SECT. 1] 



CONTINUOUS KEFLECTION PROFILING 



69 



be interpreted to give the true depth and attitude of its interface by the well 

 known arc-swinging technique. Where a sub-bottom slope is concerned, the 

 geophysicist must determine whether he is required to consider the velocity 

 structirre in constructing arcs. This technique in echo-sounding is based on 

 the assumption that the water has constant velocity. These rather tedious slope 

 corrections are frequently by-jjassed unless the exact attitude and location of 

 the reflecting surface is critically important. 



DEPTH 700 

 IN 



FA THOMS 

 tC-800 



FM/SEC) 800 



900 



1 0001— 



-1.42 NAUTICAL MILES 



700 



800 



900 



1000 



(a) 



DEPTH 

 IN 

 FA THOMS 



(c^aoo 



FM/SEC) 1 



DIRECT 

 -WATER 

 ARRIVAL 



BOTTOM 

 RErLECTION 



- — 20 



30 



105 FEET 



Fig. 21. (a) Crescent-form echo sequences from a rough bottom on the continental rise, 

 western North Atlantic Ocean, (b) Sub-bottom crescent-form echo sequences from 

 Vineyard Sound, Massacliusetts. 



After multiple reflections and side echoes have been identified and slope 

 corrections made, the jjrimary echo sequences are plotted on a structure profile. 

 If velocity information is available it may be worthwhile to convert travel 

 times to distances at this point. However, the common practice has been to 

 preserve the travel times through this step. With either choice, reflecting 

 horizons may be traced and charted by studying the whole pattern of profiles. 

 Model-making has proven helpful in several investigations. Fig. 11 from the 

 reconnaissance survey of the northwestern approach to the Strait of Messina 

 is an example. 



