Geophysicsin engineering geology engineering group of

Info icon This preview shows pages 24–27. Sign up to view the full content.

View Full Document Right Arrow Icon
Geophysicsin 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, CambridgeUniversity 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.
Image of page 24

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
25 Unit 4: Recording Seismic Signals 1.0 Introduction Instruments that record seismic signals are known as seismographs . They range from timers which record only single events to complex units which digitize, filter and store signals from a number of detectors simultaneously. 2.0 Objectives At the end of this unit, readers should be able to: (i) Understand simple seismic recorders (ii) Familiar with seismographs. (iii) Understand common instruments used in seismic signals recording 3.0 Main Content : 1.4.1 Single-channel seismographs Most single-channel seismographs have graphic displays, although rudimentary seismic ‘timers’ which simply displayed the arrival time of the first significant energy pulse numerically were once popular. On a visual display, the time range is switch or key-pad selected and the left-hand edge of the screen defines the shot or impact instant. Hard copy is not usually obtainable and times are measured directly. In some models a cursor can be moved across the screen while the time corresponding to its position is displayed. Noise levels can be monitored by observing the trace in the absence of a source pulse. Modern single-channel instruments use enhancement principles. A digital version of the signal is stored in solid-state memory, as well as being displayed on the screen. A second signal can either replace this or be added to it. Any number n of signals can be summed ( stacked ) in this way for a theoretical n improvement in signal/noise ratio. 1.4.2 Multi-channel seismographs Seismographs with 12 or 24 channels are generally used in shallow surveys, whereas a minimum of 48 channels is now the norm in deep reflection work. With multiple channels, both refraction and reflection work can be done and
Image of page 25