APPLICATION OF REFLECTION SEISMOGRAPH 35 
PROCEDURE 
As the trend of any faults that might exist wasassumed to be the 
same as that of the field on the south, east-to-west lines of depth de- 
terminations were laid out, taking advantage of roads available. When 
the data indicated the presence of a fault, the layout was changed so 
as to increase the density of shot locations in the neighborhood of the 
fault and follow its trend. The program was essentially of a recon- 
naissance nature. 
CORRELATION OF REFLECTIONS 
Two dominant reflections appear on the records. The interval be- 
tween them corresponds to that between the top of the Pecan Gap 
chalk and the basal Austin. A standard Cretaceous velocity chart was 
used in the computation of the depth determinations. Although this 
could not be expected to give correct absolute depths beyond approxi- 
mately roo feet, as it was constructed from well determinations in 
another part of East Texas, interval and relative-depth determina- 
tions made from it could be relied upon to be correct to 0.5 per cent. 
The two contour maps show the interval between the Pecan Gap and 
the Austin to be approximately constant but indicate some regional 
variation. 
A typical reflection record showing the relation of the reflection 
events to the geologic section is shown in Figure 1. The fact that a 
dominant reflection is obtained from near the base of the Austin chalk 
would indicate that the basal member of this body of chalk is consider- 
ably harder than the rest of the chalk section. Occasionally a reflection 
is obtained from the upper part of the chalk section, but it is never as 
pronounced as the basal reflection nor does it appear on the records 
with any regularity. In view of the dominant character of the two 
reflections and the regularity of interval between them, the possi- 
bility of miscorrelation is extremely small. 
The two contour maps, Figures 2 and 3, were constructed from 
reflection depth determinations entirely. No well data were used. Al- 
though the contour maps may not check similar maps made from 
well data in absolute value, they should check as regards relative data. 
Most of the wells drilled in this area are shown on the contour maps. 
A cross section showing two faults is presented in Figure 4. This 
cross section is along line AA indicated on the contour maps. The re- 
flection records from which the depth determinations were made are 
shown opposite the cross sections. Each record corresponds with the 
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