conditions preclude obtaining adequate records, and some reduction of record 

 quality under any but calm conditions. To alleviate this problem, some boomer 

 sources have been installed in vehicles that are towed under water so that 

 wave motions are significantly reduced. Boomer systems are not as portable or 

 easily set up as other tuned transducer systems; however, they can usually be 

 installed and prepared for operation in a single day. 



84. Because of their broad frequency spectrum, boomer sources can 

 achieve relatively good resolution coupled with substantial penetration. 

 Boomers of the type most suitable for borrow source exploration can, under 

 reasonably favorable conditions , produce records with a resolution of less 

 than 30 cm with a penetration of 50 m. Boomers are often used in conjunction 

 with tuned transducer systems because of their relatively greater penetration 

 and capacity to obtain data in areas where sub-bottom materials are opaque to 

 tuned transducer systems. 



Sparkers 



85. Electric arc acoustic systems, or sparkers, can only be used in 

 seawater. They are versatile units that range from relatively low- energy, 

 portable systems with characteristics suitable for use in borrow source explo- 

 ration to the high-powered units capable of achieving penetration of thousands 

 of meters used for petroleum exploration and deep stratigraphic studies. 

 Sparkers produce acoustic pulses with a broad range of frequencies by creating 

 repetitious electric arcs in the water between an electrode or array of elec- 

 trodes and a ground return. The arc causes an almost instantaneous vaporiza- 

 tion of water in its path creating a vapor- and ion- filled bubble that 

 generates an acoustic pulse. This is followed rapidly by a second pulse, 

 called the bubble pulse, that is created by the collapse of the bubble; thus, 

 each reflector on the record is shown as two closely spaced lines. 



86. In use, electric spark sources are towed behind or alongside a sur- 

 vey vessel, and paired with a hydrophone array for receiving the direct and 

 reflected signals generated by the source. Electric energy is supplied by a 

 power supply and capacitor unit that is modular and can be combined in dif- 

 ferent ways and numbers to vary the energy transmitted to the electrodes. 

 Variations of pulse characteristics can be obtained by altering the size, 

 number, and spacing of the electrodes. Sparker systems can provide a range of 

 characteristics from which to select a source configuration and energy level 

 most suited to a specific program. 



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