IO PAUL WEAVER 
faulting may cause tension at some points, compression at others, 
and through long periods of time the physical constants of the rock 
change accordingly. We have practically no quantitative data on such 
changes as may occur in gentle folding and faulting although there is 
a considerable body of information regarding the effect of high pres- 
sures such as are found in structures in more intensely folded regions. 
A careful study of geophysical surveys, particularly reflection shoot- 
ing and shooting in wells, from areas where the geology is known from 
a number of wells, and where samples are available from these wells 
for study of their physical characteristics, will contribute materially 
to the interpretation of results on new prospects. This study will 
also result in throwing much light on some of the tectonic questions 
which geologists have considered, such as the theory of elastic re- 
bound. Correlation of geophysical results by reflection shooting in 
known oil fields with the geology should be the most obvious and 
simple test of geophysical results because this method is the only one 
generally used which is not a potential function method, and is there- 
fore particularly easy to understand and to interpret from the theo- 
retical side. It is, however, one which experience shows presents fully 
as many problems on the practical side as other methods, due to 
variations in the sedimentary rocks, but these variations are the ones 
where we find the geologists have already collected so much data that 
joint effort should give solutions to many of these practical problems. 
In connection with the comparison of geophysical contour maps 
with geological contour maps based on a number of wells, there is a 
further caution necessary. Although reflection shooting is supposed 
to determine the slope and depth at a point, contours based on a 
number of such determinations are usually smoothed out and are so 
drawn as to give a most probable value to the contour. This is the 
proper procedure, if the determinations are supposed to be made on 
a warped surface, but if the horizon which is being contoured repre- 
sents an unconformity, or a line of irregular cementation, the topog- 
raphy of the surface itself may be fine-textured. Unless the determina- 
tions with the seismograph on such a rough surface are made at 
exactly the same point at which the wells are drilled, there will be a 
chance for differences between the two sets of results at certain local- 
ities equal to the irregularity of the surface which is being mapped. 
These differences do not come about on account of the resolving 
power of the geophysical method, but are due to the fact that sufficient 
points have not been determined either from geophysical survey or 
from well data to give a picture of the real roughness of the surface 
being mapped. 
532 
