ASYMMETRY OF SOUND VELOCITY 79 
The reader’s attention has already been called to the complex nature of 
the problem under discussion. It is well to emphasize again, however, that 
many factors affecting the velocity of sound in stratified rocks have not yet 
been mentioned. Indeed, many factors doubtless exist which have not yet 
been recognized. Therefore, there is still room for much experimental in- 
vestigation along these lines. At the present time, lacking a theoretical ex- 
planation which covers all phases of the phenomenon, the existence of the dip- 
effect and the validity of its use as a working method are best evidenced by 
numerous field observations; a few of which follow. 
EXAMPLES OF THE Dip EFFECT 
TaBLE II]. Material: Tertiary shale in Venezuela. Shooting distance: 1500 meters. Station spacing: 





250 meters. 
NW Shots SE Shots 
Stations Velocities Stations Velocities 
112-111 2450 106-107 2660 
111-110 2360 107-108 2720 
110-109 1980 108-109 3120 
109-108 2080 109-110 2600 
108-107 2320 110-111 2500 
107-106 2400 111-112 2720 
106-105 2400 112-113 2200 
105-104 2600 113-114 2180 
Average 2324 2588 

The velocity values shown in Table II were recorded in the course of 
surveying a line which was laid out approximately at right angles to the 
general strike of the area. The total distance covered by the stations is two 
kilometers, and the stations have been so chosen as to represent the same 
portion of the subsurface in the northwest and southeast shot groups, after 
making allowance for the probable amount of throwback of the reference 
point. The shale, in this case, was covered by clay, but the evidence of various 
nearby test wells pointed to the cover being fairly uniform in thickness. Con- 
sequently, the difference in the velocity averages indicates a regional dip to the 
southeast. The variations of the individual velocity values show the effect 
of local folding. 
We have already mentioned the fact that some relatively unstratified 
formations show asymmetry due to dip. The following data were gathered in 
areas of indistinctly bedded sands and clays in Western Venezuela. 
The values in Table III cover a distance of ten kilometers with an allow- 
ance for throwback as in the case of Table II. The difference between the 
average velocities is small, but in view of the circumstances a larger differ- 
ence would hardly be expected. These shots were taken at a distance of about 
_ twenty kilometers out from the foothills of the Perija mountains. The dip 
represented by the averages is simply the normal depositional dip of water 
borne sediments laid down along the mountain front. The dip is toward the 
223 
