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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45 MODULE 3 Unit 1 Seismic Refraction Unit 2 Field Interpretation Unit 3 Limitation of Refraction Methods UNIT 1: SEISMIC REFRACTION 1.0 Introduction Large primary voltages are needed to produce measurable IP effects. Current electrodes can be plan metal stakes but non- polarising electrodes must be used to detect the few millivolts of transient signal. 2.0 Objectives At the end of the unit, readers should be able to; (i) Understand the importance of power source in seismic refraction. (ii) Know that important fundamentals in refraction seismic refraction. 3.0 Main Contents 3.1 Refraction Surveys Ideally the interfaces studied in a small refraction survey should be shallow, roughly planar and dip at less than 15 . Velocity must increase with depth at each interface. The first arrivals at the surface will then come from successively deeper interfaces as distance from the shot point increases. 3.1.1 The principal refractors P-wave velocities for common rocks were shown in Figure 1.1. In shallow refraction work it is often sufficient to consider the ground in terms of dry overburden, wet overburden and weathered and fresh bedrock. It is very difficult to deal with more than three interfaces. The P-wave velocity of dry overburden is sometimes as low as 350 ms 1, the velocity of sound in air, and is seldom more than 800 ms 1. There is usually a slow increase with depth, which is almost impossible to measure, followed by an abrupt increase to 1500– 1800 ms 1 at the water table. Fresh bedrock generally has a P-wave velocity of
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46 more than 2500 ms 1 but is likely to be overlain by a transitional weathered layer where the velocity, which may initially be less than 2000 ms 1, usually increases steadily with depth and the accompanying reduction in weathering. 3.1.2 Critical refraction and the head wave Snell’s law This implies that if, in Figure 1.2, V 2 is greater than V 1 and if sin i = V 1 /V 2, the refracted ray will travel parallel to the interface at velocity V 2. This is critical refraction . After critical refraction, some energy will return to the ground surface as a
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