SOUND CHANNEL AXIS DEPTH LEARNING  OBJECTIVES:  Recognize subsurface  oceanographic  features  conducive to   deep   and   shallow   channel   conditions. Evaluate  deep  sound  channel  axis  (DSC)  and shallow   sound   channel   axis   (SSX)   depth products.  Identify  the  graphic  and  tabular outputs of each. In this section we will discuss both the deep and shallow channel axis products. First, let’s look at the deep  sound  channel  axis. DEEP SOUND CHANNEL AXIS DEPTH  (DSC) A deep sound channel occurs when the deep sea is warm  on  top  and  cold  below.  The  surface-warming effect is not sufficient to extend all the way to the bottom and is limited to the upper part of the water column, below which it forms the main thermocline. The main thermocline exhibits a decrease in temperature at a moderately  rapid  rate  with  depth.  Below  the  main thermocline,  the  sea  is  nearly  isothermal  about  38°F) and therefore has a positive sound speed gradient due to the  effects  of  pressure. Sound  Ray  Refraction The DSC axis is located at the depth of minimum sound speed in the deep sound channel. This sound speed minimum causes the sea to act like a kind of lens, as expressed by Snell’s law, where sound rays above and below the minimum are continuously bent by refraction toward the DSC axis. That is, as the ray enters the deep sound  channel  from  above,  the  sound  speed  follows  a negative  gradient  and  the  ray  bends  downward  toward the  depth  of  the  minimum  sound  speed,  the  axis. Conversely, after the ray reaches the axis, the sound speed gradient is positive and the ray bends upward toward the axis. This refraction pattern forms the low-loss deep sound channel, as a portion of the power radiated by a source  in  the  deep  sound  channel  remains  within  the channel  and  encounters  no  acoustic  losses  by  reflection from the sea surface and bottom. Because of the low transmission loss, very long ranges can be obtained from a source of moderate acoustic power output, especially when it is located near the depth of minimum velocity, the  axis  of  the  sound  channel.  Note  that  not  all propagation paths in the DSC are entirely refracted paths. When the source or receiver or both lie beyond the limits of the channel, only reflected paths that encounter either the surface or bottom or both are possible. Ocean Variations The  ocean  by  no  means  is  laterally  uniform. Because the temperature structure of the ocean varies with location, the axis depth ranges from 4,000 feet (1,225  meters)  in  mid-latitudes  to  near-surface  in  polar regions. As the channel axis becomes shallower, low values of attenuation can be reported. For example, the channel axis becomes shallower with increasing latitude northward  from  Hawaii,  so  a  shallow  source  finds  itself closer to the DSC axis as it moves northward. As a result, the transmission becomes better than it would be if the DSC axis were at a constant depth. Also, signals in the DSC can be found to reach a maximum and then begin to decrease with increasing range instead of the normal  linear  decrease.  This  effect  is  attributed  to  poor sound  channel  conditions  along  part  of  the  path.  The horizontal variations of the DSC axis can be readily observed on the DSC product. Sound Fixing and Ranging (SOFAR) Channel The deep sound channel is sometimes referred to as the SOFAR (sound fixing and ranging) channel. Its remarkable  transmission  characteristics  were  used  in  the SOFAR  system  for  rescue  of  aviators  downed  at  sea.  In SOFAR a small explosive charge is dropped at sea by a downed  aviator  and  is  received  at  shore  stations thousands of miles away. The time of arrival at two or more stations gives a “fix,” locating the point at which the detonation of the charge took place. More recently, the ability to measure accurately the arrival time of explosive  signals  traveling  along  the  axis  of  the  deep sound  charnel  has  been  used  for  geodetic  distance determinations and missile-impact locations as a part of the Missile Impact Location System (MILS) network. EXAMPLE  OUTPUT There is one graphic output available with the DSC product. It is a shaded deep sound channel axis depth display. The amount of shading indicates the range of depth in feet. See figure 9-9. Clear c 1,500 feet Light 1,500 – 3,000 feet Medium 3,000-4,500   feet Heavy M,500 feet 9-8