Parasnis ds 1996 principles of applied geophysics

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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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30 MODULE 2 Unit 1 Seismic reflection Unit 2 Reflection Survey UNIT 1: SEISMIC REFLECTION 1.0 Introduction The seismic reflection method absorbs more than 90% of the money spent world-wide on applied geophysics. Most surveys are aimed at defining oil bearing structures at depths of thousands of metres using hundreds or even thousands of detectors. However, some reflection work is done by small field crews probing to depths of, at most, a few hundred metres. The instruments used in these surveys were originally very simple but may now have as much in-built processing power as the massive processing laboratories of 20 years ago. Field operators need to have some understanding of the theory behind the options available. 2.0 Objectives At the end of the unit, readers should be able to (i) Give a descriptive treatment of the more important aspects, concentrating on the developing fundamental understanding of these methods and the physical principles on which they are based. (ii) Know the instruments used in this seismic survey . (iii) Have understanding of the theory behind seismic reflection. 3.0 Main Contents 3.1 Reflection Theory Ray-path diagrams, as used previously, provide useful insights into the timing of reflection events but give no indication of amplitudes. 2.1.1 Reflection coefficients and acoustic impedances The acoustic impedance of a rock, usually denoted by I , is equal to its density multiplied by the seismic P-wave velocity. If
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31 a seismic wavefront strikes a planar interface between two rock layers with impedances I 1 and I 2 at right angles ( normal incidence ), the amplitude of the reflected wave, as a percentage of the amplitude of the incident wave (the reflection coefficient , RC ) is given by: RC = ( I 2 I 1 ) / ( I 2 + I 1 ) If I 1 is greater than I 2, the coefficient is negative and the wave is reflected with phase reversed, i.e. a negative pulse will be returned where a positive pulse was transmitted and vice versa. The amount of energy reflected first decreases and then increases as the angle of incidence increases. If the velocity is greater in the second medium than in the first, there is ultimately total reflection and no transmitted wave (Section 1.1.5). However, most small-scale surveys use waves reflected at nearly normal incidence.
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