Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 404, 1992 Aug; :1-2
Objective: Validate the premise that there is measurable seismic activity associated with abandoned mine lands and that this activity can be used to determine stability of the abandoned mine openings, failure cycle of these openings, and susceptibility of the area to surface subsidence. Approach: The U.S. Bureau of Mines conducted field investigations at two geologically contrasting sites, Colorado Springs, CO, and the Allegheny Portage Railroad Historical Site near Cresson, PA, to demonstrate that developing monitoring technology can be applied to any site. An array of seven geophones (velocity-measuring sensors) were strategically located to encompass an area at each site where failure in the underground mine openings could be expected, based on historical, geological, and engineering information. How It Works: Bureau research has effectively demonstrated that stress changes within a rock mass can stimulate the development of cracks and fractures, resulting in the release of strain energy that generates seismic waves. These seismic waves, traveling outward from their source, can be detected, and the point of origin can be located in three-dimensional space. Furthermore, these seismic signals contain information that defines the source mechanism and the conditions between the source and the receiving sensor. In the field investigations, the data acquisition system operated in a continuous mode, recording information on magnetic tape in analog form. When a seismic event occurred, it was detected by the array of geophones. The signal was sent to a central recording station via a transmission cable, where it was filtered, amplified, and recorded. At the end of a 24-hour period, the data tape was replaced and the recorded data tape was reviewed for seismic activity. When an apparent event was observed, it was reproduced at the recorded speed. If it appeared on at least five data channels within an acceptable time window, it was digitized and stored on digital magnetic tape for processing and analysis. The time resolution of the digitized signal was based on the analog record-reproduce tape speeds, the size of the digital data block (number of sample points), and the digital sample rate. The "event-select" time window was established using the maximum array dimension and the lowest anticipated media velocity. Cross-correlation data processing techniques were used to establish a nonarbitrary method to obtain arrival time differences from widely variant quality seismic signals. The derived arrival times were then used in mathematical algorithms to obtain seismic event locations. Research conducted by the Colorado School of Mines under Bureau contract has successfully demonstrated that seismic activity can define various types of mine opening failures. The character (seismic signature) of the activity can be used to determine the stability of the mine opening as well as susceptibility for surface subsidence. Test Results: Seismic activity was observed at both field sites. Seismic events from both sites were located very near to or within the perimeter of the geophone array, and these events have an uncommon similarity in character and frequency spectra. Since both sites are associated with old, abandoned underground coal mines, and both sites are experiencing surface subsidence, it is possible to conclude that the seismic events observed are related to initial failure or collapse of the underground openings and could be activity precursive to surface subsidence. Concurrent research at the Colorado School of Mines has resulted in generating seismic events that are descriptive of a mine roof failure, using a repetitive, controlled seismic source. These events are strikingly similar in character to those recorded by the Bureau at the Pennsylvania and Colorado sites. This similarity strongly indicates that the seismic events recorded in the field could be activity that is precursive to actual underground subsidence and eventual surface subsidence. Benefits: The fact that locatable seismic activity associated with subsidence was observed at both sites is strong evidence that passive seismic monitoring is a viable technique to assess the stability of abandoned mine land sites. The refinement and application of this technology could significantly reduce exploratory and abatement drilling costs, since it has the capability to accurately locate the active area, define the stage of subsidence, and determine the areal extent of the subsidence zone. This same approach could be used to evaluate abatement and control methods.
Denver, CO: U.S. Department of the Interior, Bureau of Mines, TN 404