56 



HKRSEY 



lOHAP. 4 



[— 



DEPTH 

 IN 



FEET 



100 



(C^4800 

 FT /SEC) 

 200 



300 



400' — 



-0.58 NAUTICAL MILES 



— 200 



300 



400 



Fig. fi. Refloftion profile across a buried cliannel in Narragansett Bay, Rhotle Island. 

 Brief intcMvals of half-wave rectification iilusti-ate the enhanced clarity of recordings 

 that di.splay phase correlations over tho.se displaying envelope correlation. 



3. Operations at Sea 



An effective continuous profiler can be mounted in almost any craft, ex- 

 cepting jDossibly a canoe. Low-power profilers have proven very convenient 

 for inshore, shallow- water work ; on seagoing ships one has room for more 

 power supply and other complications which result in deeper sub-bottom 

 penetration, which is usually needed offshore, becoming possible. The research 

 ship or boat commonly tows the sound source and hydrophone (or hydrophone 

 array) a considerable distance astern. This is done partly to clear the ship's 

 wake, whose bubbles will absorb the sound, and partly to place the hydrophone 

 farther from the acoustic noise sources on the ship. Recently, successful 

 experiments have been carried out at the Woods Hole Oceanographic Institu- 

 tion with small hydrophones towed from a boom ahead of the bow. Towing 

 speeds are commonly 3 to 6 knots. Source and hydrophone depth vary, but are 

 commonly adjusted to be about a quarter wavelength for the lowest principal 

 frequency in use. (That is, an arbitrary frequency near the low cut-off of the 

 lowest filter.) 



For sounding sub-bottom structures, source and receiver are set close 

 together; say, 10 ft. For measuring seismic velocities, source and receiver are 

 gradually separated while operating. Oblique reflections, refractions, or both, 

 may be recorded. Often the drift of the observing ship is enough to carry it 

 from the cabled hydrophone, or the hydrophone may be deliberatel}' towed 

 away from the ship (see Fig. lb). Obviously, considerable variety is possible, 

 including placing source, receiver or both remote from the observing ship 

 without connecting cables. Then information can be recorded remotely or 

 relayed by radio or underwater sound. 



4. Results 



Continuous profiling has been used for about five or six years. Applications 

 were made first to shallow water and shallow unconsolidated sediment struc- 



