146 F. GOLDSTONE 
the surface weathering as understood by geologists is only a fraction 
of the thickness of this low velocity zone. We sometimes have recorded 
a thin surface layer which has a velocity of propagation lower than 
air and presumably is the actual weathered zone. The determination 
of the velocities of all but the upper 100 feet is best made by correla- 
tion with subsurface data obtained by drilling, but it is possible to 
derive an approximation by direct observation if a very long reflexion 
profile is shot; the slopes of the #?—d? curves plotted for reflexions from 
STRATIGRAPHIC COLUMN VELOCITY CURVE 
HUNTON LS 19000 FT / SEC 




7] SYLVAN SH 







Fic. 6.—Typical stratigraphic column and velocity curve in Oklahoma. 
various depths give a first approximation to this velocity-depth rela- 
tionship. Velocities determined by refraction shooting will naturally 
almost invariably be too high. 
Since it is obvious that an error in the assumed thickness of the 
surface zone will reproduce itself in the computed depth to a horizon 
below the Pennsylvanian multiplied by a factor of over six, determi- 
nation of the thickness of this low velocity surface layer is a critical 
factor. This is best done by shooting a small charge close to the last 
seismometer on a spread and computing this zone from the resulting 
observed time-distance curve. Figure 7 demonstrates such a case; 
806 
