of reflector surfaces. Several devices are available that reduce or eliminate 

 the effects of wave -induced motions. One of these devices uses an accelerome- 

 ter to sense the motions and to apply an appropriate correction to the timing 

 of the acoustic pulse intervals. These devices can only be used where the 

 acoustic source and receiver are directly mounted on the survey vessel or 

 towed vehicle such that both are moving in tandem. Where these components are 

 towed separately, the motions can be compensated for by use of an electronic 

 device that senses the frequency of the waves and selectively filters out the 

 incident motion effects. This device can be incorporated in the seismic 

 reflection system aboard the survey vessel or used later in recreating the 

 records from taped data. A more direct means of reducing wave effects is 

 available in the form of a vehicle for the acoustic source and receiver that 

 is towed beneath the surface at depths that are less affected by surface 

 waves . 



100. Signal stacking. Signal stacking is a technique for improving the 

 signal-to-noise ratio. The process involves storing the data on two or more 

 sequential acoustic pulses and return signals in an electronic buffer that 

 permits displaying of the summed output on the record. The data for true 

 signals adds constructively, increasing the signal strength while extraneous 

 noise, being random, is reduced by the summation process; thus, there is con- 

 siderable improvement in the signal-to-noise ratio. Signal stacking can be 

 used during the actual survey or applied to later reprocessing of the taped 

 raw acoustic data. 



101. Record annotators . Geophysical records require frequent annota- 

 tion to provide essential information on navigation control and time that is 

 used to make tidal corrections . The annotators are keyed to event marks on 

 the record made at intervals of 2 or 3 min. Record annotators automatically 

 mark the time, navigation fix numbers, and other types of information. This 

 has the advantage of freeing the monitor from a need to make frequent nota- 

 tions and prevents systematic errors caused by entering an erroneous fix 

 number . 



Side-Scan Sonar 



102. Side-scan sonar is an acoustic technique for visualizing sea floor 

 features (Williams 1982; Clausner and Pope 1988). Acoustic signals, from a 



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