204 



TRANSMISSION OF EXPLOSIVE SOUND IN THE SEA 



1000 1500 2000 



RANGE IN YARDS 



•COMPOSITE OF ALL DATA TAKEN OVER A TWO MONTH PERIOD, AVER- 

 AGED BY RANGE GROUPS, AND WITH EXCLUSION OF ALL SHOTS MADE 

 BEYOND THE SHADOW BOUNDARY AS COMPUTED FROM BATHY- 

 THERMOGRAPH DATA 



OSHOTS MADE AT 100 FOOT DEPTH, MORNING OF APRIL 3, 1942, WITH 



HYDROPHONE AT 54 FEET 



iSHOTS HADE AT 50 FOOT DEPTH, AFTERNOON OF APRIL 3, 1942, 



WITH HYDROPHONE AT 54 FEET 



Figure 9. Dependence of time of rise on range, show- 

 ing influence of the shadow zone. 



observed and computed shadow boundaries are 

 marked on the figure. The abrupt increase in time of 

 rise on crossing the observed shadow boundary is 

 quite conspicuous, and was noticed in UCDWR ex- 

 periments on all days when strong downward refrac- 

 tion was present. For comparison, Figure 9 also shows 

 as a full curve the average values given in Table 1 

 of Section 9.2.1, which at each range represent the 

 time of rise when refraction conditions are such that 

 the hydrophone is in the direct zone. It will be seen 

 that out to the shadow boundary all values agree to 

 within the fluctuations of the data. 



The sharpness of the increase in time of rise as the 

 shadow boundary is crossed suggests using a plot 

 like Figure 9 to determine the location of the shadow 

 boundary. Table 2, taken from reference 9, gives a 

 comparison of the range to the shadow boundary de- 

 termined in this way with the range as deduced from 

 the bathythermograph measurements, and also with 

 the range as deduced from a plot of peak intensity 

 against range, such as Figure 5. It will be seen that 

 time of rise and peak pressure always give very 

 nearly the same position for the shadow boundary, 

 but that this position does not always agree well with 

 that given by the bathythermograph. This is not sur- 

 prising, since such things as surface waves and small 

 changes of temperature very close to the surface can 



* Depth of hydrophone, 54 ft in all cases. 



based on the concept of a horizontally stratified 

 medium will prove inadequate to explain the experi- 

 mental results, and that some more complicated proc- 

 ess must be considered. 



Figure 9 shows how the time of rise to the first 

 pressure maximum is affected by crossing the shadow 

 boundary. The lower dashed curve is for the same 

 shots as Figures 5 and 7, while the upper dot-dash 

 curve is for shots at shallower depth at a different 

 time on the same day. Velocities and ray diagrams 

 for this day have been given in Figure 6. Since the 

 shadow boundaries computed from the temperature 

 data do not agree very well with the boundaries de- 

 termined empirically from the behavior of sound in- 

 tensity as a function of range (see Table 2) both the 



have quite an appreciable influence on the position 

 of the shadow boundary, and the shadow boundary 

 is determined by the distribution of temperature over 

 a large area of the sea, while the bathythermograph 

 measures temperatures on only one vertical line. 



Many of the oscillographic pressure-time records 

 obtained of explosive pulses are much less simple and 

 comprehensible than the examples which have been 

 selected for discussion in the preceding paragraphs. 

 Some of the irregularities can apparently be explained 

 in terms of multiple ray paths, while others are more 

 puzzUng. Figure 10 shows some typical oscillograms 

 obtained on a day when the bathythermograph 

 showed that there were alternate layers of large and 

 small temperature gradients, which should have pro- 



