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

Abstract

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.

Fault Detection with Crosshole and Reflection Geo-Radar for Underground Mine Safety

“…By varying the position of the antennas in the boreholes and measuring the time it takes for the signal to reach the receiver, one can create velocity models in between the boreholes [1,2]. Alternatively, it is possible to extract amplitude variations and create attenuation models of the underground [3,4].…”