APPLICATIONS OF DIP SHOOTING 123 
of straight lines intersecting approximately at the center of interest. 
This was the first type of survey originated, and is still used by many 
operators. 
Shot points and recording positions alternate along some conven- 
ient straight line, so that records are taken on each geophone position 
from adjacent shot points. Using an appropriate velocity, the in- 
dividual reflections are plotted on a one-one scale. After beginning at 
some convenient depth the profile of a “phantom” horizon is projected 
by use of the dips from near-by reflections. The elevations of this hori- 
zon are carried from one profile to another at the points where the 
traverses intersect, and the even hundred-foot elevations of the 
horizon are spotted on a map. By use of a contour interval of ap- 
proximately 1oo feet, the structure is then contoured on the resulting 
phantom horizon. 
Such a survey is simple, and rarely leads to complications in inter- 
pretation, since with no closing traverses any error simply results in 
freak contouring. A simple survey of this type is quite satisfactory for 
symmetrical, unfaulted structure. 
While, as pointed out later, errors inherent in the dip method tend 
to cancel each other over large areas, for accurate detail small closed 
traverses must be used, and the error in each must be distributed 
around it. We have therefore borrowed the torsion-balance technique, 
such as described by Roman.‘ This distribution of the apparent errors 
must precede any attempt at contouring. It can generally be made by 
inspection, or the whole traverse least squared by any one of a num- 
ber of methods developed for use in torsion-balance surveys. 
STATISTICAL ANALYSIS OF ERRORS 
One hundred fifty-one closed traverses shot by the dip-reflection 
method, having a total length of 786 miles and involving 2,458 dip 
determinations, were used for the investigation. On these traverses 
the difference in elevation of an ephemeral horizon had, in large part, 
already been computed from the dips. The differences in elevation 
were summed up in a clockwise direction. An increase of elevation in 
that direction was given a positive sign, a decrease a negative sign. 
The residual found on closing the traverse has been termed the “‘mis- 
closure.”’ It is designated in feet and carries the sign resulting from an 
algebraic addition of the differences in elevation around the traverse. 
The misclosure of each of the traverses used has been plotted with- 
4 Irwin Roman, “Least Squares in Practical Geophysics,” Geophysical Prospecting 
(1932), P. 491. 
645 
