26 



SHOR 



[chap. 2 



Fig. 4. Photograph of tiHJ hydrophone system. 



Is not adequate or if for some reason it is not possible to steam (as in the case 

 where the dredge is down and currents are pulhng the hydrophones), cable is 

 paid out from the winches into a heap on deck and paid out rapidly at shot 

 ■fime for the most distant shots only. 



A block diagram of the SIO shooting and receiving system is shown in Fig. 5. 

 The hydrophones used are normally either Brush Electronics Model AX-58C 

 or Research Manufacturing Co. Model R-100. The former uses Rochelle salt 

 crystals immersed in castor oil, the latter barium titanate ceramic cylinders 

 cast in plastic. Both types produce a signal of approximately 100 [xV per 

 dyne/cm2 for signals between 2 c/s and 20 kc/s. The output is at high im- 

 pedance and must be matched to the cable with a cathode follower pre- 

 amplifier (current amplifier) at the hydrophone; the SIO, LGO and WHOI 

 pre-amplifier circuits are shown in Fig. 6. Batteries to operate the pre-amplifier 

 are located not in the hydrophone but at the weight (actually a brass box 

 filled with batteries) 50 ft away ; this eliminates the need to open the hydro- 

 phone case or to re-balance at times of battery change. Hydrophones turn on 

 when the battery case is right-side-up. The least signal from sea noise in calm 

 weather in the low-frequency (2 to 20 c/s) range of interest for seismic refraction 

 is of the order of 10 (xV hydroi)hone output ; first stage noise must therefore be 



