banks which build up and intermittently discharge high energy electrical 

 pulses in response to triggering commands from the recorder. In small, port- 

 able units, the power is usually supplied by batteries, and the unit may have 

 several functions (e.g., power supply and recorder) combined in a single hous- 

 ing. In larger systems, the power supply is separate and electrical input is 

 provided by a portable, deck-mounted generator. Some power supply units have 

 the transformers and capacitor banks in separate modules that can be combined 

 in different numbers to produce different power levels. 



91. Because of the high energy of the power supply output, it must be 

 protected from rain or spray, and from contact with personnel. It should be 

 attended by a qualified electronic technician familiar with safe installation, 

 operation, and maintenance of seismic reflection systems. 



Signal processors 



92. Seismic signal processors are used to amplify the signal received 

 by the hydrophones and to selectively filter out signals in preselected fre- 

 quency ranges . These functions may be performed by separate components or 

 combined in a single unit. The simplest amplifiers increase the strength of 

 all acoustic signals equally. In this mode, setting the amplifier gain high 

 enough to increase the strength of the weaker reflections may cause excessive 

 burning of the chart at the usually strong bottom reflector. To allow the 

 weaker signals to be increased without excessive burning, many amplifiers have 

 automatic or time variable gain controls. Automatic gain controls selectively 

 increase amplification of the weaker signals on all parts of the record. Time 

 variable gain controls increase amplification progressively as the stylus 

 crosses the chart paper, thus increasingly reinforcing the deeper reflectors 

 that are most likely to be weak because of signal spreading and attenuation. 



93. Band pass filters are used to reduce extraneous acoustic noise 

 unrelated to the sound source. Extraneous noise is usually produced by the 

 survey vessel, turbulence created by towing the hydrophone streamer, and ambi- 

 ent noise generated by waves or other natural sources in the water. Band pass 

 filters function by filtering out acoustic signals with frequencies outside 

 preset high and low cutoff frequencies. The cutoff frequencies are selected 

 on the basis of the frequency range of the acoustic source and the character- 

 istic frequencies of the extraneous signals. Band pass filters are especially 

 useful in playing back raw acoustic data taped during the survey when differ- 

 ent band pass frequencies are tried to obtain maximum record quality. 



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