156 



SHALLOW-WATER TRANSMISSION 



of the sea for upward refraction, but there was no such 

 effect at the lower frequencies. With downward re- 

 fraction the state of the sea did not influence the 

 transmission. Except at the lowest frequencies and 

 over the softest bottoms, the type of bottom did not 

 appreciably affect the transmission loss. Bottom types 

 were MUD, SAND, and GRAVEL. Shoal areas and 

 areas over sea valleys showed high transmission 

 losses. The attenuation was virtually independent of 

 depth for flat bottoms. Some correlation was found 

 between the empirical value of n and refraction con- 

 ditions; the power of spreading tended to assume 

 large values in isothermal water. 



Table 7. Summary of shallow-water results (BI,FI) 

 number of fits in indicated classification. 



6.3.2 Pacific Ocean Measurements 



Twenty transmission runs were made in the coastal 

 waters off volcanic islands in the Pacific (see refer- 

 ence 11). Measurements were made of overall trans- 

 mission in the 1- to 3-kc band over SAND and 

 CORAL bottoms. These measurements permitted the 

 following conclusions. In the 1- to 3-kc band, the 

 transmission loss from 100 to 3,000 yd could best be 

 fitted by n equal to 1.5 and a equal to 2.5 db per kyd, 

 under most hydrographic conditions. These condi- 

 tions included slight upward refraction in water of 

 depth about 200 ft, sUght upward refraction over 

 sloping bottoms, and downward refraction in water 

 of depth about 100 ft. For downward refraction over 

 a sloping bottom, however, the transmission loss at 

 ranges above ],000 yd was much greater. For this 

 case, the best fit above 1,000 yd was estimated to be 



10 db per kyd for the attenuation with the spreading 

 factor n uncertain. This result is in agreement with 

 ray theory,!^ which predicts that sound multiply re- 

 flected from the bottom under these conditions should 

 run downhill, following the bottom slope and leaving 

 a shadow zone near the surface. 



Also, some runs were made in the Thirteenth Naval 

 District. '^ These runs were made in water less than 

 300 ft deep over coarse gravel or rocky bottoms. 

 Velocity gradients were slight and the sea calm. No 

 correlation with computed limiting ranges was ob- 

 served. The majority of the runs were best approxi- 

 mated by zero attenuation and n equal to the values 

 given in Table 8. One run through a tide rip was best 

 approximated with n equal to 1 and the values of a 

 given on the right-hand side of Table 8. 



Table 8. Summary of shallow-water results (Thirteenth 

 Naval District). 



6.3.3 



Summary 



Recent experiments carried out by UCDWR with 

 pulses of sonic single-frequency sound have not yet 

 been reported; they are, therefore, not included in 

 this summary. This summary lists conclusions which 

 were reached in the spring of 1945 on the basis of 

 data available then.'^" It should be pointed out, 

 however, that none of the conclusions reached at that 

 time have been invalidated by later information. 



In shallow water, a distinction must be made be- 

 tween transmission over MUD bottoms (which re- 

 sembles deep-water transmission) and transmission 

 over all other bottom types. No significant differ- 

 ences were discovered in sonic experiments between 

 any of the other bottom types including MUD- 

 AND-SAND. Over sloping bottoms, a significant 

 dependence on refraction pattern has been observed : 

 with downward refraction transmission tends to be 

 poor, while in isothermal water it is as good as in deep 

 water. 



Over level bottoms, with isothermal water or in the 

 presence of downward refraction, the transmission 



