EXPLOSIVES FOR GEOPHYSICAL PROSPECTING 31 
sary. The character of the pressed explosive material may vary 
widely, however, depending upon the design of the electric blasting 
cap. In some types, only a single charge is employed, while in others 
two loads are used, first, a base charge of a high speed, high strength 
compound, and, second, a superimposed primer whose function is to 
initiate detonation of the high velocity load. The firing circuit con- 
sists of two leg wires, connected within the cap by a very fine wire 
known as the bridge wire. When the leg wires are connected to a 
source of electrical current, the bridge wire fuses and causes ignition 
of the surrounding material. This ignition composition may be either 
a loose charge, introduced into the shell in the same manner as the 
pressed leads, or a solid pellet formed around the bridge wire. The 
waterproofing column is located above the electrical circuit and pre- 
vents water from entering the interior of the cap. The sulfur seal 
fastens the waterproofing compound and leg wires in place. 
In establishing the time vs. distance values in seismograph ex- 
ploration, it is essential that a record be made instantly of the dyna- 
mite explosion which. initiates the earth vibrations picked up by 
the geophones. To obtain this record, the seismograph set-up includes 
a time Circuit and suitable means for breaking this circuit simultane- 
ously with the dynamite explosion. 
Several methods are employed for producing this time break. In 
one of them a wire is wrapped around the dynamite charge and is 
broken by the explosion. This system has one very desirable feature 
in that the time and electric blasting cap circuits are entirely separate; 
therefore, the speed of the cap is not a factor in the time values. On 
the other hand, such a procedure necessitates two sets of wires leading 
to the dynamite charge. This is disadvantageous to a certain extent, 
since, with the reflection type of seismograph work now almost uni- 
versally employed, the relatively deep shot holes are at best none too 
easy to load and the use of two circuits increases the possibility of 
broken wires and poor connections. 
The second time break method utilizes two electric blasting caps. 
One detonator is lowered into the shot hole with the dynamite charge 
and serves only to initiate its detonation. The second electric blasting 
cap, which is hooked in series with the first cap and is placed on the 
ground adjacent to the hole, is brought in firm contact with a wire 
connected in the time circuit, this wire being broken when the shot is 
fired. This procedure, which has the disadvantage that two caps are 
used for each shot, requires, for proper performance, that there be 
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