Large mining blasts can complicate the identification and discrimination of small underground nuclear explosions and may offer evasion opportunities. Mining blasts typically show a unique spectral signature: spectral reinforcements associated with time-delayed detonations between adjacent shot holes or rows of shots. Discrimination of a nuclear detonation that is simultaneous with a mining blast must depend upon recognizing significant spectral or waveform abnormalities within seismic signals from the mining blasts. In this investigation, large, simultaneous detonations within mining blasts are simulated for observed explosions from the Mesabi Range in Minnesota and for a series of quarry blasts at the Kaiser Permanente Quarry in Cupertino, California, which included a simultaneous detonation conducted by Lee et al. (1989). The Mesabi explosions are examples of large, ripple-fired blasts with known blast patterns (Smith, 1989).
The models suggest that a large, single, deeply buried explosion dominates the waveform signature if it contains more than 5 to 15% of the total explosive in the mining blast. Spectral signatures of these combined explosions still show periodicities characteristic of ripple firing; however, their amplitude is greatly reduced. Inclusion of a deep simultaneous shot accentuates the high-frequency spectrum. If single explosions are sufficiently close to the combined quarry blast, their application as empirical Green's functions can isolate the simultaneous explosion within the blast. If empirical Green's functions are within 0.5 km of quarry blasts, individual explosions can be retrieved if delays are 100 msec between shot holes and signals extend to 40 Hz. Identification of large, simultaneous detonations within a blast may depend upon knowledge of the mine's blasting practices and its variability from blast to blast.