30 SHOK fCHAP. 2 



receiving lieriods. The receiving ship lies to, securing all machinery possible and 

 operating essential receiving equipment and radios from the ship's batteries 

 and from l^attery-driven a.c. converters located near the equipment they 

 su])i)ly. With the ship thus silenced the need for streaming the hydrophones 

 away from the ship and for filtering out the 20 c/s mechanical noise and 60 c/s 

 electrical pick-up disappears. This was the first method used; the SIO system 

 was developed to meet the problem of ships that are difficult to silence and the 

 presence of other scientific programs requiring power during stations. 



In the LGO-WHOI system, hydrophones are identical with those at SIO; 

 the pre-amplifiers are similar but not identical, as shown in Fig. 6, B and C. 

 In a recent version of the LGO pre-am])lifiers the filament batteries have been 

 put into the hydrophone and power is turned on when the plate voltage is sent 

 down the cable. The hydrophones are not balanced ; instead, a series of five or 

 six floats is fastened to the cable (which is heavier than water) at increasing 

 distances of 10 to GO ft from the hydroj^hone. The floats are so arranged that 

 the system has slight positive buoyancy when about 200 to 300 ft of cable is in 

 the water. Prior to arrival of the refracted wave, extra cable is paid out to 

 make the system heavy and the hydrophone then sinks slowly during arrival 

 of the shot, as shown in Fig. 3b. Submergence of the floats eliminates wave 

 noise ; the slack bight of wire prevents tugging from the ship, and flow noise is 

 greatly reduced by the slow steady sinking of the hydrophone. In order to 

 determine the hydrophone depth, a bourdon-tube depth gauge is placed on the 

 hydrophone or near it on the cable in a separate housing. Two hydrophones are 

 usuaUy used, one at the bow and one at the stern ; more would be difficult to 

 handle. Cables are pulled and slacked by hand. Systems of amplifying and 

 recording have varied. Broad-band amplifiers of a number of different types 

 have been used in the past ; the common feature has been the use of several 

 frequency bands and amplitude levels. WHOI has in recent years standardized 

 on their "Suitcase" amplifier (Dow, 1952), a battery-powered, accurately 

 calibrated, wide-band, high-gain, low-noise, dual-channel amplifier designed 

 for general underwater sound use. It has a jDass band between 6 dB points of 

 8 c/s to 50 kc/s ; interstage jilug-in filtering permits selection of any desired 

 response band. A companion power amplifier drives a hot-wire (Sanborn) 

 recorder. All frequency bands used in the SIO system can be covered by the 

 WHOI "Suitcase" system. In addition, on some operations, a set of standard 

 land-type multi])le-channel refraction amplifiers with variable filtering has 

 been used on the output of the "Suitcase" to provide as many as 12 different 

 frequency and amplitude channels per hydrophone. Tape recording of the 

 seismic signals has been done with the "Suitcase" system, primarily for analysis 

 of the reflected and direct water arrivals. 



Work at Lamont since about 1952 has been done with an amplifier system 

 with three frequency bands (3.5-135 c/s, 30-250 c/s and 620-14,000 c/s recti- 

 fied), each at five outj^ut levels, described by Sutton (1952). The most recent 

 LGO instrumentation has been a completely transistorized set of portable 

 amplifiers providing three frequency bands at two output levels for each of 



