Where the source is a heavy weight dropped from a

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Where the source is a heavy weight dropped from a considerable height, a relay switch can be attached to its top surface but may not trigger if the drop is not absolutely straight. A crude but more reliable home-made device which can be attached to any dropping weight is shown in Figure 1.5. Figure 1.4 ‘Post-office relay’ impact switch on the back of a sledgehammer handle . Figure 1.5 Weight-drop contact switch. On impact the inertia of the bolt compresses the spring and contact is made with the upper surface of the weight .
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17 Time-break pulses may be strong enough to produce interference on other channels ( cross-talk ; Section 1.3.5). Trigger cables and circuits should therefore be kept well away from data lines. 4.0 Conclusion It is possible (although not common) for a detonator to be triggered by currents induced by power lines or radio transmissions but this is less likely if the leads are twisted together. Triggering by static electricity is prevented if the circuit is closed. The shorted, twisted, ends of detonator leads should be parted only when the time comes to make the connection to the firing cable, which should itself be shorted at the far end. Explosives should not be handled at all when thunderstorms are about. 5.0 Summary Explosive charges need to be matched to the holes available. Large charges may be used in deep holes with little obvious effect at the surface, but a hole less than 2 m deep will often blow out, scattering debris over a wide area. Only experience will allow safe distances to be estimated and even experienced users can make mistakes; safety helmets should be worn and physical shelter such as a wall, a truck or a large tree should be available. Heavy blasting mats can reduce blow-outs, but their useful lives tend to be short and it is unwise to rely on them alone. A point where a shot has been fired but no crater has formed should be regarded with suspicion. The concealed cavity may later collapse under the weight of a person, animal or vehicle, leading to interesting litigation. 6.0 Tutor Marked Assignment Q1. Write short notes on the following in relation to seismic work. a) Hammer b) Explosives c) Safety d) Time Breaks. e) crater 7.0 References/ Further Readings :
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18 Kearey, P., Brooks, M. and Hill, I. (2002) An Introduction to Geophysical Exploration (Third Edition), Blackwell Science, Oxford, 262 pp. John, M. (2003) Field Geophysics (Third Edition). John Wiley and Sons Ltd. England, 249pp McCann, D.M., Fenning, P. and Cripps, J. (Eds) (1995) Modern Geophysics in Engineering Geology, Engineering Group of the Geological Society, London, 519 pp. Mussett, A.E. and Khan, M.A. (2000) Looking into the Earth: An Introduction to Geological Geophysics, Cambridge University Press, Cambridge, 470 pp. Parasnis, D.S. (1996) Principles of Applied Geophysics (Fifth Edition Chapman & Hall, London, 456 pp.
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  • Winter '12
  • JOHN
  • Geophysics, Seismic Waves, Cambridge University Press, Love wave, Rayleigh wave, P-wave

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