John m 2003 field geophysics third edition john wiley

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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. 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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60 Unit 3 LIMITATIONS OF THE REFRACTION METHOD 1.0 Introduction First-arrival refraction work uses only a small proportion of the information contained in the seismic traces, and it is not surprising that interpretation is subject to severe limitations. These are especially important in engineering work; in low-velocity-layer studies only a time delay estimate is sought and short shots alone are often sufficient. 2.0 Objectives At the end of the unit, readers should be able to (i) Understand prominent practical limitations of the geophysical methods. (ii) Be exposed to different techniques applicable in low velocity layer environment. 3.0 Main Contents 3.1 Direct waves The ground roll consists of a complex of P and S body waves and Love and Rayleigh surface waves travelling with different but generally slow velocities. There is often some doubt as to which component actually produces the first break, since conventional geophones respond only poorly to the horizontal ground motions of direct P-waves. Close to the source, enough energy is associated with the P-waves for the response to be measurable, but at greater distances the first breaks may record the arrival of S-waves, surface waves or even the air wave. 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. A more important reason may be that the amplifier gains at geophones close to the shot point have been set so low that the true first arrivals have been overlooked (Figure 3.7). Full digital storage of the incoming signals should allow the traces to be examined individually over a range of amplifications, but if this is not possible, then the most reliable velocity estimates will be those that do not treat the origin as a point on the line.
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61 3.2 Vertical velocities However much care is taken to obtain valid direct-wave or refracted-wave
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