GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1333 
the result is equal to the square root of the sum of the squares of the individual 
errors, the probable error from these causes is + .004 sec. or + 25’ at 5,000 feet. 
2. Inability to determine the same point on reflected impulse from record 
to record. 
It is to be noticed that the selection of arrival time of the reflections as 
shown in figure 12 is not the moment of first arrival of the reflected impulse, but 
a point somewhat later. It is difficult to select this same point on the reflected 
impulse in every record. An error of a full period, or about 120 feet in depth, 
is most frequently made, but ordinarily can be detected in contouring. An 
error of a half period, or about 60 feet, may pass unnoticed. 
3. Variation from true time of point selected for arrival of a reflected 
impulse when interference with the preceding impulse occurs. 
In the lower illustration of Figure 12, the second impulse on each trace 
interferes with the preceding impulse and the reflection is selected at the in- 
terference pattern. The position of this point may vary depending on the ratio 
of the amplitude of the preceding impulse to the incoming impulse. Theoret- 
ically this point may vary 0.005 second from the true time. This is partly an 
instrumental characteristic and may change with different seismic equipment. 
4. Discrepancy between time signal of explosion and actual explosion of 
dynamite. 
This may be the result of defective blasting caps or blasting equipment 
and can be easily eliminated. However, such errors may exist for some time 
before being detected. 
5. Errors in time-measuring device of seismic equipment. 
Because of the present excellence of most of the timing instruments this 
error is now negligible. Formerly it was a serious and abstruse error. 
In summary, each of the foregoing possibilities a error might give a false 
position for the crest of a structure. 
III. Insufficient data accurately to correlate reflections from point to 
point. 
A reflected wave shows on the seismic record as an impulse. In appearance 
on the record there is nothing to distinguish an impulse received from the 
Checkerboard limestone from an impulse received from the Dewey limestone. 
If two reflecting horizons occur close together, as shown in Figure 12, this 
“character” of impulse is retained as long as the upper limestone and the 
intervening shale remain constant in thickness. If either of these changes 
appreciably the “character” changes. ‘‘Character” correlation by itself, 
therefore, may be misleading. 
Reflections are ordinarily obtained from limestones in excess of 30 feet 
which are overlain by shale. It is customary to obtain several reflections on 
each record and to correlate these groups of reflections from one position to 
another by means of the intervals. It happens, for no reason known at present, 
that a reflection which is strong on one record may fail to appear on the next. 
The correlation of these seismic logs is similar to the correlation of well logs 
on which some lime tops only were indicated with driller’s log accuracy. It is 
a difficult task where intervals vary within short distances. Where several re- 
flections occur close together, errors in correlations may be made if all reflections 
do not occur on every record. It is impossible to indicate what error might be 
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