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HYDROGRAPHIC MANUAL 



Page 570 



two values from the table — the nautical miles for heights corresponding to D-d and 

 D-d' , assuming a uniform depth of water throughout the range. It should be noted 

 that the distance along the direct linear path will be- slightly shorter than the distance 

 measured on the earth's surface, but for such distances as are involved in subaqueous 

 sound ranging this difference is quite small and may be ignored. 



However, it should be noted further that this ideal case of uniform velocity will 

 never be approached closely enough in practice to warrant consideration of this maxi- 

 mum range of unreflected linear propagation from any other point of view than that of 

 illustration. Moreover, interference (see 6223) will reduce the intensity of a direct, 

 linearly propagated sound to such an extent that it could not be detected at such a 

 great distance from the source, in practice. 



If a graph is made by plotting velocity with reference to depth, for use in sub- 

 aqueous sound ranging, the shapes of the curves for most regions of the ocean will be 

 quite similar. In general, the velocity will be relatively high at or near the surface and 

 will decrease with depth to about 200 or 300 fathoms where the velocity will start to 

 increase due to pressure and will continue to increase nearly linearly to the bottom. The 

 relatively high velocity near the surface is due to atmospheric surface warming of the 



water. Sound originating from a 

 point source near the surface in 

 water with such a velocity gradi- 

 ent, will follow a path which is 

 concave downward until it reaches 

 a depth corresponding to that 

 where the velocity starts to in- 

 crease, and below that depth the 

 path of the wave is concave up- 

 ward. If the vertical velocity 

 gradient in a body of water is uni- 

 form over a large area, the shape, 

 of the path of sound will be sym- 

 metrical wdth reference to the midpoint between the sound source and w^here it again 

 reaches a depth equal to the source depth. If the velocity changes linearly with depth, 

 but at different rates in different depth layers, the path of the soimd wave will be com- 

 posed of arcs of circles of different radii, a different arc for each layer. The radius of 

 curvature will be greater where the change of velocity with depth is small, and the radius 

 will be smaller where the change in velocity with depth is large. In other words, the 

 path of somid will bend only slightly through layers where the change in velocity is small 

 per unit of depth, but will bend more where the change in velocity per unit of depth is 

 large. 



The velocity-depth curve is not always as simple as that described above. In 

 certain regions subsurface layers of warm water will cause a change of sign of the velocity 

 gradient. In figure 126, a case is illustrated where the velocity decreases from the sur- 

 face to a depth where it increases because of a subsurface layer of warm water. Below 

 this layer, the velocity again decreases until depths are reached where the velocity 

 starts to mcrease due to pressure. On the right side of figure 126, is shown the path 

 of the refracted sound through each layer for a given angle of emergence of the sound. 

 (Angle of emergence and initial angle are used herein interchangeably referring to the 

 direction of the sound wave as it leaves its source.) 



Figure 126.— Path of sound through water of nonuniform velocity. 



