The Effects Of Sparging On P- And Sh- Vertical Seismic Profiles

While the introduction of pressurized air into an unconsolidated, coarse-grained fluvial aquifer might<br>well be expected to affect the P-wave velocity profile below the water table, we have found that S-waves are<br>also sensitive to changes induced by air sparging. In a study spanning over a year of sparging, observations of<br>both P- and S-waves were conducted by Vertical Seismic Profiling (VSP). While the primary objective was to<br>characterize the aquifer, we have found that air sparging has significantly affected both P- and S-wave<br>propagation. Below the water table we have observed as much as a 54% decrease in P-wave velocity, and as<br>much as a 31% increase in S-wave velocity after continued sparging. Above the water table, we observe only<br>small changes in both P- and S-wave velocities. This pattern of velocity change (decreasing P, increasing S)<br>may be due to an increase in the amount of trapped air below the water table.<br>Published laboratory studies in the small strain regime have shown P-wave velocities to be sensitive to<br>void ratio, fluid content, and confining stress. On the other hand, most similar studies of S-waves have only<br>been conducted on either dry or saturated samples. However, one recent laboratory study suggests that shear<br>modulus and shear velocity may increase significantly at partial water saturations (due to capillary forces). Data<br>from our in-situ survey supports this more recent lab work. We have observed that S-wave propagation may be<br>significantly altered by fluid content when soils are partially saturated with water (where trapped air may exist,<br>producing a 3-phase fluid-frame system). In addition, we have observed changes in the propagating wavelet.<br>This may be an indication that viscous damping is also affected by partial water saturation. We conclude by<br>observing that S-waves may prove to be an attractive alternative for mapping the effects of air sparging.