Reynolds jm 1997 an introduction to applied and

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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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19 Unit 3: Detection of Seismic Waves 1.0 Introduction In this unit, detection of seismic waves are discussed. Land seismic detectors are known as geophones , marine detectors as hydrophones . Both convert mechanical energy into electrical signals. Geophones are usually positioned by pushing a spike screwed to the casing firmly into the ground but it may be necessary to unscrew the spike and use some form of adhesive pad or putty when working on bare rock. 2.0 Objectives At the end of this unit, readers should be able to: (i) Differentiate between geophones and hydrophones (ii) Apply geophones on land and hydrophones on water (iii) Know that geophones are referred to as seismometer or detectors. 3.0 Main Contents 3.1 Geophones A geophone consists of a coil wound on a high-permeability magnetic core and suspended by leaf springs in the field of a permanent magnet (Figure 1.6). If the coil moves relative to the magnet, voltages are induced and current will flow in any external circuit. The current is proportional to the velocity of the coil through the magnetic field, so that ground movements are recorded, not ground displacements. In most cases the coil is mounted so that it is free to vibrate vertically, since this gives the maximum sensitivity to P waves rising steeply from subsurface interfaces, i.e. to reflected and refracted (but not direct) P waves. P-wave geophones that have been normally connected give negative first-arrival pulses ( breaks ) for refractions and reflections, but may break either way for direct waves. In reflection work using large offsets, or in refraction work where the velocity contrasts between overburden and deeper refractors are small, the rising
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