28 G. H. LOVING AND G. H. SMITH 
pedoes. Less powder was consumed in these methods of loading and 
shooting, but no additional explosive requirements were introduced, 
except that of suitable water resistance. Subsequently, the reflection 
method of prospecting, employing deep holes and relatively small 
charges, has become very popular and, at present, is in almost uni- 
versal use. 
In the reflection system of seismic exploration, the shot holes 
are generally 3 to 5 inches in diameter and from 25 to 500 feet deep, 
the usual depth being approximately 75 feet. In following the normal 
shooting practice, the explosive charge may vary from an electric 
blasting cap alone to 100 pounds of dynamite, but generally ranges 
from 1 to 10 pounds. Several charges may be fired in the same hole to 
obtain check records or to secure data on the strata in different 
directions from the shot point. The holes are not cleaned out between 
shots and considerable difficulty is occasionally experienced in loading 
successive charges, due to sand, muck, or stones obstructing the hole. 
Water is used as stemming to confine the shots, the depth of water 
being as much as several hundred feet in very deep holes. Most charges 
are detonated almost immediately after loading, but occasionally the 
explosive may be left in the hole for several days before shooting. 
From this brief description of conditions, it can be seen that this 
type of shooting requires an altogether different explosive, at least 
as regards physical characteristics, than does the refraction method. 
As stated before, refraction shooting demands only that the ex- 
plosive detonate at the desired speed and, in certain instances, be 
of good water resistance. With the reflection method, these two 
qualities are essential and, in addition, the dynamite must be capable 
of shooting under fairly high water pressures, and be of such con- 
sistency or stiffness that the cartridge will not bulge or buckle when 
forced down a hole which has become obstructed from a previous shot. 
The most popular explosive for this work has been 60 per cent 
ammonia gelatin. It detonates at the rate of about 4,700 meters or 
15,400 feet per second; it is very water resistant; and it is entirely 
satisfactory under wet conditions which are not too severe. Until 
quite recently, the 60 per cent ammonia gelatin for the seismograph 
trade was identical with that furnished the mining, contracting, and 
other explosives consumers. In other words, it was the standard 60 
per cent ammonia gelatin packed by machine into light weight, paper 
shells. For machine packing, a gelatin dynamite must be of the proper 
consistency to allow extrusion through a nipple into a shell in much 
272 
