This study utilized seismograms from the U.S. Geological Survey seismological station near Ely, Minnesota, at which 39 delay-fired explosions at the U.S. Steel Minntac iron mine in Mountain Iron, Minnesota, were recorded. The waveforms were analyzed in both the time and frequency domains to characterize amplitude and energy levels as recorded over the regional travel path. The measurements were then related to delay-firing parameters as furnished by Minntac.

The seismograms were filtered in five one-octave bands from 0.5 Hz to 16 Hz, and peak velocities were measured for the P and the Rg/Lg arrivals in each frequency band. Peak velocities for each phase in each frequency band were plotted against total yield, yield/delay period, yield/hole, and spall. Only highly scattered, poor correlation between peak time-domain measurements and any blast parameter could be observed.

Spectral amplitude modulations (scalloping) in the 0.3-3.0 Hz range were observed for all of the recorded mining explosions. Each spectrum also showed an unusual increase in P-wave spectral amplitude with increasing frequency beginning at about 3 Hz.

Using blast parameters provided by the mine, a linear superposition model successfully predicted the major features of the observed spectra, including the existence and variability of the spectral modulations, which in turn account for the poor correlation of peak ground measurements with yield. The model results also indicate that the unusual increase in P-wave spectral amplitude beginning at about 3 Hz may be due to the high P-wave velocity in the area being mined.

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