The velocity changes of electromagnetic waves in anisotropic rock were monitored through georadar tomography to investigate rock behavior due to drill & blasting excavation during construction of an underground laboratory at Yongin-City, Kyunggi-Province, Korea. Four horizontal boreholes spaced 1.4 m apart were drilled parallel to the test tunnel before excavation, and crosshole georadar tomography using an approximately 500 MHz transmitter source was performed at the pre- and post-excavation phases about three months apart. Velocity tomograms were generated which included the correction of borehole deviation effects. A least squares method with a smoothness constraint was applied as an inversion technique, based on the model of elliptic anisotropy with an arbitrary symmetric axis. Three different inversion algorithms (isotropic inversion algorithm without correction of anisotropic effects, isotropic inversion algorithm with correction of anisotropic effects, and anisotropic inversion algorithm) were applied and their results compared. The maximum velocity tomogram from the anisotropic inversion algorithm presented the best result. In tomographic images at the pre- and post-excavation phases, significant differences in the pattern of the velocity distribution were not observed. The direction of anisotropy, however, was affected by excavation. Additionally, the velocity of electromagnetic waves decreased slightly and the low velocity zones expanded after excavation. The velocity reduction might be attributed to micro-cracks in the rock that were created due to excavation and saturated with groundwater.
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Jung et al. (1999) studied this question.