BOTTOM REFLECTION. SHALLOW-WATER TRANSMISSION 



219 



Table 6. Attenuation coefficients for explosive sound received in various frequency bands. 



Location of shots 



Frequency limits of 

 recording channel 

 6 db below peak 



sensitivity in 

 cycles per second 



Attenuation in 

 db per kiloyard 



Number of 

 observations used 



Spread of 

 ranges used 

 in kiloyards 



attenuation figures is rather uncertain, since the re- 

 ceiving channels each cover a fairly broad band and 

 since it is likely that the attenuation varies strongly 

 with frequency. Moreover, it can hardly be decided 

 yet whether the attenuation is due to absorption, to 

 scattering, or to variations in the depth of the sound 

 channel with geographic position. The latter factor 

 would have an influence on the observed intensities 

 similar to that of changing the depth of the explo- 

 sion, the important variable being merely the distance 

 of the explosion from the axis of the sound channel. 

 In spite of all these uncertainties, however, the figures 

 in Table 6 probably do give a significant upper hmit 

 to the order of magnitude of the absorption at sonic 

 frequencies. 



Measurements of a similar sort carried out on the 

 bottom-reflected pulses of the first series of experi- 

 ments give values for the reflection coefficient of the 

 bottom which will be presented later in Table 7 (see 

 Section 9.4.1). In these experiments no difference 

 could be noticed between pulses of the same group 

 whose ray paths differed by one in the number of sur- 

 face reflections undergone. This shows that the reflec- 

 tion coefficient of the surface was unity, to within an 

 accuracy of five or ten per cent, at the angles of inci- 

 dence involved, which ranged from nearly normal 

 incidence down to about 10 degrees from the hori- 

 zontal. 



It has been suggested that triangulation based on 

 the times of the last soimd channel arrivals at several 

 stations might be of practical use in the accurate loca- 

 tion of a boat or plane on the ocean. Extrapolation of 

 the intensities so far measured for the crescendo 

 formed by the last sound channel arrivals suggests 

 that a few pounds of high explosive may be heard 

 above background at ranges of ten or twenty thou- 

 sand miles or more, if shoals or land masses do not 

 intervene to cast a shadow.^" 



RANGE IN KILOYARDS 

 200 400 600 800 1000 1200 1400 1600 1800 2000 



- I 



UJ 



S 

 ►- 0.7 



SO.5 

 a: 



h- 



>-0.3 



o 



q: 



UJ 



SO.2 



— 

 ° "v. O 



tf>^— 



~0 



200 400 600 800 1000 



TRAVEL TIME OF LAST SOUND CHANNEL ARRIVAL 

 IN SECONDS 



1200 



Figure 22. Sample plot of the dependence on range of 

 the total energy recorded for all sound channel arrivals. 

 Source: 4-lb TNT bomb. Location of shots: hne from 

 latitude 26° N, longitude 76° W, to latitude 39° N, 

 longitude 67° W. Recording channel within 6 db of 

 peak sensitivity in range 22 to 175. Line shov'i cor- 

 responds to attenuation of 0.0050 db/kyd. 



9.4 BOTTOiM REFLECTION AND 



SHALLOW-WATER TRANSMISSION 



The bottom of the ocean can influence the trans- 

 mission of an explosive pulse in several closely related 

 ways. When a pulse traveling through the water 

 strikes the bottom, it is partly reflected and partly 

 transmitted. If the bottom consists of two or more 

 successive strata with different acoustic properties, 

 the transmitted pulse may itself be partially reflected 



