The design of an intrinsically safe ground penetrating radar for detecting abandoned workings in underground coal mines

Underground coal mines worldwide often face significant operational risks due to the presence of unmapped or inaccurately mapped abandoned workings. Generally, these workings are below the water table and are flooded under high pressure. Present longwall or room-and-pillar operations may inadvertently breach the barrier into flooded abandoned workings and produce disastrous consequences. To mitigate this risk, legislation requires that no mining should occur within certain distances of possible historical workings. With coal mines increasing production rates to meet global demand, the restriction imposed by these buffer zones can be costly and inefficient. Geophysical applications within coal seams have been discussed in numerous works. In-seam seismic methods, though of sufficient resolution to possibly detect distant water-filled tunnels, require the use of explosives or impact sources and non-intrinsically safe equipment. Electrical methods are omnidirectional when employed underground, resulting in ambiguous readings that may originate from adjacent coal seams. Ground-penetrating radar is reasonably directional but requires well-shielded antennas to reduce the occurrence of multiple reflections reverberating from the opposing tunnel wall. Presently, the lowest frequency shielded antennas that are commercially available are centered at 100 MHz. Achieving adequate penetration and sufficient resolution to detect a distant tunnel is shown to require approximately 40 MHz center-frequency antennas. We discuss the development and application testing of a novel low-frequency intrinsically safe impulse radar system for use in gaseous coal mines. The technology is portable and deployed with one-button acquisition for use by mine personnel. Although seam rolls and faults as well as coal anisotropy can limit tunnel detectability, the preliminary results demonstrate the viability of the technical approach.