76 B. McCOLLUM AND F. A. SNELL 
TABLE I. Showing velocity of sound parallel and normal to bedding. 

Velocity parallel to bedding Velocity normal to bedding 
planes. planes. Ratio 
(Meters per sec.) (Meters per sec.) 
4450 3205 1.39 
4655 3310 1.41 
From the above figures it will be seen that the velocity parallel to the bedding 
is in the neighborhood of 40 percent higher than it is in a direction at right 
angles. 
The second type of evidence which shows the existence of this asymmetri- 
cal effect is the difference in apparent interval velocity found in shooting 
up and down the dip of stratified rocks which have been tilted out of the 
horizontal position. Before presenting the field observations which illustrate 
this aspect of the problem, let us consider what theoretical explanation can be 
advanced to account for velocity variations as produced by dip. 
It is admittedly very difficult to make an exact analysis of wave paths in 
a non-homogeneous medium like shale, and we do not attempt to offer any 
comprehensive solution here. Even a preliminary study of the problem, how- 
ever, suggests that in addition to the refraction produced by more or less 
alternating layers of different velocity, there is also refraction due to increase 
of velocity with depth in most cases. Furthermore, it seems logical to suppose 
that considerable absorption of the wave energy would occur when the wave 
path cuts across the bedding, and that this absorption would tend to dis- 
appear when the path parallels the stratification. . 
Consideration of these and similar factors affecting the travel of the sound 
wave leads to the conclusion that we can make a useful approach to the prob- 
lem by studying some of the simpler typical subsurface conditions such as 
are often met with in the field. From these simple cases it should be possible 
to extend the analysis to more complicated conditions with the aid of further 
experimental work. It is to be hoped, therefore, that additional data bearing 
on the subject will be forthcoming from localities where the dips are more 
accurately known than in many areas which the authors have investigated. 
To return, however, to those instances where it can be readily seen that 
dipping strata would give rise to asymmetrical velocities, the three clearest 
cases, perhaps, are the following: 
CASE 1 
The first example in which dip will obviously give rise to velocity asym- 
metry is illustrated diagrammatically in Fig. 4. Almost any stratified forma- 
tion has occasional hard layers which give a somewhat higher velocity than 
the average. These layers may be of the nature of “markers” which are readily 
distinguished by some peculiarity, or they may be almost unrecognizable to 
the eye while still possessing abnormal velocity characteristics. Such layers 
act as conductors for the sound wave because they offer the path of least 
time. 
220 
