Geology Reference
In-Depth Information
Figure 11.5
Electromagnetic vibrator source.
of frequencies. Typically the distance to the furthest geophone would be
about 5 m, and the frequency range would be somewhere between 5 and
200 Hz, with actual values determined by site conditions. Vibration sources
are becoming popular in surface-wave surveys (see Chapter 14).
11.2.3 Explosives
Almost any type of (safe) explosive can be used for seismic work, partic-
ularly if the shot holes are shallow and the charges will not be subject to
unusual temperatures or pressures. Cord explosives, used in quarry blasting
to introduce delays into firing sequences, are rather safer to handle than
normal gelignite and can be fed into shot holes prepared by driving metal
rods or crowbars into the ground. Detonators used on their own are excellent
sources for shallow reflection surveys where high resolution is needed.
Much of the energy delivered by an explosion can be wasted in shatter-
ing rock near the shotpoint, and seismic waves are produced much more
efficiently by shots fired in a metre or so of water. This effect is so marked
that, if the shot position is not critical, it can be worth going tens or even
hundreds of metres from the recording spread in order to put the charge in
a river. In dry areas, significant improvements can be obtained by pouring
water down shot holes.
Electrical firing is normal when using explosives but with ordinary deto-
nators there is a short delay between the instant at which the filament burns
