SECT. 1] CONTINUOUS REFLECTION PROFILING 49 



Hersey and Rutstein, 1958; Worzel, 1959). Again in deep water low-frequency 

 energy was needed. 



The bottom reflection studies in deep water, reported by Hersey and Ewing 

 (1949), showed that sound pulses of considerably lower frequency, of the order 

 of a few hundred cycles, would usefully penetrate several hundred feet of 

 bottom sediments in the open ocean. These frequencies are well above the band 

 from 20 to 150 c/s commonly used in seismic prospecting. However, at that 

 time it was necessary to use the high energy of explosions to obtain these 

 echoes. The explosions were not repeated rapidly enough over long periods to 

 correlate echoes from sub-bottom reflecting layers. Consequently, although 

 many recordings were taken in the western Atlantic between 1946 and 1950, 

 it was never possible to learn very much about deep-water sediment structures 

 from them. 



We needed a sound source which could be actuated repeatedly on a rapid 

 and regular schedule to correlate echoes from sub-oceanic reflectors over any 

 considerable distance. Earl}^ in the 1950's several groups, academic and com- 

 mercial, became interested in developing such seismic apparatus. The Magnolia 

 Petroleum Company laboratory developed an instrument, the Sonoprobe, 

 which was based on an electro-acoustic transducer generating a low -frequency 

 pulse ; it was used with a graphic recorder (McClure, Nelson and Huckaby, 

 1958). This instrument has been used both commercially and by academic 

 research groups. The United States Geological Survey, starting with some 

 studies of silting behind Hoover Dam in Lake Meade in 1947, used echo- 

 sounders employing 14.2 kc/s to measure the rate of silting there. Subsequently, 

 they made additional studies with the same technique in Lake Michigan and 

 Passamaquoddy Bay (Smith, Cummings and Upson, 1954; Smith, 1958). They 

 reduced their operating frequency to 11, and finally 6 kc/s as their work 

 progressed. At the Woods Hole Oceanographic Institution in 1952 the writer 

 and P. F. Smith made some observations in Great Harbor at Woods Hole of 

 sub-bottom echoes of the sound generated from an electric spark discharged 

 under water. This sound source was copied from the design developed by 

 Anderson (1953). We recorded on magnetic tape and at the same time observed 

 the echo sequences on a cathode ray oscilloscope. Later the writer and S. T. 

 Knott made a series of observations also in Great Harbor and later in Vineyard 

 Sound (Knott and Hersey, 1956). In all of these the spark sound source was 

 used with a graphic recorder. At the Naval Electronics Laboratory at about 

 the same time Padberg (1958) developed a directional transducer for gen- 

 erating high-energy low-frequency sound pulses from which he recorded 

 sub-bottom echoes on a graphic recorder. Beckmann et al. (1959) have rej3orted 

 successful work with a repeating gas exploder as sound source (see below). 

 Several commercial manufacturers of echo-sounding equipment have been 

 called upon to modify their equipment to radiate lower frequency pulses in 

 systems very similar to those mentioned above. Thus, considerable interest 

 has been shown in this method of studying comparatively shallow structures 

 in water-covered areas both at sea and in inland waters. 

 3 — s. Ill 



