48 



HERSEY [chap. 4 



frequency bands are effective for different studies the broad spectrum pulses 

 are the more commonly used. Previously, seismic-reflection methods for ex- 

 ])loring buried structures have depended principally on explosive sound 

 sources. The difficulties associated with explosives have limited the techniques 

 of exploration severely. Aside from problems of safety, perhaps the impracti- 

 cality of shooting many shots in close sequence has been the biggest drawback. 

 Despite these drawbacks very successful reflection ])rofiling has been carried 

 out with explosives during cruises of Vema of the Lamont Geological Observa- 

 tory (Ewing and Tirey. 1961). Several sources have been developed that can be 

 actuated rapidly and continually, and. further, have the requisite broad 

 spectrum. Any one of these that can be triggei-ed piecisely is useable, though 

 some are more limited than others. 



The method depends critically on the automatic correlation of successive 

 recordings provided by the gra])hic recorder. The first graphic recorders we are 

 aware of were used for echo-sounding by Marti (1922), and during the 1930's 

 the graphic recorder gained rapid acceptance for recording water depth con- 

 tinuously in shallow water. Prior to the use of graphic recorders echo-sounding 

 information was presented on scanning discs or other metering devices which 

 jH'ovided no permanent record and especially no record which permitted easy, 

 rapid correlation of successive soundings. Then observers often misinterpreted 

 echo returns as bottom soundings when they may equally well have been 

 echoes from objects in the water, sub-bottom echoes, or echoes from rough 

 topography. Shortly after graphic recorders came into common use, observa- 

 tions were made in shallow-water areas w^here the sound pulse had penetrated 

 the bottom to be reflected from a surface below the bottom. A well known 

 early example is a recording during trials of T.S.S. Awatea (see Veatch and 

 Smith, 1939, j^l. vi). Murray (1947) hkewise recorded sub-bottom echoes with 

 a high-frequency echo-sounder in the Gulf of Maine. Other early observations 

 are known which have not been published. One of these was made by the writer 

 and others during 1942 in the inner parts of a branch of Pearl Harbor where 

 channel dredgings had been dumped in large quantities. The dredgings were 

 known from diving operations to be very soft, watery mud and it did not seem 

 surprising that ultrasonic i)ulses could penetrate short distances, of the order 

 of 20 to 30 ft, through this watery mud to reflect from the rock surface 

 below. 



Since echo-sounders all use high frequencies one must infer from the paucity 

 of reported observations of sub-bottom echoes that sediments having low 

 absorption for high-frequency sound are rare in shallow water. For reliable 

 penetration sounds containing low-frequency energy were needed even for 

 work in shallow water. In deep water the sound pulses commonly used in early 

 sounding operations were so long as to obscure echoes from layers only a few 

 tens of feet below the bottom. Hence it is not sur]H'ising that there are no 

 reports prior to 1950 of bottom penetration in deep water from echo-sounders. 

 Since that time modest sub-bottom penetration in deep water has been observed 

 much more often M'hen short sound pulses have been used (Heezen et al., 1959; 



