Borehole GPR investigations from underground for mapping the extension of old working at a Pb-Zn Mine in Rajasthan, India

Conventional GPR reflection surveys in base metal sulphide mines are sometimes marred by scattering due to heterogeneity or presence of disseminated sulphide minerals. Surface scanning in such areas have its own limitations as the depth of investigation is adversely influenced by the highly lossy medium. Alternatively, borehole GPR investigation from underground can be an efficient tool for sensing the proximity of old workings including cavities left over by past ore extraction. In the present study two borehole GPR techniques were used from underground to establish the boundary of old workings in a Pb-Zn mine. Cross-hole transmission mode data from a stepped-frequency GPR was analysed to delineating the zone of influence created by ancient mining activity. Two parameters namely velocity and attenuation of radar waves were used to examine the intervening geological medium. Single-hole reflection survey was also carried out to scan the medium around the borehole. The result of cross-hole GPR survey is a plot of radar wave attenuation along the horizontal plane of survey. The attenuation plot clearly delineated the zone affected by past mining activity; however cavity type features were not mapped. Even though exploratory drill holes showed evidences of old workings up to 250-260 m depth from the ground surface, the present borehole GPR investigation from an underground gallery at 298 m depth confirmed that old workings did not cross the horizontal plane of investigation. Keywords—cross-hole GPR; borehole GPR profiling; water- logged working; radar wave attenuation; radargram; conductive fluid; seepage; Pb-Zn mines I. INTRODUCTION Mining for metals in India is going on for the past several centuries and relics of ore extraction over the Pb-Zn deposits of Dariba, Rajasthan typically depict mining activities from the ancient times (1). Several older workings in the form of irregular pits, trenches and small diameter shafts are seen around the area. Some of the abandoned workings are in the form of unlined/lined shafts and show signatures of the then dewatering practices. In Dariba area ancient prospectors had carried out mining up to an estimated depth of 260 m from the ground surface (500 mRL), which makes it the deepest ancient mine in Rajasthan. Delineating the exact course of the excavation and water-logging pertaining to past mining are extremely crucial for carrying out fresh mining below the old workings. Conventionally, exploratory drilling from surface as well as underground is carried out to ascertain the strata condition around a developing drive. Most of the time boreholes are spaced far apart and do not encounter old workings as they are small in size and localised. However, in the event of puncturing a water filled cavity, the fear of inundation and loss of men and machinery is high. At present, mining in the East load area of Rajpura- Dariba Mines is being carried out below the level of 100 mRL. At 200 mRL, an underground drive was excavated on the footwall at a distance of 12-20 m from the ore body. Several exploratory boreholes (inclined and horizontal) were drilled from the drive along the strike length of the ore body. One of the boreholes drilled at +15ᵒ, encountered a water filled cavity at a distance of 23 m and lead to water ingress into the drive. Drilling proved that old workings extended up to 220 mRL, however it was not possible to rule out their extension to deeper levels. In this backdrop, borehole investigation was carried out using a stepped-frequency GPR (2, 3, 4) from the 200 mRL drive to assess the rock mass condition. Nine horizontal boreholes (45-55 m deep) all crossing the ore body and extending in to the hangwall rocks were used for investigation. The distance between the adjacent boreholes varied from 25-30m. Two borehole techniques, vis-a-vis, cross-hole GPR survey and borehole GPR profiling was used for investigation. In cross-hole survey, the transmitter and receiver antennas (5) were singly placed in two adjacent boreholes and the transmission mode data was acquired. In borehole profiling both the antennas were placed in the same borehole and reflection data in common offset mode was recorded along the length of borehole. Radar wave attenuation differentiates the target better (6) and therefore was used in cross-hole interpretation. The reflection patterns from borehole profiling was used to infer the rock mass in terms of areas resembling mined out cavities/ water logged old workings.