TRANSMISSION WITH NEGATIVE GRADIENTS NEAR SURFACE 



123 



1000 2000 



RANGE IN YARDS 



Figure 41. Plot of individual anomalies for D2 between 5 and 20 feet. 



3000 



tively few points, and the individual points show a 

 large scatter. Thus, this curve is not very reliable. 



Average anomaly curves have also been computed 

 for different values of Di, the depth at which the 

 temperature is 0.1 F less than the surface tempera- 

 ture. For a fixed value of D2, these average curves 

 show no systematic variation with changes in Di. 

 With NAN patterns, gradients below 40 ft also have 

 a negligible effect on the average transmission anoma- 

 lies measured with a shallow hydrophone. 



The change of sound transmission with changing 

 Da is to be expected on theoretical grounds. It is evi- 

 dent from Figure 35 that the range to the computed 

 shadow boundary becomes much extended when a 

 shallow layer of weak gradient overhes the major 

 temperature decrease. When thermal microstructure 

 or surface reflections are taken into accoimt, sound 

 in a thin surface layer of weak gradient can evidently 

 be propagated out to substantial ranges, with some 

 sound continually being bent down into the layer of 

 sharp gradient. Thus no shadow zone is to be ex- 

 pected in this situation, and straight-line graphs of 

 the type shown in Figure 35 are likely. The high at- 

 tenuation observed in this thin layer is also generally 

 found in shallow isothermal layers, and is discussed 

 again in Section 5.4.2. 



Dependence on Hydrophone Depth 



In accordance with expectations, the range to the 

 observed shadow zone — either R^o or the range to 

 the break in the transmission anomaly curve — in- 

 creases with increasing hydrophone depth. This 

 effect, when present, results in a predicted increase 

 of maximum echo range with increasing submarine 

 depth; this increase was observed in early practical 

 echo-ranging trials.''''^ The same effect is shown 

 clearly in Figures 36 and 37; the drop in intensity 

 for the deep hydrophones is found at ranges sub- 

 stantially larger than for the shallow hydrophone. 

 When a deep isothermal layer is present below the 

 sharp surface gradient, this effect would be expected 

 to be very marked, on the basis of ray theory. A 

 sample run in one of the rare observed situations of 

 this type is shown in Figure 42, with the accompany- 

 ing temperature-depth plot. 



The statistical analysis of the data reported in 

 reference 14 provides an indication of the average 

 change of intensity with depth. The average curves 

 found for hydrophones at different depths below 50 ft 

 are reproduced in Figure 43. The upper and lower 

 plots correspond, respectively, to intervals of 0-10 

 and 10-20 ft for D2, the depth at which the tempera- 

 ture is 0.3 degree less than the surface temperature 



