Chap. 12] 



MISCELLANEOUS GEOPHYSICAL METHODS 



957 



pared with water and air, the ground is a -rather poor sound-transmission 

 medium. Attenuation of audio-frequency elastic waves in the ground is 

 much greater than the attenuation of seismic waves. Many factors are 

 responsible, such as refraction, reflection, scattering, absorption, and 

 damping. To begin with, more energy is lost in audio-frequency than in 

 seismic-frequencj^ transmission because of scattering, since the wave 

 length of audio-frequency sounds is comparable with the dimensions of 

 the interfering objects. In seismic exploration, the wave length of re- 

 flected waves of an average speed of 10,000 feet per second is 200 feet at a 



fmt 



tlufhes tcf)o Sounder f -. 16000 ~ 



mmmmmmmmm 



f««MM.tMtiviimpM«««tiiiMMnimMM 



Codfish 5hoaJ 



y.i 



Ocean ffoftom^ 



55 Johan /for/ //^ia diSMT Ifl^esrrjoeo 



loroTSN 



Fig. 12-25. Fish-shoal detection by sonic depth finder. 



frequency of 60 cycles, and the wave length of ground-roll waves of a 

 velocity of 1000 feet per second is 100 feet at a frequency of 10 cycles. On 

 the other hand, the length of geoacoustic waves of a velocity of 6000 feet 

 per second is only 3 feet at a frequency of 2000 cycles. Since the intensity 

 of sound scattered by an obstacle is proportional to the volume of the 

 obstacle and inversely proportional to the fourth power of the wave length, 

 it follows that high-frequency sounds may readily be scattered several 

 hundred thousand times more than low-frequency seismic waves, other 

 conditions being equal. This accounts for the limited range of high-fre- 

 quency sound waves in the ground. The relative range of seismic and 



