ECHOES OF BATS AND MEN 



some waves travel close to the surface, others through 

 deeper layers of rock, while still others travel hundreds 

 of miles below the surface. Careful study of the times 

 of arrival of such waves at different Ustening stations 

 has enabled geologists to learn much more than they 

 could have determined by any other method about the 

 composition of our planet. (The Science Study book 

 How Old Is the Earth goes into this subject in more 

 detail.) 



The actual waves recorded by a seismograph are of 

 quite low frequency, and they are usually so irregular 

 that it is difi&cult or even meaningless to describe them 

 in terms of frequencies. Major components vary from 

 about 0.5 to 5 c.p.s. They also differ from sound waves 

 in air or water in that they involve motion in directions 

 other than the direction of wave propagation. There are 

 several different types of seismic waves, classified accord- 

 ing to the relative degrees of motion in various direc- 

 tions. By painstaking analysis of recordings made at 

 various points above and below the ground and in dif- 

 ferent directions from the place of a test explosion, 

 geologists can locate many kinds of rock structures be- 

 low the surface. This procedure has been of great use in 

 prospecting for oil, or rather for the types of rock and 

 salt deposits that are commonly associated with it. Much 

 of our industrial economy has been made possible by 

 the success of this method for echolocating oil. 



Echoes versus X-rays 



Sound waves have also come into widespread use for 

 harmless testing of materials such as metals and rubber. 

 If the material is pure and homogeneous, it transmits 

 sound waves in a smooth and orderly way. But if there 

 are discontinuities, such as air bubbles in castings or 



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