Structural picture as reflection it provides data on

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structural picture as reflection, it provides data on the velocity of the refracting beds. The method made it possible to cover a given area more quickly and economically than with the reflection method, though with a significant loss of detail and accuracy. Despite its disadvantages, refraction is now rarely employed in oil exploration because of the larger-scale field operations required. Also, the reflection method has developed to the point that it can now yield nearly all of the information that refraction shooting could produce as well as relatively unambiguous and precise structural information unavailable from refracted waves. 5.0 Summary Refraction is particularly suitable where the structure of a high-speed surface, such as the basement or top of a limestone layer, is the target of geological interest. If the problem is to determine the depth and shape of a sedimentary basin by mapping the basement surface, and if the sedimentary rocks have a
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51 consistently lowe r seismic velocity than do the basement formations, refraction was in the past an effective and economical approach for achieving this objective. Airborne magnetics and, to some extent, gravity have replaced seismic refraction for such purposes. Because velocities in salt and evaporites are often greater than in surrounding formations, refraction has been useful in mapping diapiric features such as salt domes. Under favourable circumstances, this technique has been used to detect and determine the throw of faults in high- speed formations, such as dense limestone and basement materials. 6.0 Tutor Marked Assignment Q1. What do you understand by the Principal Refractors Q2. Explain with the aid of a diagram critical refraction at two interfaces. Q3. Write short notes on the following a) Critical distance b) Annotation of field records c) Length of refraction spread d) A spread 7.0 References/ Further Readings 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. Reynolds, J.M. (1997) An Introduction to Applied and Environmental Geophysics, Wiley, Chichester, 796 pp. Sharma, P.V. (1997) Environmental and Engineering Geophysics, Cambridge University Press, Cambridge, 475 pp. Telford, W.M., Geldart, L.P., Sheriff, R.E. and Keys, D.A. (1990) Applied Geophysics (Second Edition), Cambridge University Press, Cambridge, 770 pp.
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52 Whitely, R.J. (Ed.) (1981) Geophysical Case Study of the Woodlawn Orebody, New South Wales, Australia, Pergamon Press, Oxford, 588 pp.
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