. Early geophysical papers of the Society of Exploration Geophysicists . ; «»eci «tco«o«*c W/MW//M//////M//////M^ Fig. 2.—Elements of reflexion seismograph unit. INSTRUMENTS Instrumental design has followed widely separated paths, but there is evidence of convergence of these paths as the objective sought is more clearly visualised. Electrical detectors, usually referred to as seismometers, are uni- versally used, some employing moving coils in electromagnetic or permanent magnetic fields, some the carbon button microphone, and others the piezo-electric type of detector. Amplification of the w
. Early geophysical papers of the Society of Exploration Geophysicists . ; «»eci «tco«o«*c W/MW//M//////M//////M^ Fig. 2.—Elements of reflexion seismograph unit. INSTRUMENTS Instrumental design has followed widely separated paths, but there is evidence of convergence of these paths as the objective sought is more clearly visualised. Electrical detectors, usually referred to as seismometers, are uni- versally used, some employing moving coils in electromagnetic or permanent magnetic fields, some the carbon button microphone, and others the piezo-electric type of detector. Amplification of the weak electrical image can be obtained by audio-frequency vacuum-tube amplifier circuits employing from one to many stages. Recording of the image is through either oscillographs or string galvanometers. In average commercial work timing of the interval between the explosion and the arrival of an event at a seismometer must be accurate within limits of This accuracy is readily obtainable by means of an electrically maintained tuning-fork which may be arranged either to drive a synchronous motor or a shutter which periodically interrupts the light beam of the galvanometer. Experience has shown that the greatest energy is carried by the component of the reflected longitu- dinal wave having a frequency ranging from 30 to 60 cycles per second, and it is standard practice to peak the frequency amplification 797
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