SEISMIC METHODS 891 



to use, since the standard sticks of explosive commercially available provide 

 a wide range of sizes of charges. Charges of this type ranging in size from 

 one-third of a pound to 20 pounds have been effectively used. 



The arrangement and spacing of the charges in the pattern is a function 

 of the area being surveyed and will vary from a few charges arranged in a 

 straight' row to a triangularly spaced pattern of 7 or 13 charges. Figures 

 560 and 561 illustrate some of the patterns which have been used. The 

 spacings likewise will vary from ten feet to more than seventy feet. 



The separate shock waves merge into a single large flat wave of suffi- 

 ciently low energy per unit area. Because of the velocity at which these 

 wave fronts travel, it is necessary that all of the charges be detonated 

 simultaneously. This is accomplished by connecting the caps in series. 



A condenser-discharge type of blasting machine serves very well, or a 

 conventional blasting machine can be used by connecting an 80-micro farad 

 condenser across the generator (not across the terminals of the blasting 

 machine, because the residual charge on a condenser so connected is apt to 

 fire the next charge as soon as the condenser is connected). 



Any arrangement of an offset shot-point can be used in which the dis- 

 tance from the shot-point to the closest geophone is great enough to permit 

 all the desired reflections to arrive 0.3 to 0.4 second ahead of the air wave. 

 The first arrivals are of sufficiently low energy so that a pre-set suppresser 

 circuit is not necessary, and a properly designed automatic gain control will 

 give the desired uniformity in amplitude. If the system includes a pre-set 

 suppresser or mixer operated by the first arrivals, it will usually be necessary 

 to increase the sensitivity of this circuit. A seismometer placed on the 

 ground beneath one of the charges will provide a satisfactory time break. 



Due to the absence of high frequencies as a result of this method of 

 initiating the seismic wave, an increased band width filter setting can 

 be used. 



The air-shooting technique does not produce sufficient ground roll to 

 damage ordinary structures, and only the airborne blast effects need be 

 considered. Due to the horizontal spacing of the charges, the risks involved 

 are greatly reduced. By far the most likely harm to building structures is 

 the breakage of windows. Although any real damage is visible and limited 

 to a relatively short range, the noise, of course, can be heard for a long 

 distance. 



Multiple Reflections 



The phenomenon of multiple reflections is of common occurrence in 

 nearly all forms of wave energy, and is most familiar in light when two 

 mirrors are placed substantially parallel to each other. Since much of the 

 phenomena of geometrical optics manifests itself in seismic prospecting, 

 it seems most reasonable that multiple reflections are to be expected, and 



