RAY DIAGRAMS AND INTENSITY CONTOURS 



61 



50 ft in a distance of one mile. Figure 22 is the ray 

 diagram for this case. 



Isothermal Layer above Thermocline 



The most common temperature-depth distribution 

 observed in the ocean possesses a surface layer of 

 reasonably constant temperature, which overUes a 



TEMPERATURE-F 



J 1000 



RANGE IN YARDS 



5000 10,000 



I ^ 



G° 



Figure 22. Ray diagram for deep isothermal water. 



layer where the temperature decreases rapidly with 

 increasing depth, called a thermocUne. A ray diagram 

 for such a temperature-depth pattern, with the sur- 

 face mixed layer extending down to a depth of 100 ft, 

 and an underlying thermocline is shown in Figure 23. 

 It will be noted that all the rays which issue from the 

 projector at higher angles than 1.44 degrees remain 

 entirely within the top layer; the rays become hori- 

 zontal at some depth less than 100 ft and bend back 

 to the surface. All the rays leaving the projector at 



TEMPERATURE-F 



FiGUKB 23. Ray diagram for isothermal layer above 

 thermocline. 



angles lower than 1.44 degrees reach the thermocline 

 while still inclined downward; the thermocline pro- 

 gressively increases this downward bending. For 

 theoretical reasons, then, the beam should split into 

 two parts; one heads back toward the surface and the 

 other heads down into the thermocline. Between these 

 two beams the sound intensity should be very low 

 according to the ray theory. 



All velocity-depth patterns for which the ray 

 theory predicts such a splitting of the beam have 

 been called split-beam patterns. The existence of the 



predicted low-intensity zone which lies between two 

 zones of higher intensity has been verified in experi- 

 ments with explosive sound. The experiments are 

 described in Chapters 8 and 9. However, experiments 

 with single-frequency sound, which are designed to 

 test whether or not the beam splits when predicted, 

 have frequently failed to indicate any splitting of the 

 beam at all. The reasons for this discrepancy are not 

 completely understood. Diffraction of sound into 

 the low-intensity zone, although predicted to a 

 limited extent by wave acoustics, is not sufficient to 

 explain why the beam does not split. Possibly the 

 sound in the predicted low-intensity zone may be 

 largely due to scattering of sound either by in- 

 homogeneities in the predicted path of the rays, or 

 by roughness of the sea surface, or by irregularities 

 of the temperature distribution in the ocean. 



Strong Negative Gradient 



Negative temperature gradients are a frequent oc- 

 currence near the sea surface, especially when the sur- 

 face is receiving more heat than it is losing. Under 

 such temperature conditions the rays are bent 

 strongly downward. Figure 24 gives a ray diagram 



TEMPERATUHE-F 



RANGE IN YARDS 

 1000 2000 



Figure 24. Ray diagram for strong negative tempera- 

 ture gradient. 



drawn for a case where the negative temperature 

 gradient amounted to about 8 F per 100 ft of depth. 

 It is clear from the figure that the ray which left the 

 projector horizontally has been bent down 400 ft by 

 the time it has covered a horizontal range of 1,000 yd. 

 The most important quality of the ray diagram for 

 this case is the indicated shadow cast by the surface. 

 It is clear from the figure that no ray leaving the pro- 

 jector can possibly penetrate into the zone marked 

 shadow zone if the water is deep. All rays lower than 

 the ray leaving the projector at a climbing angle of 

 4.8 degrees stay within that ray and are bent down- 

 ward. The 4.8-degree ray itself becomes tangent to 

 the surface and then bends downward. All rays higher 

 than this "limiting ray" are reflected by the surface 



