LONGITUDINAL SEISMIC VELOCITIES 3 
obtained for the first 2,000 feet of section, these curves do not indicate 
clearly the velocity of the section between 1,000 and 2,000 feet deep. 
This can be obtained for a vertical path by taking the difference in 
times of arrival of the energy at the 1,000- and the 2,000-foot depths 
and dividing it into the difference between the two depths. This 
velocity is termed the differential velocity of that particular section. 
On the other hand, the average velocity of the section is measured 
from the surface to any given depth. 
It is the differential velocities which are significant in showing 
the manner in which velocity varies with depth. The results given in 
this paper are differential velocities and are determined from actual 
measurements rather than from a smooth curve drawn through the 
velocities measured. For the most part, these differential velocities 
represent the velocity of a section about 1,000 feet thick. 
To compare velocity in sections of different geologic age, it is 
necessary that the sections differ from one another as little as possible 
in all respects except age. Sections of shale and sand were used chiefly 
for comparison. A section logged as ‘‘caicareous”’ will give velocities 
considerably higher than a normal shale and sand section. Curve 
CC (Fig. 1), for example, is a curve of average velocities in Permian 
and Pennsylvanian beds of calcareous sand and shales. The velocities 
observed are considerably higher than those in the normal Pennsy]- 
vanian section represented by BB. This example discloses the need 
for a careful selection of similar sections so that comparisons of 
velocities may be significant. 
As is shown later, the amount of overburden on a given section 
influences the velocity of that section. This fact requires that com- 
parisons of velocity in sections of different geologic age be made under 
the same conditions of overburden. 
The data available on limestone velocities is scarce and subject to 
greater errors than the shale velocities. Velocities of limestone at the 
surface have been measured by short refraction profiles on the lime- 
stone outcrop. These results are open to some question, however, 
since some alteration in velocity is to be expected due to the weather- 
ing of the limestone at the surface. 
Measurements of limestone velocities in wells are difficult. Most 
of the limestone sections available for measurement being less than 
400 feet thick and the velocities being high, the error in velocity deter- 
minations is considerable. It was not possible to secure a comparison 
of velocities of limestone sections at exactly the same depth below 
the surface. 
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