INFORMATION OBTAINABLE FROM REPORTED RANGES 



85 



imitually perpendicular leaves in a direction exactly 

 opposite to the incident direction. The action of a tri- 

 plane can, tiierefore, ho compared with that of a 

 single plane perpendicular to the incident rays. That 

 is why a triplane reflects a larger percentage of the 

 incident energy back into the transducer than any 

 other hody of equal size. 



Figure 15. Triplane. 



Another type of artificial target is the so-called 

 echo repeater. This is a device which acts es.sentially 

 as a relay. It consists of a transducer with power out- 

 put proportional to the incident sound energy re- 

 ceived bj' a hydrophone. Echo repeaters have been 

 used only for training purposes. A full description of 

 the echo repeater can be found in two UCDWR re- 

 ports.*' 



4.5 INFORMATION OBTAINABLE FROM 

 REPORTED RANGES 



The research methods described in Sections 4.3 and 

 4.4 of this chapter are of comparatively recent origin. 

 During the first year of the war, the only information 

 available consisted of observed maximum echo and 

 listening ranges obtained by surface ships on escort or 

 patrol duty and by research vessels on ocean cruises. 



In testing the performance of echo-ranging gear, 

 several workers recognized the strong variability of 

 achieved maximum echo ranges.'""'^ Vessels at- 



I ached to the West Coast Sound School at San Diego 

 found in i)racticc maneuvers that ranges in the after- 

 noon coin]}ared unfax'orably with ranges in the morn- 

 ing (if t he same day. These observed ma.ximum ranges 

 gave the first clue to the existence of shadow zones in 

 the presence of sharp downward refraction. Maxi- 

 mum ranges obtained by echo ranging on submarines 

 above and below the depth of the layer revealed the 

 existence of a layer effect (see Section 5.3.3). Also, the 

 tabulation of observed ranges over various types of 

 ocean bottom in shallow water gave clues as to the 

 effect of the bottom on sound transmission. 



Because of the unexplained variability of observed 

 maximum ranges, it was decided to set up the investi- 

 gatioii of the underwater sound field as a research 

 program, and the quasi-laboratory methods of trans- 

 mission runs and echo runs were developed. Since the 

 inception of the transmission program, observed echo 

 ranges have rarely served as scientific e\'idence; they 

 have continued to serve as a stimulus for the investi- 

 gation of new problems and as signposts on the road 

 to solutions. The SS Nourmahal, a converted yacht 

 with a deep projector and with imusually quiet 

 machinery, has reported extreme echo ranges in the 

 presence of very deep isothermal layers; as a result, 

 the sound field in deep mixed layers was investigated. 

 To gi\'e another example, earlier experience had 

 shown that the sound field in shallow water over 

 MUD bottoms is very nearly the same as the sound 

 field in deep water, because MUD reflects sound 

 rather poorly. Unexpectedly long echo ranges in 

 certain areas in which the bottom was classified as 

 MUD led to a new program aimed at a differentia- 

 tion of the various bottom sediments now called 

 MUD. Also, data obtained at WHOI seem to show- 

 that attenuation increases at very high wind forces. 



These observed ranges have been obtained both by 

 naval vessels in regular operations and by research 

 vessels. A number of naval vessels have sent to WHOI 

 records of observed maximum echo ranges along with 

 bathythermograph slides. Additional observed ranges 

 were obtained by research vessels on extended cruises 

 in various parts of the world. The range data thus 

 obtained do not permit any detailed conclusions con- 

 cerning the transmission loss as a function of range 

 but serve to indicate whether sound transmission was 

 good, fair, or poor. A summary of the theory of 

 maximum echo ranges is presented in Volume 7 of 

 Division 6. 



