TRANSMISSION WITH NECiATIVE CKADIENTS NEAR SURFACE 



133 



TEMPERATURE -F 



RANGE IN YARDS 

 2000 



3000 



4000 



300 



Figure 53. Sound channel ray diagram, extreme case. 



shows the corresponding rapid rise in the transmission 

 anomaly. 

 23 This is a much the same as above except that the gradient 

 in the upper 10 ft is somewhat weaker. This curve and 

 the preceding one are clcsely similar. 



33 This is a NAN, MIKE, or CHARLIE pattern, but in 

 any case the thermoclinc is shallow and the attenua- 

 tion high. 



34 The main thermocline is deeper here since the tem- 

 perature is within 1 F of the surface temperature down 

 to at least 40 ft. The hydrophone may be below or 

 above the thermocline. 



35 The top of the main thermocline is not much above 

 80 ft, and the hydrophone is either close to the top or 

 above it. However, there are gentle gradients above 

 the hydrophone, and these act to reduce the sound 

 intensity. The reduction in sound intensity produced 

 by the weak gradient between the projector and the 

 hydrophone may be regarded as an example of layer 

 effect. 



45 The gradients above the hydrophone are weaker and 



transmission is improved. 

 55 The water is virtually isothermal down to 80 ft, and 



the results discussed in Section 5.2 are applicable. The 



deviation of this curve from a straight line is probably 



not significant. 



Sound Channels 



When the sound velocity at the projector is less 

 than the velocities above and below, rays leaving the 

 projector at sufficiently small angles will, in theory, 

 curve back and forth within two fixed depths of equal 

 sound velocity, giving rise to the curious ray diagram 

 shown for an extreme case in Figure 53. This situa- 

 tion is called a sound channel, and should in theory 

 give rise to high sound intensity at long ranges. When 

 sharp negative temperature gradients are present 

 over sharp positive gradients, such sound channels 

 should be persistent and very marked . However, very 

 few measurements have been made with positive 

 temperature gradients present in the water. 



In the absence of positive temperature gradients, 

 the effect of pressure on sound velocity can produce 

 a positive velocity gradient below the projector. How- 

 ever, this gradient is very small, and a temperature 



decrease of only 0.3 F (at 60 F) between the projector 

 and the isothermal layer will bend the sound rays 

 down so sharply that an isothermal layer 100 ft thick 

 is required to bend the rays back up again. More- 

 over, as a result of this small gradient, rays bent down 

 into the isothermal layer would return to the surface 

 only at ranges of many thousands of yards. This 

 bending is so gradual that the presence of thermal 

 microstructure might be expected to mask com- 

 pletely any sound channel effects resulting from up- 

 ward bending in nearly isothermal water. However, 

 since some striking acoustic effects are observed with 

 shallow gradients overlying isothermal layers, and 

 since thermal microstructure has never been measured 

 under such conditions, it is instructive to examine 

 what the sound field would be like in truly isothermal 

 water underlying slight gradients at the surface. 



If the projector were in such a hypothetical layer 

 of completely isothermal water, the effects of the 

 sound channel would not be particularly noticeable 

 since the sound that has curved first up into the 

 negative gradient and then down into the isothermal 

 layer would be indistinguishable from the rays that 

 have traveled through the isothermal layer for their 

 entire path. In fact downward bending by a very 

 shallow surface gradient above the projector is proba- 

 bly very similar to reflection by the surface. 



To produce marked effects the negative tempera- 

 ture gradient at the surface must extend below the 

 projector depth, so that the entire sound beam is bent 

 downward, resulting in low sound intensities meas- 

 ured by a shallow hydrophone at short range. Then 

 when the rays are curved back to the surface thou- 

 sands of yards out, the sound intensity should show 

 a marked increase. On the basis of the simple ray 

 theory, which neglects thermal microstructure and 

 diffraction, the theoretical intensity at the projector 

 depth is infinite at the range where the axial ray from 

 the projector becomes horizontal again; this singu- 

 larity results from the crossing of many adjacent rays 

 at this point. Although of course diffraction and ther- 



