50 



HKRSKV 



ICHAP. 4 



2. Design 



Methods adapted by the various separate groups using the continuous 

 profiling technique are similar. A schematic diagram of a typical instrument 

 is shown in Fig. 2. The elements of design are quite simple. Sound a])sorption 

 per unit distance is known to increase rapidly with frequency both in water 

 and sediment. Further, absori)tion in sediment is much higher than in water at 

 all frequencies of interest. Thus low frequencies will be required to penetrate 

 sediment (or rock). The question how low? we have had to decide empirically. 

 We know that absorption is small enough in some water-filled sediments to 

 permit ultrasonic echoes (c«. 24 kc) to be recorded through at least 30 to 40 ft 

 of soft mud. 6-14 kc/s have been used successfully to penetrate bottom sedi- 

 ments to well over 100 ft. Hersey and Ewing (1949) recorded apparent sub- 



HYDRO- 

 PHONE 



Fig. 2. Schematic diagram of continuous profiling electronics. 



bottom echoes from reflectors down to 2400 ft in a 300-600 c/s band. The 

 experience of many years of commercial exploration supports the estimate 

 that 20 to 150 c/s is the most practical band for deep penetration. Hence for 

 the needs of one problem or another there is a very wide frequency range of 

 proven utility so far as penetration is concerned. However, when fine detail is 

 needed, as in studying thin-bedded sediments, there is a lower limit to the 

 frequency band that will serve. Therefore both low and high frequencies may 

 be needed ; a good design will provide a wide choice of frequency bands, or 

 several discrete frequencies. 



Except for adaj^tations of conventional echo-sounders, which do use discrete 

 frequencies, all designers have chosen a short, broad-band pulse as their sound 



