73 



, ^ ' %»,,4^w ''' """ '' g' ' iag ' pg i "' 



U M4!gg»M ' '' ' '?y- ' ''yS»#^1W^ 



nWMm 



-"'■^'"^^— ""■ — ■" -■■■•^^'^Mfir'''^''^'''Tn il iiBiM^^ 



Fig. 3. Hydrophone suspension. 



frequencies of the order of 10 cps used in seismic refraction work most of the 

 noise is "dangling noise" produced by motion of the hydrophone and its suspen- 

 sion in the waves and currents. This is greatly reduced if the hydrophone and 

 the outer 50 to 100 feet of line are buoyed to neutral buoyancy, as shown in Fig- 

 ure 3. This assembly then streams out from a weight suspended at the de- 

 sired depth, with liberal use of rubber shock cord, from the spar buoy shown on 

 the upper part of Figure 3. As the weak refracted waves at long range arrive 

 nearly vertically, the optimum hydrophone depth is one quarter wave length of 

 the dominant frequency. At this depth the surface reflected wave will be in 

 phase with the direct wave. 



The frequency spectrum of the refracted wave consists largely of low 

 frequencies. At the scale of distances prevailing in deep sea observations, 

 negligible energy above 50 cps is propagated through the bottom. Hence the 

 problem of obtaining maximunn utilization of explosive energy is to find the prop- 

 er depth of shot and hydrophone at which the dominant low frequency of the ex- 

 plosion has nnaximum effects. 



The only published work adequate for calculation of the energy spectrum 

 of an explosion is the fine work of Arons and Yennie. Figure 4 shows the pres- 

 sure-time curve as measured by thenn for a TNT explosion at 500 ft. depth in 

 the sea. The distance in feet from the charge center was wV^/O.352 where 

 W is the charge weight in lbs. The low frequency part of the fourier energy 

 spectrum of this pressure time curve is shown in Figure 5. At frequencies up 

 to 100 sec"Ub^/-^ the points are spaced closely enough to delineate a continuous 

 curve. Above this frequency the points oscilate rapidly, and are sensitive to 

 errors in the pressure time curve. In this region the dashed curve represents 

 the average level. At frequencies above ZOOO sec~Ub^/3 most of the sound en- 

 ergy resides in the initial sharp pressure pulse whose form can be approximated 



