GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1327 
0.004 or 0.005. This relation of time of arrival is the principal criterion 
for the identification of reflections. The differences in arrival time of 
any other type of wave are considerably greater. For example, the incre- 
ment of time for the directly transmitted wave is approximately 0.92 
second. For refractions from shallow beds to be confused with the re- 
flections would require exorbitantly high velocities in these shallow beds. 
Beds showing such velocities do not exist. 
As a matter of fact there is no great difficulty in identifying retlec- 
tions on the record. Any difficulties that may present themselves are 
ordinarily those caused by the presence of too many retlecting strata too 
close together and the consequent appearance on the record of too many 
reflections. This, of course, can not be altered without altering the sub- 
SHOT FECOROLERS 
Us 
Fic. 10.—Reflections from sloping surface. 
45 
