Vestnik MGTU (Mar 2019)
Evaluation of rock disturbance by GPR sensing using water saturation for contrast
Abstract
he most important element concerning efficiency and safety of developing mineral deposits is the consideration of the main structural features of the rock massif. The paper considers methodological issues of the GPR sensing application to study the internal structure of rocks. GPR surveys have been performed on a working bench of the Zhelezny mine, Kovdorsky GOK. Initially, the sensing has been performed by longitudinal profiling of the rock mass in its natural state to the depth of 20 m in combination with a telemetric survey of rock fracturing in the borehole located on the profile. Then the borehole was completely flooded with water, and GPR sensing studies have being repeated after 15, 30 and 60 minutes simultaneously with the registration of a decrease in the water level in the borehole. The GPR study results have been analysed in order to identify the effect of rocks' water-saturation on the recorded electromagnetic characteristics, and to determine the relationship of structural disturbance of rocks and their dielectric constant before and after water-saturation. It has been revealed that rock water saturation is an indicator that allows for more contrasting detection of rock disturbance zones due to the relatively insignificant difference in dielectric constant in the undisturbed and disturbed dry rock massif. The rock disturbance in an open pit slope has been estimated by GPR sensing, using water-filled borehole for contrast of measurements. The assessment has shown that water-saturated, porous, permeable, fractured rocks have higher dielectric constant values than dry / monolithic, weakly fractured rocks that have lower dielectric values permeability. It has been established that due to the water-saturation of rocks changes in the values of dielectric constant increased twice, and amounted to 0.6 units, while before saturation these changes were only 0.3 units, i. e. were two times lower. In order to substantiate the assessment of rock disturbance by GPR sensing, a comparative analysis has been made of GPR data and void of rocks in the vicinity of the borehole calculated on the basis of the filtration coefficient. The analysis has shown that the dielectric constant increases with increasing voids in the rock and, accordingly, with increasing water-saturation
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