ABSTRACT:

Significant hazards to miners are created when active workings approach mined-out areas of either the same mine, or mines located adjacent, above, or below the active mine. Potential hazards include ground collapse and water or toxic gas inundation. These previously mined-out areas may be unintentionally penetrated if information pertaining to their location is not accurate or available to mine operators. To mitigate intersection risks, geophysical techniques offer means to detect the presence of old mine workings before they are encountered. The reliability of cross hole seismic tomography (XHST) was evaluated through a void imaging demonstration at the Colorado School of Mines Edgar Mine experimental facility. Construction of an isolating bulkhead permitted imaging comparisons between air-filled and water-filled void conditions. Despite rather small errors in void location, it is concluded based on imaging results that XHST can offer a reliable and sufficiently accurate method of void imaging for the purpose of mitigating the hazards related to intersecting abandoned mine workings.

1. INTRODUCTION

Significant hazards to miners are created when active workings approach mined-out areas of either the same mine, or mines located adjacent, above, or below the active mine. Potential hazards include ground collapse and water or toxic gas inundation. These previously mined-out areas may be unintentionally penetrated if information pertaining to their location is not accurate or available to mine operators. Although there are current regulations at the state and federal level addressing the accurate surveying and mapping of mine workings as well as the long-term archival of mine maps, this was not the case prior to approximately 1970. Many of the abandoned mines were in operation prior to the regulations and have not been accurately surveyed, mapped, or documented. In addition, many of the maps that have been created cannot be located.Mine Safety and Health Administration (MSHA) records show that since 1995, there have been over 100 reported incidents where active mines have inadvertently cut into mined-out areas. Unavailable, inaccurate, or incomplete mapping of older abandoned mines is typically responsible. Many additional incidents have not been officially reported because no injuries or other significant consequences resulted. These incidents continue to occur as mine operators attempt to recover reserves that may be located near abandoned mines. A mine operator is presently required to identify any adjacent mine that will be within 1,000 feet of the projected workings of the proposed mine. However, investigation of recent inundation incidents has found that maps of abandoned mines have been off by as much as 3,000 feet, meaning that maps alone cannot reliably ensure that inundation will not occur. Geophysical techniques offer means to detect the presence of old mine workings at a resolution that may be far superior to old mine maps. In this paper, the results of void imaging using cross hole seismic tomography (XHST) are summarized. These surveys were conducted at the Colorado School of Mines (CSM) Edgar Mine experimental facility, located in IdahoSprings, Colorado. Construction of an isolating bulkhead permitted imaging comparisons between airfilled and water-filled void conditions.

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