Measuring fluid-induced changes in near-surface shear velocity using surface waves

Michael West and William Menke

Lamont-Doherty Earth Observatory and Department of earth and Environmental Science of Columbia University
Palisades, New York 10964
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Abstract. Seismic methods can monitor transient ground water by detecting changes in seismic velocity. Compressional and shear wave velocities respond to changes in the bulk modulus, shear modulus and density caused by the presence of water. We present three methods that allow these small perturbations in seismic velocities to be tracked through time in a field setting. Compressional velocities are tracked using differences in first arrival traveltimes. The shear velocities and subsequent perturbations are calculated from surface wave frequency components. A simple inversion scheme allows these velocities to be tracked as a function of depth below the surface. Using these strategies, several observations are made discussing how fluid saturation can be derived from seismic properties. We confirm prior observations of a large compressional velocity change associated with full saturation. Contrary to prior lab studies, we find shear velocities in an unconsolidated sandy environment to be quite sensitive to moisture content. Partial saturation can be detected from increased shear velocities resulting from greater cohesion between grains. We also observe the influence of pore pressure changes below the water table. This suggests that shear velocities might be used to monitor pressure changes in a fluid reservoir.