104 



DEEP-WATER TRANSMISSION 



o 

 <n 



(A 



s 



50 



70 



80 



90 

 100 



10 



20 50 



RANGE IN YARDS 



200 



500 



1000 



Figure 16. Measured and computed intensities in shallow water. 



the bottom is independent of depth and location. 

 There is no evidence for this assumption, especially 

 since the nature of the bottom in the deep ocean is 

 believed to be somewhat different from that in shal- 

 low water. Moreover, the attenuation coefficient 

 measured at considerable depths has no necessary 

 connection with the coefficient in the surface layers of 

 the sea. Thus, these data are of little use, except for 

 predicting the levels of sound reflected vertically 

 from the bottom in different depths of water." 



Values of the attenuation coefficient have also been 

 found from measurements of horizontal transmission 

 in shallow water. Since these depend on the numerical 

 value of the bottom-reflection coefficient, they do not 

 give an accurate indication of the attenuation in deep 

 water of constant temperature. However, at high 

 frequencies, the error introduced into the results by 

 an incorrect value of the bottom-reflection coefficient 

 becomes small, and these values are relatively trust- 

 worthy. 



An investigation along these lines is described in a 

 report issued by UCDWR.'* In this work, sound was 

 transmitted from a dock in San Diego harbor through 

 water 30 to 50 ft deep. As the range wis changed, 

 from 10 to 1,000 yd, the recorded sound intensities 

 fluctuated rapidly, since the direct, surface-reflected, 

 and bottom-reflected rays interfered, first construc- 

 tively, then destructively. The exact computation of 

 these interference effects would be very complicated. 

 Instead, only the highest peaks reached by the 



fluctuating soimd were considered; for these peaks it 

 was assumed that the different rays involved were all 

 in phase, all interfering constructively. These meas- 

 ured values were then compared with theoretical 

 curves, computed without regard for absorption. 

 These curves were found by adding the calculated 

 direct sound to the calculated sound from all the 

 images resulting from successive surface and bottom 

 reflection; the directivity of the sound projector was 

 taken into account, and all the different rays were 

 assumed to arrive in phase. The difference between 

 the observed and computed curves was then used as 

 a measure of the absorption. A sample plot showing 

 the observed peaks of the sound intensity and the 

 theoretical curves for different values of ja, the 

 amplitude reflection coefficient of the bottom, is 

 given in Figure 16 for a sound frequency of 100 kc. 

 For comparison, the inverse square curve expected in 

 a deep, ideal ocean is also shown in the figure. Un- 

 fortunately, no temperature measurements were made 

 during this work. Measurements made in the same 

 location one year later showed that the temperature 

 gradients were usually small because of strong tidal 

 currents. The temperature difference between and 

 20 ft was found to be less than 0.2 F, 94 per cent of 

 the time, and less than 0.1 F, 75 per cent of the time. 

 Thus, it is probably legitimate to take the attenua- 

 tion coefficients of this study as representative of 

 isothermal or nearly isothermal water. 



At 60, 80, and 100 kc, this work gave attenuation 



