RADAR IMAGING OF FRACTURES AND VOIDS BEHIND THE WALLS OF AN UNDERGROUND MINE

2-D and 3-D rock-penetrating radar data were acquired on the wall of a pillar in an underground limestone mine. The objective was to test the ability of radar to image fractures and karst voids and to characterize their geometry, aperture, and fluid content. Strong radar reflections in the raw field data correlate with fractures and a cave exposed on the pillar walls. Large amplitude and rugged pillar wall topography were included in the Kirchhoff migration algorithm because standard elevation corrections are too inaccurate. Large angles between the fractures and the pillar wall on which the data were acquired necessitated a steep-dip migration algorithm. The depth-migrated 250-MHz radar images illuminate fractures, karst voids, and the far wall of the pillar up to ~25 m depth into the rock, with a spatial resolution of <0.5 m. Higher frequency radar improved image resolution and aided the interpretation, but came at the cost of less depth of penetration and considerable extra acquisition effort. Most of the fractures exposed on the pillar walls have apertures much thinner than a radar wavelength. However, they produce strong reflections due to the very strong contrast in physical properties between the rock and the fluid filling the fractures. Water-filled fractures with mm-scale aperture and air-filled fractures with cm-scale aperture produce strong reflections at 250 MHz. Strong variation in reflection amplitude along each fracture is interpreted to be due to variable fracture aperture, including open voids, and to non-planar fracture structure such as gaps, step-overs, and minor bends. Fracture apertures were quantitively measured, but distinguishing water from air filling the fractures was impossible due to the complex radar wavelet and fracture geometry. Two conjugate fracture sets were imaged. One of these fracture sets dominates rock mass stability challenges throughout the mine. All of the detected voids and a large cave are at the intersection of two conjugate fractures, indicating preferential water flow and dissolution along conjugate fracture intersections. Detecting, locating, and characterizing fractures and voids prior to excavation can enable miners to mitigate potential collapse and flood hazards before they occur.