STRUCTURE OF THE EARTH — HODGSON 



289 



graphs be properly designed, we can also discern the arrival of later 

 impulses — points on the common radius through the source, of the 

 familiar circles-within-circles which can be seen when gravity waves 

 radiate from a stone thrown into still water and are reflected from the 

 shore or other obstacles. 



A little consideration is sufficient to show that, near the source, the 

 arrival of the wave at successive stations will be at the rate Fi, if we 



-— 7 





/ / 



/ / 



/ / .' / .' .' .' / / ' ' -^ / / / / / / ' 



\.l -^ '' ^ / ' -^ ^ / ^ '' ^ / -^ ^' / / '' / / '' / 

 y^< I. L/-.<.j../ / ^. ^ ^ ^ ' ^' ' '' ' / '' / 



/ / / / / / 



FiGORE 5. — Diagram illustrating tlie refraction method of probing surface layered structure. 

 Velocities in the upper and lower layers are respectively Fi and Fa. At distance C the 

 energy by surface and subsurface paths arrives simultaneously. For lesser distances, 

 the earlier arrival is by the surface path. For greater distances, the earlier arrival is by 

 the subsurface path. The critical angle a depends on the velocities Y\ and 7- which are 

 given by the two graph slopes. Having Yx, Vs, C, and a, the thickness of the upper 

 layer may be computed. 



may so designate the velocity in the upper layer. At a particular dis- 

 tance from the source is some station at wliicli the energy, traveling 

 along the surface with velocity Fj, will arrive at the same instant as 

 that which has traveled down to the surface of discontinuity at ve- 

 locity Fi along the under surface of that layer at velocity F2, to a 

 point from which it rose again to the surface with velocity Fi. The 

 greater distance traveled is just compensated for, by part of the path 

 being traversed at greater velocity. 



We can readily see that to get from one point to another in New 

 York City for short distances we can travel to better advantage by 



