ASYMMETRY OF SOUND VELOCITY 73 
of profiling found wide application in many parts of the world, and still 
possesses much merit as a means of subsurface investigation in many local- 
ities. Its technique is new the most elementary part of the stock-in-trade of 
the oil seismologist. The procedure for cbtaining a complete cross-sectional 
profile usually consists in shooting a line of detector stations from both direc- 
tions. Generally the overall time of travel of the sound wave is recorded from 
each shot to its appropriate stations, and complementary time-distance 
curves are plotted for the series of shots from each direction. 
Whatever may be the subsequent steps in making use of the primary time- 
distance data, all the calculations must necessarily be based on the deviation, 








Apparent velocity (m/sec. 








0 2 4 6 6 WO 2 
Values of Y (Degrees) 
Fig. 2. Chart for showing relationships between apparent velocities when shooting upslope 
and downslope. Y=angle of slope; Vi taken as 2500 m/sec.; V2 taken as 4500 m/sec. 
or asymmetry, of the apparent velocity relative to the true velocity of the 
rock which is being profiled. This asymmetry of the upslope and downslope 
values is best shown by comparing the station-to-station velocities derived 
by dividing the horizontal distance from one station to the next by the 
interval time, At. Fig. 2 shows the striking asymmetry which results from the 
slope effect in an ordinary case, while in salt dome work it is well-known that 
infinite apparent velocities are not uncommon. 
The above described method, as is well understood, gives the slope of the 
contact surface between the high velocity bed and the overlying low velocity 
deposits. In many cases this contact surface, where it has not been subject 
to irregular erosion, will conform to the structural outlines of the subsurface 
geology. In such cases the simple method above outlined is adequate to give 
the desired information on the subsurface geology. There are many cases, 
however, where this situation does not exist, and in such cases the method 
217 
