Source Scaling of Contained Chemical Explosions as Constrained by Regional Seismograms

Quantification of source coupling and characterization parameters for contained, single-fired chemical explosions observed at a regional seismic array are quantified. The explosions were conducted in a mine in northeast Wyoming where delay-fired explosions occur regularly. The primary observational data sets consist of close-in measurements in the mine and regional observations at the International Monitoring System Primary array at Pinedale, Wyoming (PDAR). The single-fired explosions ranged in size from 5500 to 50,000 lb and were separated from one another from 30 m to over 4 km. Peak amplitude measures at a single element of the regional array are well matched with a power law dependence on explosive weight. Similar scaling constants were determined for each of the dominant regional phases: P_n, 0.84 ± 0.14; P_g, 0.84 ± 0.09; and L_g, 0.91 ± 0.08. Peak amplitudes across PDAR show a factor of 3 variation for an individual explosion across all array elements. This amplitude variation is accompanied by a decrease in spatial correlation with increasing station separation indicating the importance of near-receiver structure. The regional signals were further characterized by computing frequency-dependent envelope functions at each array element. Consistent source scaling factors at a single array element were obtained from the envelope functions. A high degree of spatial variability as a function of frequency was documented for the envelope functions. Spectral ratios are used to investigate source scaling relations for the different explosions. Four similarly prepared shots with total explosive weights between 5500 and 6000 lb were detonated to investigate source repeatability. Spectral ratio analysis identified two of the four shots as only partially detonating, resulting in a reduction of long-period spectral level by factors of 10 and 50. These long-period reductions were accompanied by increases in source corner frequency, consistent with a smaller source size. Spectral ratios of waveforms from the larger explosions that fully detonated illustrate that P_g and L_g source functions are similar. The empirical source ratio is well modeled with a Mueller–Murphy source model. These experiments and the resulting data analysis suggest that simple, single-fired explosions of modest explosive weight can be used to calibrate regional seismic arrays, networks, and individual stations.