Geology Reference
In-Depth Information
through, which usually provides the time reference, and the time at which
the main charge explodes.
Zero-delay
detonators should be used for seismic
work, and total delays through the entire system, including the recorders,
should be routinely checked using a single detonator buried a few inches
away from a geophone.
Explosives involve problems with safety, security and bureaucracy. They
must be used in conformity with local regulations, which usually require
separate secure and licensed stores for detonators and gelignite. In many
countries the work must be supervised by a licensed shot-firer, and police
permission is required almost everywhere. Despite these disadvantages, and
despite the headaches that are instantly produced if gelignite comes into
contact with bare skin, explosives are still used. They represent potential
seismic energy in its most portable form and are virtually essential if signals
are to be detected at distances of more than 50 m.
A variety of explosive-based methods are available that reduce the risks.
Seismic waves can be generated by devices that fire lead slugs into the
ground from shotgun-sized cartridges, but the energy supplied is relatively
small and a firearms certificate may be needed, at least in the UK. Another
approach is to use blank shotgun cartridges in a small auger that incorporates
a firing chamber, combining the shot hole and the shot. However, this seldom
provides more energy than a blow from a well-swung hammer, and is less
easily repeated.
11.2.4 Safety
Large amounts of energy must be supplied to the ground if refractions are
to be observed from depths of more than a few metres or reflections from
depths of more than a few tens of metres, and such operations are inherently
risky. The dangers are greatest with explosives - but nor is it safe to stand
beneath a half-ton weight that is about to be released.
Explosives should only be used by experienced (and properly licensed)
personnel. Even this does not necessarily eliminate danger, since experts in
quarry blasting often lack experience in the special conditions of seismic
surveys. If there is an accident, much of the blame will inevitably fall on
the party chief who will, if he is wise, keep his own eye on safety even if
specialists are employed for that purpose.
The basic security principle is that the shot-firer must be able to see
the shot-point. Unfortunately, some seismographs have been designed so
that the shot is triggered by the instrument operator, who can seldom see
anything and who is in any case preoccupied with checking noise levels. If
such an instrument is being used, it must at least be possible for firing to
be prevented by someone who is far enough from the shot-point to be safe
but close enough to see what is happening. This can be achieved if, after the
