82 B. McCOLLUM AND F. A. SNELL 
employed to check the location of the recognized folds and to search for others 
which were not exposed on the surface. 
The results tallied with the geological picture in a manner which could 
hardly be questioned. It is not possible, here, to present all the data which 
were gathered, but one example may not be out of place to illustrate this 
special application of the method to exposed or very thinly covered folds 
which have been planed by erosion before being buried. Fig. 6 shows the 
apparent velocities given by shots from opposite directions plotted in the form 
of velocity curves. The line of survey crossed one of the known shale struc- 
tures already mentioned. The cross-section of the fold is shown diagram- 
matically at its proper location near Station 106. The northwest flank of the 
anticline is considerably steeper than the southeast flank. In shooting from 
the NW, note how the velocity rises to an abnormally high value due to the 
steep dip of that flank. A corresponding minimum value is given by the south- 
east shots, and the reverse occurs in both cases on the other side of the fold 
after crossing the crest. Doubtless a part of the velocity asymmetry is due to 
slope effect on the surface of the shale under the clay cover, but repeated ob- 
servations seem to demonstrate that this will not account for all of the 
velocity variation which is found in such cases. 
APPLICATION OF Dip SHOOTING 
The main essential in utilizing the dip effect is to shoot all stations from 
opposite directions. Generally the stations are set along a straight line which 
is laid out, as nearly as may be, at right angles to the strike of the area in ° 
order to get the maximum effect from the dip. Shooting distance, station 
spacing, etc., must be determined according to what is known of the surface 
and subsurface conditions, as with any other method. Often special prelim- 
inary shooting is necessary to reveal this information. 
It is possible that satisfactory results might be obtained from the use of 
overall velocities, derived from the total time of travel of the wave from shot 
to detector, but the authors have always preferred to employ station-to- 
station velocities because they not only localize the dips better, but also bring 
out the velocity asymimetry more sharply. 
In accordance with this practice, then, the interval velocities given by 
the shots from both directions may be tabulated and the complementary 
values compared. It is necessary to make allowance for the throw-back of the 
reference point as is customarily done in all profile work, although here the 
exact amount to allow for is indeterminate on account of the variable form 
of the wave path. The fact that the wave may penetrate to an appreciable 
depth in the shale, and that all parts of the path are generally curved, 
renders it impossible to do more than make an approximate calculation of the 
throw-back, but this will usually suffice for practical purposes. 
The velocity asymmetry observed on any setup may, of course, be due to 
two causes. First, the slope of the contact between the shales and the overly- 
ing formations, and second, the velocity asymmetry in the shales themselves. 
In order to determine and eliminate the former, it is usually best to shoot a 
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