GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1313 
mostly reflected as in the previous illustration. B is, of course, a much 
better reflecting point than C, because the wall is much more rigid than 
the rope. It should be borne in mind that at a junction point where re- 
fractions and reflections occur, the sum total of the energy in the re- 
fracted.and reflected waves equals the energy in the original wave. A 
good reflector point, therefore, means one at which a large part of the 
arriving energy is reflected back while a small part travels on as a re- 
fracted wave. 
In the previous illustration we dealt with points of contact between 
two media. Practically, we are concerned with planes of contact between 
solid earth materials of different elastic properties. Though this case is 
obviously more complicated, the same fundamental theory applies. The 
device of the ropes admirably illustrates the occurrence of reflections, 
but does not demonstrate the special case of refractions, of greatest value 
in seismography. 
Figure 2 illustrates a simple condition of a soft low-speed and homo- 
geneous earth material overlying a hard high-speed rock which is also 
homogeneous. The detonation of a charge of explosive at the surface of 
the ground generates an elastic wave of spherical wave front. What 
happens to the energy in various parts of this wave front is to be consid- 
ered. Instead of considering the wave front, it is simpler to use rays which 
are normal to the wave front. There are, of course, an infinite number of 
these rays radiating into the earth from the shot point A. 
As stated previously, the velocity of a wave through any medium 
depends only on the physical constants of the medium, specifically the 
elasticity and the density. It is true that the velocity does depend to a 
certain extent on the coefficient of absorption of the medium, but this is 
in general of second order in importance and is here neglected. The co- 
efficient of absorption determines the loss of energy in a wave caused by 
molecular friction. 
The velocity of a disturbance may be expressed simply: 
yen ee 
D 
where V is the velocity, Z is the appropriate coefficient of elasticity, and 
D is the density. In non-rigid media, such as fluids, E is simply the in- 
compressibility, whereas in rigid media E is a function of both the in- 
compressibility and the rigidity coefficient and may be expressed thus: 
/1+4,K 
Vv D 
V= 
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