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The shot holes are shallow and the charges will not

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the shot holes are shallow and the charges will not be subject to usual Figure1.3 Impact source. A half-ton weight being dropped from a portable crane during a survey of the low-velocity layer . 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. Often, much of the energy delivered by an explosion is wasted in shattering rock near the shot point, 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 shot position is not critical, it can be worth going tens or even hundreds of metres away 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.
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14 Electrical firing is normal when using explosives but with ordinary detonators there is a short delay between the instant at which the filament burns through, which provides a 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 which reduce the risks. Seismic waves can be generated by devices which 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 which incorporates a firing chamber, combining the shot hole and the shot. Even this seldom provides more energy than a blow from a well-swung hammer, and is less easily repeated. 3.4 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 dropping from a height of 4 m. Explosives
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