142 



SHALLOW-WATER TRANSMISSION 



RAY DIAGRAM AND BOTTOM PROFILE 



200 



300 



SOUND FIELD DATA 



< 



o 



m 20 



< 

 cc 



40. 



7^v= 



r 



4890 4940 4990 



SOUND VELOCITY IN 

 FEET PER SECOND 



2000 



4000 

 RANGE IN YARDS 



6000 



Figure 2. Transmission run over sand showing linear transmission anomaly. 



the presence of negative gradients have furnished the 

 most valuable information on the reflectivity of sea 

 bottoms, because of the large contribution of bottom- 

 reflected sound under these conditions, even at 

 relatively short ranges. Fortunately, the Pacific 

 Ocean off southern California has sharp thermoclines 

 most of the year, and the bulk of shallow-water trans- 

 mission runs by UCDWR off San Diego were made 

 in the presence of downward refraction. Figure 1 is 

 a data sheet from a typical transmission run in 

 shallow water over a SAND bottom. On the sheet, 

 the sound data are plotted as transmission anomaly 

 against range. ''The transmission anomaly vs range di- 

 agram would be a horizontal straight line if the sound 

 field intensity obeyed the inverse square law. If the 

 transmission obeyed a law of the form of equation (2), 

 the transmission anomaly would be represented by 



'' It will be recalled that transmission anomaly is defined as 

 the excess of the transmission loss in decibels over the value 

 computed in accordance with the inverse square law of 

 spreading. 



a slanting straight line, whose slope would be a, the 

 coefficient of attenuation. It has already been men- 

 tioned that in many cases the transmission anomaly 

 can be approximated reasonably well by a straight 

 line. Such a straight line, fitted by inspection, is 

 drawn as the dashed line of Figure 2. The slope of this 

 line is approximately 4.5 db per kyd. 



Experience has shown that reasonably linear trans- 

 mission anomalies are typical of well- and fairly well- 

 reflecting bottoms. The slope of the transmission 

 anomaly curve depends markedly on the degree of 

 reflectivity of the sea bottom, at least for supersonic 

 sound, but much less on the exact shape of the tem- 

 perature distribution, as long as the downward re- 

 fraction is strong enough to force the direct sound 

 field out of the depth of the receiving hydrophone. 



Effect of Velocity Gradients 



This section deals with an analysis of several 

 hundred shallow-water transmission runs, which 

 were obtained by the UCDWR and by the WHOI 



