1324 EUGENE McDERMOTT 
mined as shown in Figure 6A. The method is not very accurate, as 
second order quantities in time are being used. All of these methods, 
however, combined with experience, permit a fair estimate of the ve- 
locity. 
The accuracy of the method requires that allowance be made for 
the thin weathered layer at the surface, as shown exaggerated in Figure 7. 
A representative thickness for this weathered layer is approximately 30 
feet. As the velocity in it is very low—z2,o00 feet per second as com- 
pared with 8,coo in the unweathered layer below it—it can not be av- 
eraged with this layer but must be separated, as it varies considerably 
in thickness. This is accomplished by shooting a small charge at the 
shot and recorder points. The thickness of this layer and the time con- 
sumed in it are thus determined by the refraction obtained from the un- 
SHOT RECOROER 
WEATHERED LAYER 
PPEFLECTION 
POINT 
Z Ui 
Fic. 7—Weathered layer (exaggerated). 
42 
