1318 EUGENE McDERMOTT 
The essentials of this method are illustrated in Figure 4. ‘The re- 
fracting layer indicated may be a part of the top of the salt dome. It 
was mentioned previously that beyond a certain angle of incidence all 
the energy in an arriving disturbance is thrown back as reflected energy 
and none is refracted into the second medium. This angle of incidence 
is known as the critical angle. The ray shown in Figure 4 is assumed 
to be arriving at the contact of the salt and sediments at this angle. A 
part of the energy is consequently refracted along the contact surface. 
Some of this energy returns to the surface and registers on the recorder 
as shown. If the distance between the shot and recorder points is known 
and the velocity in the refracting layer is also known, by measuring 
the time necessary to travel over the path indicated it becomes a very 
simple problem in geometry to determine the depth to the refracting 
salt layer. By placing several recorders in a line at known distances 
from the shot point, called profiling, it is possible to determine the 
velocity of the refracting stratum. If the layer is not horizontal, it is 
still possible, by reversing the profile direction, to determine its velocity 
and at the same time determine the slope. By this procedure the config- 
uration of the top of the salt may be accurately determined. Obviously, 
the method may be extended to any high-speed stratum other than salt 
domes. The thicker and more uniform the stratum, the greater the ease 
with which it may be worked. The method has in fact been extended to 
the determination of the configuration of several successive strata. The 
method is particularly adaptable to such regions as West Texas, though 
less applicable to a region such as Oklahoma. 
FRECOROER 





SHOT REAC RECORDER 
CRITICAL ANGLE 
BPEFRACTING LAYER 
Ms 
Fic. 4.—Path of wave at critical angle refraction as used to determine structure 
by refraction method. 
36 
