GEOPHYSICAL METHODS OF INVESTIGATING 235 



methods of location by means of time-travel curves or 

 course-time curves depend. They also depend very much 

 on the relation of the load to the deformation, i.e., Young's 

 modulus E. The relations of E and a- on the one hand and 

 Lame's coefEcients X and ^ on the other are as follows:^ 



_ a _1 E ^ 1 1 



^ ~ (1 +0(2 - (r)'^'^ 2*l + (r''' 2*X + m' 



E = --^{?>\ + 2,) (16) 



and when disturbances in the interior set up the usual 

 longitudinal waves with velocity V and transverse waves 

 with vibration velocity V of the particles at right angles 

 to the direction of propagation, we have^ 



T/ - / ^ + 2m _ \E 1-0- y, ^ jl, 



^ ~^~~P Vp (1+0(1 -2a)' Vp' 



= iMzm (17) 



Vp 2(1+0 ^ '^ 



Only two equations are presented for determining these 

 most important quantities, E, a and p, or X, fx and p, so that 

 with reliable results a third relation can be found, otherwise 

 suitably complete assumptions must be made. Compara- 

 tively little is known of the vagaries of earth wave motion, 

 especially in the case of artificial earthquakes which are 

 liberated for seismic ground research and borehole surveys. 

 It will be seen that the direct study is very closely allied to 

 seismology as applied to actual earthquakes, but the study 

 of the behavior of purposely produced ground concussions, 

 as by the explosion of dynamite, gives rise to many features 

 which are not recorded of natural or long-distance earth- 

 quakes, such as surface air and sound waves, certain 

 types of strata limit reflection, etc. 



1 ZoEPPRiTZ, K., Repts., Imp. Soc. Sci., p. 66, 1919, and p. 121, 1912, 

 Gottingen ; also Davison, C, " Manual of Seismology," Cambridge University 

 Press. 



* Ambronn, op. cit., p. 152; also Haddock, M. H., Colliery Guardian, p. 

 333, 1927. 



