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(iv)Understand the uses of explosives in seismic works 3.0 Main Content: 3.1HammersA 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 where 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.
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12 3.2 Other impact sources More powerful impact sources must be used in larger surveys. Weights of hundreds of kilograms can be raised by portable hoists or cranes and then dropped (Figure 1.3). The minimum release height is about 4 m, even if a shorter drop would provide ample energy, since rebound of the support when the weight is released creates its own seismic wavestrain. A long drop allows these vibrations to die away before the impact occurs. Tractor-mounted posthole drivers, common 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. Relatively small (70 kg) weights falling in evacuated tubes have sometimes been used. The upper surface of the weight is exposed to the air, and effectively several hundred extra kilograms of atmosphere are also dropped. The idea is elegant but the source is difficult to transport because the tube must be strong and therefore heavy and must be mounted on a trailer, together with a motor-driven compressor to pump out the air. Vibration sources are widely used in large-scale reflection surveys but produce data that need extensive and complex processing.