Where these are the built in printers and assuming

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Where these are the built-in printers, and assuming the instrument also has the capacity to store data digitally, it is worthwhile having a separate lap-top computer coupled to a reasonable printer at the field base. It would be foolhardy, however, not to produce, and preserve, the field hard-copy. Powerful microcomputers are incorporated into most modern instruments, with high-capacity hard drives for data storage. Bewildering numbers of acquisition and processing options are available via menu-driven software. So versatile are these instruments that it is sometimes difficult, or at least time consuming, to persuade them to carry out routine, straightforward survey work. 4.0 Conclusion Seismographs that allow signals to be displayed and summed are obviously superior to mere timers, and can be used to study events other than first arrivals. However, they are generally only useful in shallow refraction work since it is difficult to distinguish between direct waves, refractions and reflections on a single trace. Hammer sources are universal, since it would be expensive and inefficient to use an explosive charge to obtain such a small amount of data. 5.0 Summary The seismograph The earliest known instrument for indicating the arrival of seismic tremor from a distant source is reputed to have been invented by a Chinese astronomer called Chang Heng in 13AD. The device consisted of eight invented dragons placed at equal intervals around the rim of vase. Under each dragon sat an open-mouthed metal toad. Each dragon held a bronze ball in its mouth. When a slight tremor shook the device, an internal mechanism opened the mouth of one dragon, releasing its bronze balls, which fell into the open mouth of the metal toad beneath, thereby marking the direction of arrival of the tremor. The principle of this instrument was used in the 18 th century European devices that consist of brimful bowl or mercury with grooved rims under which tiny collector bowl
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29 were placed to collect the overflow occasioned by a seismic tremor. These instruments gave viable evidence of a seismic event but were unable to trace a permanent record of the seismic wave itself. They are classified as seismoscopes. 6.0 Tutor Marked Assignment Q1. What are seismographs? Q2. List and Explain different types of seismograph you have learnt in this unit . 7.0 References/ Further Readings Kearey, P., Brooks, M. and Hill, I. (2002) An Introduction toGeophysicalExploration (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 Geophysicsin 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.
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