1334 EUGENE McDERMOTT 
possible from mistakes in correlation, but it is necessary in evaluating a seismic 
structure to realize such possibilities. 
The percentage of relative error is here understood as the ratio of the ab- 
solute error in determining the difference in datum between two points to the 
mean depth of the two points. This is obviously a function also of the distance 
between the two points, since lateral velocity variation is a function of distance. 
With this qualification, it is true that,in a small area with many wells where seis- 
mic depth points are close together, the relative error may be only o.5 per cent. 
However, with the usual mile control in an area with few wells, the relative 
error is probably of the order of 1 to 1.5 per cent for the better points. It is of 
indeterminate amount where reflections are poor and correlations questionable. 
Despite the foregoing possibilities of error, it must be reiterated that most 
men who have followed the results of seismic work during the past year are 
convinced of its great value. There is reason to expect improvements in equip- 
ment and technique which will increase the present efficiency. 
In order to obtain knowledge of the velocity variations throughout the 
Mid-Continent region so that correct geologic seismic correlation can be made, 
it is suggested that seismic data at drilled wells be exchanged by companies 
engaged in reflection shooting. This type of codperation would be similar to 
an exchange of well samples. 
Pau WEAVER, Houston, Texas (written discussion received, August 21, 
1931): Mr. McDermott calls attention to the importance of determination of 
the thickness of the ‘‘weathered layer” in accurate reflection work. In many 
places the thickness’ of this layer is greater than that which the geologist calls 
the ‘‘zone of weathering,” and pending a more thorough analysis of the problem 
of the low velocity of seismic waves near the surface, it seems advisable to use 
the expression suggested by McCollum, “surface correction zone,’ because 
for it a “surface correction” is applied to the travel time. This zone has a low 
velocity, even in areas where diluvium, alluvium, et cetera, are thin or absent, 
such velocity extending much deeper than the deepest level of the ground water. 
The velocity is much lower than that of the beds immediately below, even when 
the geologist fails to detect a change. _ 
It is suggested that, as overlying beds are removed by erosion, the lower 
beds thus released from pressure may manifest an “elastic rebound,” and cause 
a change in density, which manifests itself in a greater porosity near the surface 
than in the same lower beds where under greater cover. 
I hope that seismologists will measure the thickness of this ‘‘surface correc- 
tion zone,” and compare the number of feet thus obtained with the number 
determined for the depth to lowest ground water, that is, the zone of weathering 
which the geologists consider. 
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