a guide and is raised by a pulley system
connected to the tractor power take-off. Another alternative is the Propelled
Energy Generator (PEG), which uses an elastic band to propel a mass of a
few tens of kg at high velocity from a height of just half a metre.
