Low-Frequency Vibrations Produced by Surface Mine Blasting Over Abandoned Underground Mines

摘要

Complaint-producing blasting vibrations resulting from an active surface mine operating over abandoned mine workings beneath the surface mine and the nearby town of Blanford, IN were studied. Bureau researchers analyzed over 500 blasting records collected over a period of 9 months at 7 Blanford residences. To characterize generated and propagated seismic waveforms, the Bureau set up two extended instrument arrays and monitored five production and two specially fired single-charge blasts. Ground vibrations were of high amplitude and dominated by unusual and more structure response-producing low frequencies (4-8 Hz) than in previous studies at other sites. These conditions resulted primarily from the geologic structure as the vibration propagating medium. Either the natural horizontal layering and/or extensive horizontal room-and-pillar network underlying the region were trapping seismic energy and determining its character. Blast designs were partly responsible, with high level casting and complex multidelayed blasts producing wave interaction and constructive reinforcement. The traditional 8-ms criteria for separating charge weights per delay appears insufficient for this low-frequency site.