APPLICATIONS OF DIP SHOOTING 127 
tion of direction becomes obvious in the profile and the velocity may 
be corrected accordingly. The phenomenon of alternate opposite errors 
in dip is commonly known as ‘‘wash-boarding.”’ 
A more serious velocity error enters where there is a rapid but uni- 
form change of velocity horizontally. This results in a displacement 
of the image point in the direction of increasing velocity. In conse- 
quence, the computed dip of the sediments will be rotated downward 
away from the direction of increasing velocity, as compared with the 
true dip. Therefore, if the velocity in the sediments over a deeply 
buried salt dome increases over the uplift, the uncorrected dip picture 
will tend to exaggerate the structure. Refraction surveys have demon- 
strated that many of the more deeply buried domes produce such 
velocity increases in the overlying sediments, as at Iowa. 
This type of velocity change will produce “‘wash-boarding”’ in 
those portions of the area where the velocities are notably different 
from the standard velocities used for various depths. But the error 
here referred to will not be compensated by the ‘‘wash-boarding,”’ 
as it always rotates the dip downward away from the direction of in- 
creasing velocity, regardless of the direction of the shot. 
In dip shooting it is assumed that the plane of the incident and 
reflected wave-path is vertical. Obviously, this is true only when the 
shot point and seismometers a= lined up directly along the direction 
of dip. If they are lined up in any other direction the plane of the wave 
path lies not vertical but at right angles to the reflecting bed. The 
dip measured is then the component of the dip in that plane rather 
than in a vertical plane. The error introduced by this inclination of the 
reflection plane, generally known as “‘side-swipe,”’ is negligible in the 
case of low dips, but where the dip is steep and the strike variable, it 
might produce a perceptible misclosure. This would be especially 
likely if the form of the closed traverse departed far from a parallel- 
ogram. In that case the errors made by shooting diagonally across the 
dip downward would not be compensated in shooting diagonally 
across the dip upward on the opposite side of the traverse, or vice 
versa. 
A factor which might be expected to control the amount of mis- 
closure is the spacing of the dip determinations. In drawing an ephem- 
eral horizon the dip between the points where it has actually been 
determined must be interpolated. This interpolation is, of course, 
based on the assumption that no radical variations in dip occur be- 
tween the actual determinations. This assumption is obviously a 
possible source of misclosure, and accordingly, closer spacing of dips 
should tend to reduce misclosure. 
649 
