Time-Dependent Monitoring of Seismic Wave Velocity Variation Associated with Three Major Seismic Events at a Deep, Narro
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Time-Dependent Monitoring of Seismic Wave Velocity Variation Associated with Three Major Seismic Events at a Deep, Narrow-Vein Mine Setareh Ghaychi Afrouz 1
&
Erik Westman 1 & Kathryn Dehn 3 & Ben Weston 2
Received: 31 January 2020 / Accepted: 1 October 2020 # Society for Mining, Metallurgy & Exploration Inc. 2020
Abstract One of the most difficult challenges in underground mining is to forecast significant seismic events prior to their occurrence in order to support the safety of miners and to minimize damage to the underground operation. Passive seismic tomography is a tool to image the seismic velocity changes in a rockmass, which can potentially reveal the rockmass behavior before and after a significant seismic event. In this paper, the changes in seismic velocity of the rockmass before and after three major seismic events are investigated to determine whether there are identifiable precursory velocity changes associated with the major seismic events. Conventional mechanics imply that the induced stress at the hypocenter would continue increasing until failure, in the form of an induced seismic event, occurs. With passive seismic tomography we can image changes to the seismic wave velocity distribution within a rockmass and hence infer the changes to the induced stress near the hypocenter. This paper evaluates the hypothesis that induced stress at the hypocenter (as inferred by the p-wave velocity) increases until the seismic event occurs. In addition to analyzing p-wave velocity changes near the hypocenter, changes to the p-wave velocity at “zone centers” were also analyzed. These “zone centers” are regions within the rockmass that consistently have a p-wave velocity that is much higher than the average p-wave velocity for the rockmass. It is found that the p-wave velocity did not increase at any of the three hypocenters prior to the seismic event occurring, however the p-wave velocity did increase in the closest “zone center” for two of the three events. Keywords Passive seismic tomography . Underground hard rock mining . Induced seismicity . Seismic velocity . Microseismic monitoring
1 Introduction As an underground ore deposit is mined, the excavation process results in concentrations of increased induced stresses. Accumulation of this stress can cause instabilities that can be hazardous. The partial or general instability of the roof and ribs in underground mines is the main cause of ground falls, which have resulted in multiple injuries and fatalities [1]. Rockbursts are any volumetric displacement in underground rock that causes damage with any magnitude within the mine [2, 3]. Although the number of fatalities associated with * Setareh Ghaychi Afrouz [email protected] 1
Virginia Tech, Blacksburg, VA, USA
2
U.S. Silver Corp, Wallace, ID, USA
3
Northern Star Resources Limited, Delta Junction, AK, USA
rockbursts is significantly less than other types of underground hazards such as fires or inundation, they are considered one of the more significant potential hazards due to their perceived random
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