87. In general, electric arc sources do not achieve the degree of reso- 

 lution that can be obtained by tuned transducer and electromechanical sources. 

 However, their flexibility is a useful trait, especially in areas where it is 

 difficult to obtain adequate subbottom penetration. Paired with another 

 source capable of obtaining high resolution of the uppermost sub-bottom 

 reflectors, electric arc sources are suitable for borrow source investigations 

 in saltwater environments. 



Hydrophone receivers 



88. Unlike tuned transducer systems that both transmit and receive 

 acoustic signals, electromechanical and electric arc profilers require a sepa- 

 rate receiver component. These components consist of one or more hydrophones, 

 usually composed of piezoelectric material, that can convert acoustic signals 

 to an electric voltage of the same frequency and proportional strength. Volt- 

 age is processed by a signal processor component and displayed on a graphic 

 recorder. 



89. Hydrophone receivers may consist of a single element or an array of 

 electrically connected multiple elements contained in an oil-filled, plastic 

 tube that is towed behind the survey vessel. Since hydrophones must be sensi- 

 tive to weak signals in order to obtain data from deeper reflectors and low 

 acoustic impedance interfaces, they also respond to acoustic energy from other 

 sources such as other geophysical instruments, ship noise, turbulence created 

 by towing the hydrophone array, and noise created by waves. These extraneous 

 signals are picked up by the receivers, and generate marks on the graphic rec- 

 ord that obscure or complicate the patterns generated by actual reflectors. 

 Some reduction in extraneous noise can be made by varying the length of the 

 hydrophone array, and the number and spacing of elements, in such a way as to 

 enhance the reception of valid signals. Other measures such as using fre- 

 quency filters, and physical methods such as increasing the distance between 

 the vessel and towed array and using reduced boat speeds, are also effective 

 ways to suppress unwanted noise. 



Energy sources 



90. Energy is supplied to electrically driven acoustic sources by a 

 power supply component consisting of transformers and capacitor banks that 

 convert input from a standard generator or battery supply to high energy elec- 

 tric pulses that are used to drive the acoustic sources. The transformers are 

 used to boost the line voltage and supply electrical energy to the capacitor 



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