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Fluid-Induced Changes In Shear Velocity From Surface Waves
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 13th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems, Feb 2000, cp-200-00139
Abstract
Seismic methods can monitor transient ground water by detecting changes in seismic velocity.<br>Compressional and shear wave velocities respond to changes in the bulk modulus, shear modulus<br>and density caused by the presence of water. We present three methods that allow these small<br>perturbations in seismic velocities to be tracked through time in a field setting. Compressional<br>velocities are tracked using differences in first arrival traveltimes. Shear velocities and<br>subsequent perturbations are calculated from surface wave frequency components. A simple<br>inversion scheme allows these velocities to be tracked as a function of depth below the surface.<br>Using these strategies, several observations are made addressing how fluid saturation can be<br>derived from seismic properties. We confirm prior observations of a large compressional velocity<br>change associated with full saturation. Contrary to prior lab studies, we find shear velocities in an<br>unconsolidated sandy environment to be quite sensitive to moisture content. Partial saturation can<br>be detected from increased shear velocities resulting from greater cohesion between grains. We<br>also observe the influence of pore pressure changes below the water table. This suggests that<br>shear velocities might be used to monitor pressure changes in a fluid reservoir.