Fluid-Induced Changes In Shear Velocity From Surface Waves

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 modulusand density caused by the presence of water. We present three methods that allow these smallperturbations in seismic velocities to be tracked through time in a field setting. Compressionalvelocities are tracked using differences in first arrival traveltimes. Shear velocities andsubsequent perturbations are calculated from surface wave frequency components. A simpleinversion scheme allows these velocities to be tracked as a function of depth below the surface.Using these strategies, several observations are made addressing how fluid saturation can bederived from seismic properties. We confirm prior observations of a large compressional velocitychange associated with full saturation. Contrary to prior lab studies, we find shear velocities in anunconsolidated sandy environment to be quite sensitive to moisture content. Partial saturation canbe detected from increased shear velocities resulting from greater cohesion between grains. Wealso observe the influence of pore pressure changes below the water table. This suggests thatshear velocities might be used to monitor pressure changes in a fluid reservoir.