Short duration increases, less than some 15 minutes, are 

 produced by siflall boats, mostly fishing vessels. The longer ones are 

 produced by passing ships, usually tankers and freighters. To produce 

 an increase of 3 db or more, it is estimated that small boats pass 

 within a distance of a half male from the hydrbphone. The corresponding 

 distance for ships is some 2 to 5 miles. Because ships usually pass 

 seaward of the deep h^/drophone and the recorder has a linear scale, 

 large deflections on the deep hydrophone chart often correspond to hardly 

 noticeable deflections on the chart of the shallow hydrophone although it 

 is only a m.ile inshore from the deep hydrophone. The spikes that rise 

 above the mean noise level trace are short pulses of sound such as are 

 produced by marine animals. They consistently occur more frequently on 

 the shallow than on the deep hydrophone. 



The number of occurrences of increases caused by small boats 

 that exceeded 3.0 db , are shown in the upper chart of Figure 23. The 

 number is the average, over seven days, of the total occurrences in 

 night and day periods of 12 hours each. A small boat passed by about 

 once every two hours in daytime. During night time, there was little 

 activity. Corresponding occurrences of the longer duration increases 

 attributable to ships, are shown in the lower chart of Figure 23, As 

 might be expected, the difference between day and night activity is less 

 than that for small boats. 



(c) Propagation 



A series of propagation tests, using one half pound dynamite 

 charges, were made during rhe period 27 April to 5 May, 1961.^ The 

 charges xvere electrically detonated at depths of 5 feet and 1^ feet and 

 at distances of 2.5 to 43.0 nm from the hydrophones. The received 

 levels were measured with a Sanborn chart recorder employing a log- 

 amplifier. The build up time of the recorder was 0.01 seconds and the 

 levels were based on the maximum deflections. A source level of 124.0 

 db^id at 1 yard, was calculated for the shot at 2.5 nm, assuffling spherical 

 radiation. On this basis, the transmission loss across the Gulf Stream, 

 from Fowey Rocks to Bimini, a distance of 43 nm, was 110 db. The cor- 

 responding loss for spherical spreading is 98 db. 



The levels received at the deep and shallow hydrophone were 

 about the same. The appearance of the arrivals however, was quite 

 different . 



It is planned to make a more complete study of the possible 

 transmission paths and the spectrum levels of the received pulses by 

 means of magnetic tape recordings. 



Sound velocity versus depth profiles and the bottom profile 

 from Fowey Rocks to Bimini and from. Virginia Key to Bimini were measured 



Propagation Tests Across the Gulf Stream, Memorandum for File, May 25, 

 1961, W. C. Green. 



= 11- 



