BUCK: ARCTIC ENVIRONMENTAL LF ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



release surface where the noise sources are not directly overhead, as 

 is the case of gravity waves in the open ocean, but are arriving from 

 a distance through rays that are fairly close to the horizontal. 



Lacking a long vertical hydrophone array to make a proper measure- 

 ment of vertical directionality of noise, we attempted to do the next 

 best thing by matching the data of Figure 14 to the Lloyd's Mirror 

 function (sin [2Tr (D/A) sin 3])- The results (see Figure 15) of this 

 rather crude analysis are best described by an example. At 20 Hz 

 the measured increase of noise level with depth shown in Figure 14 

 can be closely approximated by a ray bundle arriving 20° to 24° below 

 a horizontal, perfect pressure release surface. Similarly, at 40 Hz 

 the angle is 13° to 25°, etc. 



The data of Figures 14 and 15 were taken in one day and are too 

 small a sample to be necessarily representative. However, in con- 

 sideration of the fact that 100 percent of the energy from long-range 

 targets is arriving in the sector 5° to 17°, the possibility of gain 

 through a vertically directive array is at least interesting. To do 

 any good at the lower frequencies the array would have to have on the 

 order of 10 elements and extend to below 1,000 feet. 



Two low-frequency DIMUS sonars have been installed at ice island 

 T3, one for a short experiment in January 1960 and the second in 1965. 

 The arrays for both of these were mounted through sea ice adjacent to 

 the island (Figure 16). In 1967 a 1,200-foot diameter, 32-element 

 array was installed through the 100-foot thick ice of the island 

 itself in the approximate location shown in this figure. These 

 systems were used to investigate feasibility of large-aperture, 

 digitally processed arrays in the Arctic. It is believed that they 

 still hold the record for being the largest circular arrays ever 



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