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Abstract

It is shown that, up to a certain limiting weight, the seismic amplitude from underground explosions increases in direct proportion to the weight of charge fired. Above this limit the amplitude soon becomes closely proportional to the cube root of the charge weight. It follows that if weights greater than this limit, which is commonly 200–300 lb, are to be fired, it is more efficient to divide the charge into several portions, each well separated from its neighbor. This applies both to surface and to buried charges.

The limiting weight depends on the elastic properties at the shotpoint and on the predominant frequencies in the observed refracted pulse. It may be found by experiment, or less precisely, by calculation. The calculated limit for buried shots is obtained from the equation W1/3 = Vo/(2πKf), where W is the limiting weight in pounds, Vo is the velocity at the shotpoint in ft/sec, f is the highest frequency of interest in cps (say, twice the predominant frequency), and K is a constant which depends on rock type and is usually between 3.5 and 4.5.

It is shown why underwater explosions are usually more efficient than underground explosions, and Table 2 gives these efficiencies as a function of charge weight and frequency band.

Because surface charges generate a large amount of shear-wave energy they may cause the wave SPP to be recorded with large amplitude and may, therefore, present an interpretation hazard.

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