THEORY OF SEISMIC PROSPECTING 41 
increasing time in a uniform manner. This picture is a good one to 
keep in mind as it involves merely the propagation of the wave front. 
OUTCROP 

1SO-TIME CT SS 
= SS 
CRITICAL ELLIPSE 

Fic. 4 
In Fig. 4 we have shown the conic section K inside of which the 
iso-time curves are circles and outside of which the iso-time curves 
are conic sections for the case a= 30°, 3=20°, and H= 1000 feet. 
285 
