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6 m e end d 1 12 0 077 870 33 5 m thickness of

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6 m E end: d 1 = 12× 0 . 077 × 870 = 33 . 5 m Thickness of intermediate layer (d 2= 12 [ t i −2 d 1 / V 1,3 ] × V 2,3): W end: d 2 = 12× {0 . 060 25 . 2 / 790} × 1910 = 26 . 8 m D = 26 . 8 + 12 . 6 = 39 . 4 m E end: d 2 = 12× {0 . 149 67 . 0 / 790} × 1910 = 61 . 3 m D = 33 . 5 + 61 . 3 = 94 . 8 m Stage (4) Reciprocal time interpretation (example using Geophone 8) Reciprocal time (t A + t B t i) W end: t R = 101 + 254 = 295 ms E end: t R = 233 + 208 149 = 292 ms Average = 293 ms
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65 Depth conversion factors at short shots (F = 2 × D /t i) W end: F = 2 × 39 . 4 / 0 . 060 = 1310 ms 1 E end: F = 2 × 94 . 8 / 0 . 149 = 1270 ms 1 F at G8 (by interpolation) = 1280 ms 1 Depth at G8 (D = tA + t B t R) D = 12× ( 0 . 174 + 0 . 213 0 . 293 ) × 1280 = 60 . 2 m 4.0 Conclusion Despite the limitations of refraction surveys, interpretations are not always wrong when they disagree with drill hole data. Only a very small subsurface volume is sampled by the drill, and many drill tests of drift thickness have been terminated in isolated boulders some distance above the true top of bedrock. It is always important that explanations are found for any differences between drilling and seismic results. 5.0 Summary The complex character of the direct wave may be among the reasons for the commonly observed failure of the best-fit arrival line to pass through the origin. Delays in the timing circuits may also play a part but can be determined by direct experiment, with a detonator or a light hammer blow close to a geophone. 6.0 Tutor Marked Assignments Q1. Write short notes on the following a) Direct Waves b) Hidden layers c) Blind zones d) Intermediate refractor 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
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66 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. Whitely, R.J. (Ed.) (1981) Geophysical Case Study of the Woodlawn Orebody, New South Wales, Australia, Pergamon Press, Oxford, 588 pp. Hawkins, L.V. (1961) The reciprocal method of routine shallow seismic refraction investigations. Geophysics, 26 , 806–19.
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