SPINDEL: PHASE FLUCTUATIONS, COHERENCE AND INTERNAL WAVES 



oceanographic standpoint the causal relationships between ocean param- 

 eter variability and acoustic phase variability suggest techniques for 

 measuring oceanographic phenomena. From an operational viewpoint, the 

 performance of detecting and tracking systems is strongly dependent 

 on the phase stability of the ocean transmission path. In both cases 

 the connection between ocean parameters and acoustic phase must be 

 understood. 



Fixed system studies (in which source and receiver are rigidly 

 attached to the ocean floor) conducted in the 200 to 800 Hz region 

 of the spectrum have shown that for periods less than the local 

 inertial period and greater than the local bouyancy period, internal 

 gravity waves appear to be the predominant cause of acoustic phase 

 fluctuations. These periods range from about 5 minutes to 1 day at 

 a latitude of 30°. Time scales of this order are of utmost interest 

 in array tracking and detection applications. 



PHASE FLUCTUATIONS 



Phase fluctuation data collected at Woods Hole exhibit most of 

 the features found in data obtained by the Institute for Acoustic 

 Research in Miami, the New London Laboratory of the Naval Underwater 

 System Center, the Bell Telephone Laboratories, and others. The 

 experiments conducted at Woods Hole, however, have significant 

 differences, and this is reflected in some of the observations we 

 have made. Woods Hole data are not obtained with a fixed system. 

 The acoustic source is moored at varying depth, and receivers are 

 either free-drifting, towed, or moored. Receivers are suspended in 

 mid-ocean at depths varying from 300 to 1500 meters. Receiving 

 hydrophones sweep out synthetic spatial and temporal apertures 

 several kilometers in length and several hours in duration. 



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