GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1321 
SHOT __suRFAceE FPECOROER 
POINT OF 
a REFLECTION 
Vi 
I'ic. 5.—Path of reflected wave. 
where Z is the depth, | is the average velocity from the surface to the 
reflecting stratum, 7 is the time of travel mentioned, and X is the dis- 
tance between shot and recorder points. 
The average velocity through the medium above the reflecting layer 
is a function of the depth of this layer. As a result, the path of the dis- 
turbance is not a straight line as shown in Figure 5, but is curved as shown 
by the full line in Figure 6. For simplicity the straight path is used in 
all of the diagrams. The average velocity may be accurately determined 
by lowering a recording instrument into a well and shooting charges on 
the surface. A velocity so determined may be used in a large area. Asa 
39 
