£330 EUGENE McDERMOTT 
shown in Figure 11. The profile may then be reversed, further to check 
the fault. As a result of the abrupt change in length of the reflection 
paths due to the fault, the reflections on the record show a similar abrupt 
shift. 
The average amount of dynamite used to obtain reflections is ap- 
proximately 3 pounds per shot. In some places the cost of electric blast- 
ing caps may actually equal the cost of dynamite, but generally the cost 
is so per cent of the dynamite cost It may safely be predicted that in 
the near future the average charge of dynamite for depths of 6,000 feet 
will not be more than one pound. Even at present the cost of dynamite 
is only 3. per cent of the total operaing costs. As a result of the small 
charges necessary, damages resulting from the explosion are negligible. 
As previously mentioned, the esstial problem is that of identifying 
the reflections from shot point to shot doint. In those places where there 
is but one outstanding reflection, thi: is a simple matter. Generally, 
where there is more than one reflecting stratum, the group of reflections 
has a characteristic appearance, as shovn by the group of two reflections 
obtained from each of two shot points vhich were located a mile apart 
(Fig. 12). The vertical lines on these records mark off hundredths of a 
second and, by interpolating, the arrivalof the reflection pulses may be 
determined to 0.001 second. The four taces are actuated by the four 
recording instruments already described. Although the frequency indi- 
cated in Figure 12 is 50 cycles, this may vary between 15 and 100, de- 
pending on the nature of the surface and sibsurface rocks and the depth 
of the reflector. 
DISCUSSION 
B. B. WEATHERBY, Tulsa, Oklahoma (writen discussion received, June 
15, 1931): This paper should be of considerableinterest to all geologists, but 
particularly to those of the Mid-Continent area. As a group, Gulf Coast geol- 
ogists are better acquainted with the method, nainly because it was forced 
upon them some years ago after most of the surfae had been worked for dome 
indications. 
Very little has been published previously ‘oncerning reflection work, 
which is now becoming better known in the Mid-Gntinent area. As a matter 
‘of fact, until recently, many geophysicists denied he existence of reflections. 
It was natural that after several notable successes he method should come to 
the front. Inasmuch as the seismograph is a tool b which the geologist can 
greatly increase his success, particularly when his uual methods are becoming 
more and more difficult, it behooves him to learn alhe can about the compar- 
atively new reflection method. 
This article is a good beginning in this direction As the energy paths in 
seismology are very similar to those in optics, it is dvisable to read also the 
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