Faculty members Dr Syd Peng, Dr Keith Heasley, and Dr Yi Luohave worked in collaboration with Monte Hieb and Randy Harris from the state’s Office of Miners’ Health, Safety and Training to evaluate the capability of seismic detection units in locating trapped miners in the event of an underground incident that compromises the mine’s communications infrastructure.
The first field test was completed at Dana Mining’s 4 West mine, located in southwestern Pennsylvania, in September 2006. The site had a depth of about 420 feet. A mock scenario was drawn up that included trapped miners following an accident with no communications ability or other way to determine the location or number of survivors.
While ventilation is activated and rescue efforts are underway, the only indication the surface is getting from the miners below is a series of pounding signals – 10 strikes, a 10-count pause, then 10 more strikes, then a 30-minute hold.
Terrascience Systems of Vancouver, Canada provided the seismic equipment and technical personnel in this test. Four surface geophones and two tri-axial downhole geophones were placed at the surface and a team of four remained there, while three others from the university and Hilti Corporation descended into the mine.
Once underground, the team used a series of items to pound on that would create a signal: a Hilti DXX76, a Hilti DX460, a HiltiDC462 and eight-pound sledgehammer and a crib block. The signals were attempted on the roof, roof bolts and coal ribs.
While Heasley, Luo and the team noted that the results are very preliminary, the feedback from the seismic tests was strong. “The crib block on the roof rock appeared to have the strongest signal,” said the researchers, “followed by the crib bolt on the bolt, the sledgehammer on the roof rock and the sledgehammer on the roof bolt.”
The group also noted that striking by the Hilti tools, while producing the loudest sound underground, had a minimal geophone reading.
The detectable signals received were from horizontal offsets of 0, 70 and 140. Beyond that distance, such as a later test done at 210ft, the challenge of separating background noise comes into play.
“In analyzing the seismic magnitude, it appears that just the increase in distance between the source and signal is not responsible for all of the signal attenuation that we [saw]. We hypothesize that the incident angle between the polarized source and the sedimentary layers may be causing refraction/reflection (or some other mechanism) and greatly attenuating the non-perpendicular seismic signal,” the group said at the conclusion of the test studies.
Going forward, seismic signals will be examined in more detail and the magnitudes of the signals will be further quantified. Filtering will also be introduced to the process to eliminate background noise, and a deeper mine will likely be used as the next field site.
The research group had also conducted a second seismic test at Peabody Energy’s Federal No 2 Mine in Miracle Run, West Virginia, in January. The mine depth at this site varied from 800-1000ft.
Terrascience Systems and Engineering Seismology Group, also of Canada, participated in this test, and sledgehammer and wood crib block were used as the tools to generate underground seismic signals. The research group is still analyzing the test data.