REFLEXION OF ELASTIC WAVES 149 
point was 500 feet distant from the nearest seismometer and 1,100 feet 
from the most remote. 
Figure 10 (0). The disposition of the shot point and seismometer 
spread is exactly as in Figure 10 (a), but now the shot point is located 
up dip from the recording unit, resulting in reflected events displaying 
a much slower apparent velocity of propagation. 
Figure 10 (c). Here the shot point is located midway between the 
outside seismometers. It is obvious that under these conditions events 
reflected from horizontal strata will arrive simultaneously at each 
Shot Point Seismmormcter Spresd 
—_—Se= 

tooo" 
Explanation 
200" GEE Calcareous Shale 
== 5] Sondy Shole 
Eevadal Sand 
6000 
Fic. 9.—Sedimentary conditions necessitating ‘‘dip’”’ shooting. 
equidistant seismometer; they fail to do so on the seismogram re- 
produced, indicating that reflecting plans are inclined. The shot point 
was 300 feet distant from each of the outside seismometers. 
The only evidence available on which to determine the dip is the 
time interval between arrival of a reflected event at the first and last 
seismometer. This interval is a function of the average velocity of 
propagation, distance from shot point to seismometer spread, distance 
between outside seismometers and the inclination of the reflecting 
plane. If the first three quantities are known the last can be computed. 
Figure 11 demonstrates a simple graphical method of deriving the dip 
of a reflecting surface from the running times of a reflected event to 
two points on a radial line from the shot point; it is assumed that the 
profile is laid out perpendicular to the strike. 
809 
