LONGITUDINAL SEISMIC VELOCITIES 7 
A list of velocities in limestone at a depth of 3,000-5,000 feet is 
given in Table IV. Due to the sources of inaccuracy previously men- 
tioned, these velocities may be in error by as much as 1,000 feet per 
second. 
The curve CC (Fig. 1) shows a velocity through calcareous Permian 
sediments to a depth of 2,000 feet of 10,500 feet per second, as against 
8,500 feet per second obtained from the average non-calcareous 
Permian section of this depth. Likewise, the differential velocity on 
curve CC from 2,100 to 3,500 feet is 13,300 feet per second. This is 
a section of Pennsylvanian beds of 14 per cent limestone and the re- 
mainder of the section composed of calcareous sands and shales. A 
normal section of Pennsylvanian sediments containing 14 per cent 
limestone and the remainder non-calcareous sand and shale gave 
a velocity of 11,500 feet per second. A section of 665 feet of these 
normal sediments plus 735 feet of Pennsylvanian limestone gives 
the velocity equivalent of the calcareous section. 
DISCUSSION 
The results obtained show conclusively that the velocity of non- 
calcareous sections of shale and sandstone increases with the geologic 
age of the section. That a similar condition holds for velocities in 
limestone seems to be indicated. 
Figure 3 showsthe rateof increaseof the velocity in a Pennsylvanian 
section of fixed age with the depth of that section. A comparison of 
the curve with the Pennsylvanian curve of Figure 2 shows that these 
two curves agree fairly well. This suggests that the two major causes 
of variation of velocity in fairly thick sand and shale sections are 
age and depth. If this is correct, we may consider the group of curves 
of Figure 2 as being representative of the rate of increase of velocity 
with depth in shale and sandstone of any given age. It might be 
expected that the more recent beds, being of lower velocity, would 
exhibit a greater rate of increase of velocity than the older beds. This 
appears to be true of the velocities from Cretaceous to Devonian 
which show some tendency toward approaching the Devonian veloci- 
ty-depth curve asymptotically. However, the Eocene and more es- 
pecially the younger sediments show not only a lower rate of increase 
of velocity with depth but also show a lower percentage increase. A 
possible explanation for the slower rate of increase of velocity in these 
younger sediments may be found in the manner of their deposition. 
In the wells where velocities were measured, these beds were com- 
posed mostly of land sediments. They are less tightly cemented than 
the older sediments which are chiefly marine. It might, therefore, 
667 
