GEOLOGICAL APPLICATION OF SEISMOGRAPHY 1331 
chapters on the refraction and reflection of light in any standard physics text 
book. 
Where it can be used, the reflection method has one great advantage over 
the fan shooting as it was conducted in the Gulf Coast. The older type of coast 
shooting denoted only the presence or absence of salt domes. Consequently, 
most of the data obtained were valueless to the geologist. The refiection 
method, however, gives datum figures on specific beds, and, in conjuncticn with 
existing subsurface control, permits the geologist to determine the geology of 
large areas where, at present, the best that can be done is to interpolate between 
wells. Consequently, it not only is a great asset in the discovery of petroliferous 
structures, but it is also increasing geological knowledge to a marked extent. 
G. H. WEstBY, Bartlesviile, Oklahoma (written discussion received Au- 
gust 20, 1931): During the past year many Mid-Continent geologists were 
called on to re-examine their determinations of formation datums in wells to 
permit a closer correlation with seismic reflection data. Errors were found 
in some determinations or crooked holes were suspected. Most of the seismic- 
geologic correlations could not be improved. It is probable that some of these 
men who feel certain of most of their well datums would not agree with Mr. 
McDermott’s statement that generally the relative accuracy between two 
datums obtained by the seismic method is approximately 0.5 per cent. Despite 
this disagreement, and fully realizing the possibilities of error in seismic work, 
most men who have worked with the reflection method feel that it is second only 
to the use of well samples for the determination of subsurface structure. With 
this in mind, it seems pertinent to append to Mr. McDermott’s excellent paper 
a short discussion of the present possibilities of error in the reflection method. 
Most of the relative errors in the determination of formation datum points 
by the reflection method are results of the following causes. 
Me Use of erroneous average velocity in calculations 
lI. Errors in the determination of the travel time of reflected or direct 
impulses 
III. Insufficient data accurately to correlate reflections from point to 
point 
I. The use of erroneous average velocity in calculation may be caused 
by several conditions. 
1.. Inability to detect and allow for rapid variation in average velocity. 
The average velocity to a certain stratum is a function of the stratigraphic 
section above that stratum. Abrupt lateral variation in character of beds or 
variation in thickness of limestones affects the velocity. Though variations 
in velocity from this cause are less than might be expected, it remains as a source 
of appreciable error. 
Seismic Viola datums are in many places affected by changes in the thick- 
ness of the Hunton or Mississippian limestone. For example, a change in the 
thickness of the Hunton limestone from 100 to 300 feet between two seismic 
stations introduces a relative error of approximately 60 feet in the Viola datums 
of these points. If the top of the Mississippian or Hunton affords a good re- 
flection, the influence of the thickness of these limestones on the velocity to 
the Viola can be appreciated and partly eliminated. Where the Mississippian 
limestone thins over structure, the ordinary effect is to indicate a relief on the 
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