BUCK: ARCTIC ENVIR0N^4ENTAL LF ACOUSTICS MEASUREMENTS, 

 MODELS AND PLANS 



bubble pulse arrival. The usual signal is that of the bottom illus- 

 tration. Anyway, this allowed an energy analysis of each impulse 

 separately. 



Sixty feet is considerably above Weston's "critical depth" for 

 a 1.8-pound charge and bubble migration effects can be expected to 

 reduce the energy of the bubble pulses but not of the shock wave. 

 Therefore, the bubble energies were plotted relative to that of 

 the shock wave and then compared with Weston's predictions. The 

 results indicate 3 to 8 dB less total energy below 200 Hz. The 

 previously described Central Arctic test did not indicate this much 

 difference for the 60-foot MK61, thus the issue of the shallow, 

 bubble migration zone is not resolved and additional tests are 

 called for. 



During the 1970 three-station experiment, daily shot series 

 were conducted with each ice station being used for both sending 

 and receiving. Also, ski-equipped light aircraft were employed to 

 gain short-range paths. Shot depths were 60, 200, 400, 600, and 

 800 feet; shot sizes were 1.8-pound modified MK61s and 50-pound 

 MK14s. Source yields of each shot were measured by monitoring the 

 bubble pulse frequency and Weston's equations used for source 

 energies. The floe stations each drifted well over 100 miles during 

 the 2 months and allowed paths of different bottom topography. The 

 detailed transmission loss data from these experiments were reported 

 elsewhere. 



In analyzing all of these data, it was quite apparent that the 

 Arctic as a propagating medium was strikingly similar to a low-pass 

 filter with slope and insertion loss increasing and break frequency 

 decreasing all smoothly as range increased. 



737 



