REFLEXION OF ELASTIC WAVES 139 
curve between these values. I personally prefer a unit in which this is 
done without resorting to resonance and I believe this has become 
more and more generally accepted. Since the events we are attempting 
to record are transient, it is very desirable that all components of 
a reflexion unit should be aperiodically damped. The elements of a 
reflexion seismograph unit are shown diagrammatically in Figure 2. 
Let us now examine a record obtained from such a unit; Figure 3 isa 
reproduction of a seismogram obtained by a unit designed and con- 
structed by the McCollum Exploration Company, who were one of 
the pioneers in this field. The four traces are produced by four seis- 
mometers at intervals of about 150 feet apart, the first seismometer 
being approximately 1,400 feet from the shot point. A total time of 
1.8 seconds is covered by the portion of the seismogram reproduced. 
The instant of explosion was transmitted by telephone line and is re- 
corded as the first event of the seismogram (A). The arrival of the 
longitudinal wave through the surface beds is next recorded (B), and 
after these have subsided it is obvious that much of the energy being 
recorded is reflected from considerable depths, since the apparent 
velocity of propagation is exceedingly high, indicating that the wave 
front is arriving almost parallel to the surface. This is the best criterion 
for identifying reflected events; consider the burst of energy at C on 
Figure 3 and compare its apparent velocity with the velocity of propa- 
gation of the sound wave in air at D; it will be found to exhibit a 
velocity some sixty times as great, or about 66,000 feet per second, 
which is much higher than the velocity of propagation of any elastic 
wave in rock minerals. This burst of energy must therefore have ar- 
rived at the surface subsequent to reflexion from a subsurface horizon, 
and if we know the average velocity of propagation of the wave in the 
material traversed we can compute the depth to the reflecting horizon. 
FIELD PROCEDURE 
The technique employed in the field varies with the conditions of 
the problem. In Oklahoma where this method was first developed 
to commercial success, the objective is usually to contour the top of 
Mississippian or Ordovician limestones in contact with shale members 
at depths of from 3,000 feet to 8,000 feet. It has been found possible 
to obtain satisfactory seismograms when seismometers are located 
at distances from the shot point ranging from a few feet to over 3,000 
feet; between 1,000 feet and 2,000 feet is a common range, with some 
operators preferring to use almost vertical reflexions requiring that 
seismometers be located very near the shot point. This latter procedure 
is only possible if the “ground roll” is small or if it can be more or less 
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