1320 EUGENE McDERMOTT 
If, in the method of profiling mentioned, the distances from the shot 
to the recorder points are plotted against the times of arrival for these 
distances, the graph shown in Figure 4A results. Three lines are shown 
on this graph. The points falling on the first line are the directly trans- 
mitted disturbances, whereas the second and third lines are the, result of 
refractions from each of two high-speed strata. The slopes of these lines 
represent the velocities in each of the three media. 
There is at the surface a thin layer of weathered sediments the ve- 
locity of which is very low. It is necessary to determine the thickness of 
this and the elevations of the shot and recorder points in order to secure 
the greatest accuracy. 
REFLECTIONS 
As mentioned previously, refractions and reflections are closely 
related. One phenomenon rarely occurs without the other. Therefore, 
it is merely necessary to use the proper instruments and technique in 
order to separate and make usable the reflected energy that manifestly 
must be present whenever a disturbance is initiated. Reflections have 
been identified and have proved more valuable and certainly have per- 
mitted greater accuracy than refractions. In the refraction method, 
only the initial arrival of energy at the recorder was used, because other 
than initial arrival of energy is more difficult to interpret. The use of 
reflections, therefore, obviously means a refinement of instruments and 
method. 
In the refraction method the distance between shot and recorder 
points is. several times the depth of the refracting horizon, but in the 
reflection method this distance is only a fraction of the depth to the 
reflecting stratum, as shown in Figure 5. Here is shown the path of a 
reflected disturbance. It should be noticed that the projection of the 
point of reflection on the surface is midway between the shot and recorder 
points when the reflecting layer is horizontal. This satisfies the law of 
reflections that the angle of incidence a must equal the angle of reflection 
b. By measuring the time between the explosion of the charge and the 
arrival of the reflected disturbance at the recorder and knowing the 
average velocity through the medium, the length of the travelled path 
may be determined. By measuring the distance between the shot and 
recorder points, the depth may easily be computed as follows: 
==, N/T IE 
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