When heavy machinery is operating and induced seismicity is present, an active underground mine can be a seismically active location.
Researchers have now shown how to extract and isolate the signals produced by mining activities from the local seismic background noise using data from a longwall coal mine.
Using seismic noise analysis to monitor potential hazards
(Photo : EDINSON ESTUPINAN/AFP via Getty Images)
According to Santiago Rabade, a Ph.D. candidate at the University of Utah, and colleagues, this technique, which was detailed in the Bulletin of the Seismological Society of America, might help monitor seismic occurrences and the structural integrity of the mine while activities continue.
Underground mines are called longwall coal mines to take long panels of coal from a huge wall or slice of coal.
The researchers used information gathered from a 17-geophone array on the top of one such mine for over a month.
The array was first put up to track the mine's seismicity and ambient seismic noise over time to detect potential subsurface changes.
Initially, Rabade and associates employed a cross-correlation technique, which contrasts the signals from every geophone station within 5-minute periods.
After additional investigation, the researchers were able to distinguish between periods with strong coherent signals and those without strong signals.
They concluded that the time windows with high signals were compatible with the development of mining, whereas the other windows show the region's typical, persistent seismic background noise.
The sources of the mining-dominated windows were then identified by the researchers using a geographical approach.
Their findings on time scales of 24 hours and 5 minutes were in good agreement with where the main longwall activities were located and where seismic occurrences had been seen.
If the sources are sufficiently coherent, the approach enables the discovery of several simultaneous sources, according to Rabade.
Furthermore, Rabade noted that the researchers haven't looked into whether the system can find various kinds of individual machines.
They believed that the signals coming from the longwall shearing machine dominate the position determined from the time windows with mining activities.
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Effects of coal mining
About 64% of the coal produced in the United States in 2020 came from surface mines.
These mining operations take the rock and soil away from coal seams and reserves.
In Wyoming's Powder River Basin, where coal reserves are close to the surface and may be up to 70 feet thick, are located the largest surface mines in the country.
Large portions of the Appalachian Mountains in West Virginia and Kentucky have been impacted by mountaintop removal and mining for valley fill.
Utilizing explosives, this method of mining coal removes mountaintops.
With this approach, the terrain is altered, and streams occasionally have rock and soil on top of them.
These flooded valleys may discharge water that contains toxins that endanger aquatic creatures downstream.
Surface mines often have a greater impact on the environment than underground mines. The land above mine tunnels, however, is susceptible to collapsing, and abandoned underground mines may leak acidic water.
Coal resources include methane gas, which can ignite if it gathers in deep mines. To make mines a safer place to work, this coalbed methane must be evacuated.
About 8% of the nation's overall methane emissions and 1% of its overall greenhouse gas emissions in 2019 came from coal mining and abandoned coal mines (based on global warming potential).
Coalbed methane is recovered from some mines and used or sold.
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