68 



HERSEY 



[chap. 4 



Amongst the echo sequences some represent echoes from extended interfaces 

 between materials of different acoustic impedance, while others may be echoes 

 scattered by smaller prominences (or concavities) in the bottom topograj)hy. 

 These latter echo sequences are commonly called side echoes ; usually they can 

 be recognized by their crescent form. Such echo sequences are thought to be 

 from reflectors small compared with the distance the ship moves during the 

 sequence. If so they should approximate hyperbolae. These reflectors are 

 located along the track of the ship at the top of the crescent, but a single 

 ])rofile near one of them does not locate it athwartships. Crescent-form echoes 

 are familiar in echo-sounding (Fig. 21a); also they have been recorded as 



DEPTH 

 IN 



FATHOMS 

 (C-800 

 FM/SEC) 



laaa,^ "1 BOTTOM 

 ZZ:ST J REFLECTION 



DIRECT 

 WATER 

 ARRIVAL 



BOTTOM 

 SURFACE 

 MULTIPLE 

 REFLECTIONS 



SUB-BOTTOM 



aaa -^ ' . ' ». .«i WbA ' ■ <-^ r REFLECTION 



BOTTOM 



SURFACE 

 , MULTIPLES 

 r OF THE 



SUB-BOTTOM 



REFLECTION 



Fig. 20. Mult ii)le-bottoin and siih-hottom roflection.s in Vineyard Sound, Massachusetts. 



apparent sub-bottom echoes (Fig. 21b). The latter record was taken in Vine- 

 yard Sound, Massachusetts, by the writer and others. At the time we did not j 

 prove that the crescent echo sequences came from sub-bottom reflectors, but j 

 we saw large numbers similar to those shown here, none of which rose above the 

 bottom echo. This fact suggests that the reflectors are sub-bottom, possibly \ 

 glacial boulders, which are w(;ll known on land nearby. 



Sloping echo sequences recpiire individual inter])retation before a structure- 

 profile section can be interpreted geologically. The echo-sounding literature 

 covers extensively the inter])retation of single echo sequences (for a review, 

 see Elmendorf and Heezen, 1957). Thus, sloping echo sequences known from 

 other control to have been recorded exactly normal to the strike of a slope can 



