SUPERSONIC TRANSMISSION 



141 



RAY DIAGRAM AND BOTTOM PROFILE 



BT INFORMATION 



600 



7 



SOUND FIELD DATA 



4800 4900 5000 



SOUND VELOCITY IN 



FEET PER SECOND 



4000 

 RANGE IN YARDS 



6000 



Figure 1. Typical transmission run over SAND bottom. 



traveled. This value is probably too low, and, there- 

 fore, the reflection coefficients are too low, if the at- 

 tenuation of vertical 24-kc pulses is anything like the 

 attenuation of horizontal 24-kc pulses. 



WHOI has recently developed a new method for 

 measuring bottom-reflection coefficients by means of 

 a determination of the interference pattern found at 

 short ranges when both transmitter and receiver are 

 very close to the bottom.* This experiment is carried 

 out with a cable-suspended transmitter which must 

 be nondirectional in the horizontal plane, for the same 

 reason that cable-suspended receiving hydrophones 

 must be nondirectional in the horizontal plane (see 

 Chapter 4). If the hydrophone is moved up and 

 down while a constant distance is maintained be- 

 tween transmitter and hydrophone, then the output 

 of the hydrophone goes through a series of maxima 

 and minima. If the bottom were specularly reflecting. 



then the ratio between the pressure at the maxima 

 and minima should equal (1 -f- 7o)/(l — T<i), where 

 7o is the amplitude-reflection coefficient of the bot- 

 tom; the amplitude-reflection coefficient is the square 

 root of the intensity-reflection coefficient y^ which is 

 usually employed. The method yields values for the 

 reflection coefficient which may be too low if the re- 

 flection from the bottom is not completely specular. 

 Experiments over a SAND-AND-MUD bottom 

 have led to a value of the amplitude-reflection coef- 

 ficient of 0.3 + 0.1, corresponding to an intensitj'-- 

 reflection coefficient of about 0.1. 



6.2.2 Analysis of 24-kc Trans- 

 mission Runs 



Transmission runs in shallow water and at super- 

 sonic frequencies have been made by UCDWR since 

 1943, and by WHOI since 1944. Runs carried out in 



