74 B. McCOLLUM AND F. A. SNELL 
fails to reveal the subsurface picture. Fig. 3 shows a typical case of this 
geology. Here the shale, 1, in which there exist more or less definite bedding 
planes, has been folded into a definite structural relief. Subsequent to this 
folding, extensive erosion took place leaving a surface shown by the line 2-3, 
and on this surface later deposits, 4, were laid down. In general, under these 
conditions, the shale, 1, practically always has a higher velocity than the 
recent deposits, 4. However, if we attempt to apply the simple method above 
described, we shall derive not the structural picture but merely a poor ap- 
proximation to the erosional surface between the shale and the recent de- 
posits. To further increase the difficulty there is often no definite reflecting 

eS 
—_—_—_ ——__. 
Fig. 3. Anticline hidden under unconformable beds. 
surface at greater depth which can be used to define the structure by reflec- 
tion shooting. Under such conditions the true structural picture cannot be 
revealed by any method of subsurface exploration heretofore published. These 
conditions are very frequently encountered and the purpose of this paper is 
to describe a method of exploration that is often capable of giving a correct 
structural picture where this situation exists. This method has been called 
“dip shooting” to distinguish it from the slope shooting heretofore used by 
geophysicists. 
ORIGIN oF Dip SHOOTING 
The method of profiling stratified formations which this paper outlines 
does not supercede any other method, but is simply a special development 
which is useful and applicable in certain cases which cannot be handled in 
other ways. The method is based upon an asymmetrical velocity effect which 
is analogous to the slope asymmetry which forms the foundation of refraction 
profiling. This “dip effect”, as it may be called, has been observed and util- 
ized in places as widely separated, geographically and geologically, as Canada 
and Venezuela. So far as our observations show, the phenomenon is common 
to most stratified rocks and even to some which are not ordinarily classed as 
stratified. 
Although the dip effect is analogous to the asymmetry produced by slope, 
it may arise from a different cause. The velocity asymmetry associated with 
stratification is often due to the effect of the bedding planes, dipping with 
respect to the surface, and not to a sloping subsurface. 
On this fact depend two important corollaries: First, the method neces- 
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