956 



MISCELLANEOUS GEOPHYSICAL METHODS 



[Chap. 12 



Transmitting key 

 ~ K Spiral 



Recording stylus 



Depth markfr 



Constant -speed 

 motor 



AmjAifier 



Condenser ''^ 



_JL_i Trunsmitting 



ferentiate between the sur- 

 face of the silt and the solid 

 rock on the bottom of the 

 ocean, and to pick up fish 

 shoals, determining their 

 depth and their concentra- 

 tion in relation to the topo- 

 graphy of the ocean floor 

 (Fig. 12-25). The ability 

 of high-frequency echo- 

 sounders to furnish this 

 information in addition to 

 ocean bottom contour has 

 made them invaluable in 

 commercial fishing.^*^ 



C. Geoacoustic Methods 



Geoacoustic procedures 

 are essentially short-wave 

 (or high-frequency) seismic 

 methods. They are distin- 

 guished from seismic meth- 

 ods^*" in that they involve 

 audio-frequency communi- 

 cation and direction finding 

 and not direct measurement 

 of travel times. At pres- 

 ent, geoacoustic methods are apphed in mine safety, mine rescue, mine 

 surveying, location of water pipes and water leaks, and location of enemj' 

 sappers in trench warfare. 



1. Velocity and absorption of sound waves in the ground. Since travel 

 times are not measured in geoacoustic methods, few direct data on the 

 ground velocities of audio-frequency sounds are available. However, there 

 is no reason to assume that they differ from the velocities of seismic waves 

 of lower frequencies, as discussed in Chapter 9, pages 468-472. 



Of the three sound-transmitting media — air, water, and earth — the 

 latter, particularly consolidated rock, show\s the highest velocities. Com- 



FiG. 12-24. Automatic magnetostriction echo 

 depth-recorder (after Slee). 



"9 0. Sund, Nature, 136(3423), 953 (June, 1935). 



"0 In respect to transmission frequency, dynamic soil-testing (discussed in sec- 

 tion III of this chapter) occupies an intermediate place between seismic and geo- 

 acoustic methods. 



I 



