surveys is incurred for boat rental, and operation and navigation control, the 

 modest cost of operating an additional seismic reflection system is well jus- 

 tified by the increased detail and comprehension obtained. 

 Pingers 



81. Tuned transducers are primarily used in moderate to low energy, 

 high-resolution seismic reflection profilers often called pingers. They con- 

 sist of a suitably housed element of piezoelectric material that is capable of 

 both emitting and receiving acoustic signals. It is generally used with a 

 transceiver unit that provides the electric power at the proper operating 

 frequency to drive the transducer, and amplifies the return signals for dis- 

 play on the recorder. Tuned transducers can be permanently mounted in a 

 water- filled compartment in the hull of the survey vessel, clamped to a rail 

 or frame outside the hull, or towed behind the vessel on a suitable floating 

 vehicle. Many tuned transducer systems are composed of relatively small com- 

 ponents that make them highly portable and capable of being installed on rela- 

 tively small boats . 



82. Most tuned transducer sources have a narrow frequency spectrum and 

 usually operate at a frequency of 3.5 to 7.0 kHz. Units are available where 

 the frequency can be switched so that an operator monitoring the record can 

 select the most useful frequency for a given situation. Tuned transducers, 

 especially when operated at the higher frequency modes, are capable of higher 

 resolution than other types of sources. However, their penetration is corre- 

 spondingly limited and, in some instances, they provide no sub-bottom data. 

 For this reason they are often employed as a high- resolution component of a 

 dual system survey paired with another source capable of deeper penetration. 

 Boomers 



83. Electromechanical acoustic sources, often called boomers, produce 

 acoustic pulses in water by means of a magnetically induced, rapid displace- 

 ment of a metal plate which produces a pulse of short duration containing a 

 relatively broad spectrum of frequencies. Boomer sources are usually mounted 

 on a catamaran or other suitable vehicle and towed behind or alongside the 

 survey vessel with a hydrophone to receive the signals. Because of wave 

 motions on the towed vehicle, the bottom and sub-bottom lines on the record 

 are increasingly distorted as wave height increases with a consequent deterio- 

 ration in the record quality that impairs analysis and accurate determination 

 of the actual line elevations. This causes unproductive downtime when sea 



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