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25 Unit 4: Recording Seismic Signals 1.0Introduction 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. 2.0Objectives 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 3.0Main 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