Chap. 9] 



SEISMIC METHODS 



613 



TO AMPL. 



^zzzzzzzzzza 



v/// ;;//j/ ij 



(c) Pressure seismographs. Modifications of the carbon microphone 

 were employed in the days of refraction shooting and are still in use in some 

 civil engineering applications. They have definite limitations because of 

 high noise level and packing. Their use for reflection work has been vir- 

 tually abandoned. The I.G.E.S. employed a Western Electric mining 

 detector of the carbon type for shallow refraction applications* in con- 

 nection with a string galvanometer recorder. They further used a hot 

 wire seismograph, consisting of a geophone in which the air displaced by 

 the movement of a diaphragm of fairly large diameter is forced through a 

 small orifice. In this orifice a platinum wire heated by a battery is located 

 and is cooled by the air flowing past it. The wire grid is arranged in one 

 arm of a Wheatstone bridge, the indicating device (string galvanometer) 

 is in the center arm of the bridge, the re- 

 sistance of the wire being varied in pro- 

 portion to the velocity of the motion of the 



. 68 



air. 



The Ambronn accelerometer which was 

 used extensively in the earlier refraction 

 work, notably by Sineriz in Spain (see Fig. 

 9-1 17a), is likewise based on variations of a 

 resistance due to ground vibrations. The 

 mass is suspended from two springs; two 

 contact points are arranged on the mass 

 and on a beam balanced on a knife edge. 

 The position of the beam and, therefore, 

 the contact pressure may be controlled from 

 the recording truck by varying the current 

 through a solenoid surrounding an armature 

 on the beam. The contacts are arranged 

 in a Wheatstone bridge, as shown in Fig. 

 9-1176. Individual string galvanometers 

 of high sensitivity are used at G to record the 

 variations in acceleration . The critical pres- 

 sure on the contact is adjusted on a control 

 panel in the recording truck by the circuit 

 shown on the left side of Fig. 9-1176. A 

 quadruple cable is required from the truck to each detector. 



Piezoelectric or crystal detectors have been used for both refraction and 

 reflection applications. A crystal detector with preamplifier is illustrated 

 in Fig. 9-118. A piezoelectric receiver and equipment suitable for refrac- 

 tion and reflection applications was described by Ambronn. 



Fig. 



9-118. Crystal detector 

 with preamplifier. 



*^ See p. 212 of Edge and Laby's report. 



«8 See Figs. 152, 260, and 261 in Edge and Laby. 



89 World Petrol. Congr. B.I., 165-168 (London, 1934). 



