1332 EUGENE McDERMOTT 
Viola which is less than actual relief. If the changes occur in limestones higher 
in the section, a more serious problem is ordinarily presented and error occurs 
which may escape detection. 
2. Inability to make corrections locally for gradual changes in average 
velocity. 
From the Seminole area to a point nearly 80 miles north, the average 
velocity to a depth of 3,500 feet increases approximately 1,000 feet per second. 
This change is a function of sedimentation and is probably the result of increase 
in limestone content of the section toward the north. On the assumption that 
this change is uniform, the use of one velocity chart for calculation of depth 
points in an area four townships long north and south results in contouring on 
an inclined plane and the relative error between points on the north and south 
sides of the area is approximately 150 feet. 
3. Inability to construct a velocity depth curve which will hold for ea 
ing depths to a certain formation on anticlines and in synclines. 
This inadequate knowledge of the average velocity gradient presents only 
a small error unless the relief is great. 
4. The use of a varying shot point to detector distance. 
The average velocity to a certain depth is a function also of the distance 
between shot point and detector. This is a result of change of path by refrac- 
tion. It can be eliminated by using a constant or nearly constant shooting 
distance. 
5. The use of a constant average velocity. 
Where the actual structural relief is great, considerable error may be in- 
troduced. If a velocity is ised which is less than actual, a diminishing in struc- 
tural relief will result. However, under some conditions, a constant average 
velocity affords better results than the use of an increase of average velocity 
with depth. 
6. The assumption of a 2,000-foot velocity for the weathered zone and an 
8,000-foot velocity for the unweathered. 
These are approximations which may be locally in error. 
To evaluate properly the foregoing possibilities of error, it must be noticed 
that only possibilities 1 and 6 might cause a false determination of the high 
point of structure. The other possibilities would distort the shape of a structure 
and give a false impression of the magnitude of the regional structure. 
If. Error in the determination of the travel time of reflected or direct 
impulses may be caused by the following. 
Seconds 
1. Inability to read records more closely than— 
a. Error in reading weathering record time break......... 0.001 
b. Error in reading weathering shot arrival time.......... +0.002 
c. Error in reading time break of reflection record......... +0.001 
d. Error in selecting and reading time of reflected impulse. . +0.001 

To this figure may be added +0.003 which from experience is the probable 
error in the weathering correction method. However, as the probable error of 
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