1316 EUGENE McDERMOTT 
angle c the angle of refraction. Increasing the angle of incidence finally 
leads to the limiting case beyond which all the energy is reflected and 
none is refracted. Such an angle of incidence is known as the critical 
angle. In this case: 
¢ 
; Vi 
Sine a = V, 
t 
This limiting case is illustrated by the ray AK. The refracted ray AM 
travels along the contact plane between the two media. This particular 
refracted ray is the one so much used when using the refraction method 
for determining structure and is referred to later where that method is 
discussed. 
In actual practice homogeneous strata depicted in Figure 2 are not 
encountered. There is in all cases an increase of velocity with depth, 
as a result of which the rays are slightly curved due, of course, to a grad- 
ual refraction. In the diagrams, with one exception, this curvature is 
neglected for the sake of simplicity. No serious inaccuracy can result 
from this procedure. 
REFRACTION METHOD; SALT-DOME EXPLORATION 
Seismography found its first practical application in the United 
States in the field of salt-dome exploration on the Gulf Coast of Texas 
in 1924. It was a logical and fortunate introduction of a method that was 
later to extend its usefulness to exploring other types of geologic struc- 
ture. The first domes sought were very shallow, as deep domes, later 
discovered, were not at the time known to exist. Naturally those known 
at the time were only the shallow domes which had given evidence of 
their presence by surface indications. 
The problem was ideal for the seismograph. The relatively large 
masses of high-speed salt were intruded in uniform and low-speed sed- 
imentary rocks. The velocity in salt may be almost as much as three 
times the velocity in the rocks which it has displaced. The velocity in 
salt is 16,000 feet per second, whereas the normal velocity in the surface 
sediments may be as low as 5,500 feet per second. Time anomalies of % 
second were not exceptional, because of the presence of the salt plug. 
In the upper part of Figure 3 the path of the initial impulse arriving 
at a recording instrument from the shot point is indicated. The curva- 
ture of the path is caused, as before mentioned, by the fact that the 
velocity in the sediments increases gradually with depth. If there is a 
buried salt dome between the shot and recording points, the first dis- 
turbance to reach the recorder is one which has travelled in part through 
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