240 



SUMMARY 



considerable ranges. When a sharp gradient Ues just 

 above the projector and a layer of nearly isothermal 

 water 100 ft or more in thickness Hes below, ray 

 theory predicts that the souiid bent back up by the 

 positive velocity gradient in the isothermal layer 

 should be focused at shallow depths and long ranges, 

 thus giving anomalously high intensities. Observa- 

 tions made under these conditions show that the 

 transmission anomaly on a shallow hydrophone is 

 sometimes as much as 40 db less than nornaal over a 

 narrow range interval several thousand yards away. 

 The details of these observed effects are not in good 

 agreement, however, with the exact predictions of 

 ray theory. 



Deep Sound Channels 



At a depth of several thousand feet there is usually 

 a deep sound channel. The effect of pressure on sound 

 velocity increases the velocity at greater depths, 

 and a thermocline usually present closer to the sur- 

 face increases the velocity at shallower depths. Sound 

 of frequency less than 200 cycles, for which the ab- 

 sorption is very low, has been observed to propagate 

 out for several thousand miles in such a deep channel. 

 With small explosive charges, the arrivals of the 

 different pulses agree with the different rays pre- 

 dicted theoretically. The largest number of arrivals, 

 with the highest observed intensity, occur just be- 

 fore the observed sound stops entirely; these last 

 arrivals are the rays coming almost straight along 

 the axis of the channel. 



10.3 SHALLOW-WATER TRANSMISSION 



In shallow water, the transmission of underwater 

 soimd is determined primarily by the character of the 

 bottom, and by the frequency of the transmitted 

 sound. The state of the sea is a much more important 

 factor than in deep water. Temperature gradients are 

 of secondary importance. There are two situations 

 in which sound conditions do not differ appreciably 

 from those found in deep water: (1) soft MUD bot- 

 tom; (2) strong positive velocity gradients (PETER 

 pattern) below a directional sound source. In both 

 these cases, transmission is very nearly the same as 

 in deep water with the same thermal conditions. 



10.3.1 



Sonic Frequencies 



Most of the information on the transmission of 

 sonic sound in shallow water was obtained in harbor 



surveys. The data obtained may be summarized as 

 follows. 



No systematic difference was found between dif- 

 ferent types of bottoms, with the exception of soft 

 MUD, which turned out to be a poor reflector. All 

 other bottoms apparently reflect equally well. 



Transmission over sloping bottoms in the presence 

 of downward refraction tends to be poor, in agree- 

 ment with theoretical predictions. 



Over flat bottoms, at ranges greater than the water 

 depth and out to several thousand yards, average 

 soimd transmission can be best represented by an 

 inverse 1.5th power law of spreading plus an attenua- 

 tion which appears to increase roughly linearly with 

 the frequency up to about 20 kc. The transmission 

 loss is, thus, given roughly by a formula 



H=15logr + ^{f-2)r+C, (10) 



where r is the range in yards, / is the frequency in 

 kilocycles, and C is a constant independent of the 

 range r. 



10.3.2 Twenty-four Kilocycles 



In moderately shallow water, and in the presence 

 of any bottom but MUD and spft SAND-AND- 

 MUD, the transmission anomaly can usually be 

 represented in fairly good approximation by a 

 straight line. For wind forces to 2, transmission 

 anomaUes increase with the range at a rate of 5 db 

 per thousand yards over STONY and SAND bot- 

 toms, and at a rate of 6 db per thousand yards over 

 ROCK bottoms. About half of all the rims carried 

 out yield values which differ from these average 

 values by no more than 2 db per kyd. 



The following special results are also worth noting. 

 (1) For heavy seas, transmission is somewhat worse 

 than for Ught seas. For wind force 3, about 1 db per 

 kyd should be added to the attenuation coefficients 

 given above. (2) Over sloping bottoms and hi the 

 presence of negative gradients, transmission is poor. 

 The transmission anomaly may increase with the 

 range at a rate exceeding 10 db per thousand yards. 

 (3) In shallow isothermal water, transmission is at 

 least as good as in deep isothermal water. (4) In very 

 shallow water (5 fms deep), a series of experiments 

 carried out over SAND gave very poor transmission; 

 the anomaly increased at the rate of about 16 db 

 per kyd. 



