GPR applications in mapping barrier thickness in coal mines : some case studies

Mapping of barrier thickness against old-water logged workings in coal mines is one of the most vexed problems in thie mining industry in India. Till date no reliable method has evolved to probe ahead of present mining face to give a precise estimate of the barrier thickness. Drilling at face is the only available solution practiced today. Apart from being risky, this is costliy and time-consuming also. Hence, attempts were made to map the coal barrier thickness in underground mines by using Step Frequency Ground Penetrating Radar (SFGPR). Sitespecific maneuvers were made to map the barrier under dip and rise conditions using insulated dipole antennas with a centre frequency of 30 and 50 MHz. GPR survey results showed an excellent correlation of the location of shaft and thickness of barrier with the mine plan. Though signal attenuation and water saturation of coal seams restricted the radar probing range to 30m in such cases, successful probing was achieved up to a distance of 35-40m from underground in most of the cases. Based on tlhe results of our trial experiments, GPR survey has now been notiitied by the Government of India as a safety tool for mapping of uinknown barrier thickness in coal mines. This paper describes the iresults of a few such experimental studies with GPR probing in underground coal mines.