MAPPING OF AN ABANDONED AND INUNDATED LEAD MINE USING MAGNETIC INDUCTION BEACONS AND SPECIALLY-TRAINED DIVERS

Undocumented abandoned mines pose a serious hazard to miners in nearby active mines. Maps of abandoned works can be inaccurate or absent. Since most traditional surface geophysical methods lack the penetration necessary to detect deeper mine works, drilling is often the only method available for map verification. An alternative means of abandoned mine map verification was implemented at the now-flooded Offsets Mine in Mine LaMotte, Missouri. The Offsets, an abandoned lead mine, offers the ability to detect flooded mine works ranging from 100-150 ft below ground surface. Cave divers specially trained and equipped to dive in an abandoned mine set underwater survey stations at 100-foot intervals along one of the Offset's main haul roads. At each station, divers placed a magnetic induction beacon, operating at 3496 Hz, and measured cross sections with a fiberglass tape and depth with a digital depth gauge calibrated in one-foot increments. A surveyor on the surface, using a vertically-positioned receiver loop tuned to the beacon antenna, located the divers' position relative to the surface using measurements of radiomagnetic field strength. The surface points were surveyed in using traditional land survey methods and combined with the divers' data to create an accurate three-dimensional map of the haul road and a topographic overlay. Divers were also able to document the structural stability of the mine with still photography and video.