72 



as to the nature of the earth's interior. Also, since we are concerned here 

 with oceanographic instrumentation, it is further limited to the study of ocean 

 bottom. 



Owing to the relative inaccessibility of the ocean bottom as compared to 

 the land surface, geophysics necessarily forms a more important function in 

 the study of its nature. The principal methods used in this study have been the 

 measurement of the magnetic and gravitational fields, the geothermal gradients 

 in the sea bottom, and seismic measurements. 



In interpretation of geophysical measurements it cannot be emphasized 

 too strongly that the results generally yield merely clues of and not definite de- 

 terminations of the nature of the earth's interior. For this reason a combina- 

 tion of several methods in the same region may yield a better solution than one 

 of them alone. I will not attempt to discuss the details of the magnetic, gravity 

 and geothermal measurements but will go directly to the subject with which I 

 have had direct experience -- seismic refraction and reflection measurements 

 in the Pacific Ocean. The discussants, Press and Worzel have had experience 

 in these fields as well as seismic measurem.ents in the Atlantic Ocean and I will 

 not waste your time by discussing subjects which they can treat much better. 



The kind of data obtained in seismic refraction measurements in the deep 

 sea is shown in Figure 2 which illustrates the results for a receiving station a 

 few hundred miles southeast of Hawaii. In the operation depicted, the receiving 



vessel, M/V HORIZON of the Scripps 

 Institution of Oceanography, remained 

 at rest while the firing ship, USS EPCE 

 (R)857 of the U.S. Navy Electronics c 

 Laboratory dropped charges while ap- 

 proaching and receding from the HORI- 

 ll ZON. In the upper part of the figure, 

 '|l three travel time distance curves are 

 'Is shown: (1) direct wave; (2) bottom re- 

 flection; (3) first arrival times of bot- 

 tom refracted wave for each shot. The 

 lower portion of the figure shows the 

 bottom refracted travel times corrected 

 for the time delay produced by the great 

 thickness of water. The data of this 

 lower graph are used to determine the 

 velocities and thicknesses of the sub- 

 bottom layers. 



Fig. 2. Seismic refraction observa- 

 tions at a station southeast of Hawaii. 



It is seen that a large number of 

 charges are needed. At short range 

 they must be closely spaced in order to detect breaks in slope caused by the low- 

 er velocity layers in the upper structure of the bottom. At greater range, 

 where 8.2 km/sec velocity is observed, somewhat greater spacing can be used; 

 but even here many shots are needed to determine the velocity and intercept 

 with the desired accuracy. Because all charges must be carried thousands of 

 miles from supply bases, maximum use of the explosive carrying capacity of 

 the ships requires that utmost attention be given towards making the signal-to- 

 noise ratio of the refracted waves as large as possible. 



Efforts to achieve this at the Marine Physical Laboratory have been di- 

 rected towards decreasing the background noise level and towards maximum 

 utilization of explosive sound energy. Figure 3 illustrates the hydrophone ar- 

 rangement which has been found most effective in reducing noise level. At the 



