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The detection of millimeter-wide discontinuities such as fissures in subsurface geological formations may be possible by means of GPR soundings, but establishing a law for the electromagnetic wave propagation velocity from field data in order to interpret radar data and to localize these discontinuities in depth is not easy. In order to optimize the interpretation of such radar surveys carried out at various sites, we turned our attention to the accuracy by which the electromagnetic wave propagation velocity may be determined. In a granitic quarry, with the help of a borehole cored 40 m deep, we were able to appraise the limits of velocity analysis based on normal move-out corrections. We characterized the degree of error caused by slightly dipping reflectors and we showed the instability of interval velocity calculations caused by uncertainties in reflection time and RMS velocity assessment. The velocity profile derived from laboratory measurements of dielectric permittivity of the granite samples, for various depths of cores, proved to be insufficient for an exact mapping of the fissure network. At a limestone quarry site, we showed that by working on several paths of electromagnetic waves, direct velocity determinations could help in differentiating rocks in the same formation, based on quality of the field data. Finally, velocities calculated in an anisotropic environment and in geological formations covered by a road or a concrete surface were analysed. Using data recorded in schists, we obtained an example of a non-uniform distribution of velocity in the same formation. Using data recorded in sands and in granite, we demonstrated that a surface material had negligible effect on the determination of the subjacent geological environment velocity. |
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