300 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1939 



along a path which has its end points fixed. But, not knowing the 

 variation of velocity with depth in the core, we cannot say what shape 

 the path has therein. 



However, to make a long story short, there are, as we have partly 

 indicated, a certain number of conditions which must be met if 

 an assumed variation of velocity with depth within the core can be 

 true. Over 20 such variations have been assumed by Gutenberg, 

 checked with end data, and the most probable one selected. The 

 research continues. We have still much to learn about the core. 



Returning to the surface, let us see how we may use seismographs 

 to probe the outer layers of the crust to depths which the drill may 



Figure 12. — Ray from blast to seismograph. The average Telocity from the reflecting 

 layer at R to the surface is found from shots fired in dry wells in the region, or by 

 refraction shooting. The elapsed time, shot to seismograph, is known. Hence the 

 distance BRS is linown. The distance BS is measured. Hence the depth CR may be 

 calculated. 



reach and confirm. We may use the refraction method which was 

 described; and indeed it was used to excellent effect up to about 

 1929, when researches in radio made possible a new method which 

 is now so useful that, in the United States alone, more than a million 

 dollars a month are being spent in applying it to probe the upper 

 earth strata in search of oil. 



It is known as the reflection method. A shock generated from 

 the explosion of from a quarter to a half pound of dynamite is 

 recorded on a series of seismographs placed a measured number of 

 feet apart at a measured distance from the blast. A very common 

 "spread" is to have six seismographs at intervals of 50 feet with 

 1,000 feet from the nearest seismograph to the shot point. The shot 



