Borehole seismic profiling and tube wave applications in a dam site investigation
Henriet, J.P.; Schittekat, P.; Heldens, P. (1983). Borehole seismic profiling and tube wave applications in a dam site investigation. Geophys. Prospect. 31(1): 72-86
Continuous, single-channel reflection profiling has been carried out in PVC-lined boreholes, primarily with the aim of ascertaining the position of an old subsurface gas storage tunnel on a proposed dam site.Tube wave reflection patterns thus generated have been interpreted in terms of sediment rigidity and shear wave velocity, and these results could be compared with some independent data. It is interesting to note that, within the well section penetrating Tertiary clays, the velocity of the hydraulic transients apparently was not affected by the PVC casing, which might be explained by a tight coupling between casing and clay wall. In such situations, tube waves turn out a straightforward tool for the determination of shear wave velocity and the derivation of dynamic elastic moduli of unconsolidated sediments.Further applications of the study of the distribution of seismic velocities on the dam site dealt with the consolidation history of the clays. A level of abnormally low P-wave velocities has been detected and interpreted as a gas-charged horizon which, by its coincidence with the base level of clay diapirs, might be considered to have contributed to clay flowage in past geological times. Data about maximum past burial depth, derived from shear wave velocities, turned out to be in agreement with results from consolidation testing.
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