122 SITDON HARRIS 
charge of dynamite used to produce the shock waves. The buzzer 
break at the extreme left end of this line indicates the exact instant 
the charge was fired. The second line from the top has no significance, 
but the third line is the actual seismogram; i.e., it is a record of the 
velocity of the motion of the earth’s surface in the vertical direction. 
The arrival of the first shock is plainly indicated. The vertical marks 
are time marks put on the record by a chronometer. The interval be- 
tween marks is one-fifth of a second. By careful interpolation between 
these marks it is possible to read the time interval between the firing 
of the charge and the arrival of the first shock at the seismograph to 
.oo5 of a second. This time interval for the particular record shown in 
Figure 1 is indicated on the record as 2.322 seconds; the last figure, 
of course, may be anywhere between o and 5s. 
The usual number of seismographs employed by a party is four. 
These instruments are placed at various distances from the shot point 
(the point where the dynamite is discharged), and the charge is fired; 
then the instruments are moved to new locations and another charge 
is fired. This process is continued until records are finally obtained 
with the instruments as far as six kilometers from the shot point. 
Cases are known where instruments were placed as far as 15 kilometers 
from the shot point and charges of 2,000 pounds of dynamite have been 
used. In one case, a charge of this magnitude was fired in the Gulf of 
Mexico, and the resulting shocks were recorded by instruments 15 
kilometers away on the shore. 
The records obtained are usually numbered A, Az, As, Aa, for the 
first shot; B,, Be, B3, Ba, for the second shot, and so on. For the first 
shot, the instruments are usually placed 50 meters apart and a 5- 
pound charge of dynamite is exploded; thereafter the stations are 500 
meters apart and successively increasing charges of dynamite are used. 
Figure 2 illustrates a typical profile obtained from seismic results shot 
over parallel, horizontal strata of successively increasing velocities 
of transmission of shock waves. The lower part of the figure is the 
profile of the stratification, while the upper part is the plot of the time 
travel curves obtained from the seismic data. The circles at each end 
of the surface represent the shot points Nos. 1o and 11, and the little 
triangles on the surface represent the various locations of the seismo- 
graphs. The distances of these stations from their respective shot 
points are very accurately determined, and the time-travel curves are 
obtained by plotting the time required for the first shock to arrive at 
each station against the distance of this station from the shot point. 
It will be noticed that the curves obtained are a series of straight lines 
of different slopes. If the laws of optics are assumed to hold, it is easy 
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