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While the introduction of pressurized air into an unconsolidated, coarse-grained fluvial aquifer might well be expected to affect the P-wave velocity profile below the water table, we have found that S-waves are also sensitive to changes induced by air sparging. In a study spanning over a year of sparging, observations of both P- and S-waves were conducted by Vertical Seismic Profiling (VSP). While the primary objective was to characterize the aquifer, we have found that air sparging has significantly affected both P- and S-wave propagation. Below the water table we have observed as much as a 54% decrease in P-wave velocity, and as much as a 31% increase in S-wave velocity after continued sparging. Above the water table, we observe only small changes in both P- and S-wave velocities. This pattern of velocity change (decreasing P, increasing S) may be due to an increase in the amount of trapped air below the water table.

Published laboratory studies in the small strain regime have shown P-wave velocities to be sensitive to void ratio, fluid content, and confining stress. On the other hand, most similar studies of S-waves have only been conducted on either dry or saturated samples. However, one recent laboratory study suggests that shear modulus and shear velocity may increase significantly at partial water saturations (due to capillary forces). Data from our in-situ survey supports this more recent lab work. We have observed that S-wave propagation may be significantly altered by fluid content when soils are partially saturated with water (where trapped air may exist, producing a 3-phase fluid-frame system). In addition, we have observed changes in the propagating wavelet. This may be an indication that viscous damping is also affected by partial water saturation. We conclude by observing that S-waves may prove to be an attractive alternative for mapping the effects of air sparging.

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This article was originally published by The Environmental and Engineering Geophysical Society (EEGS) in Symposium on the Application of Geophysics to Engineering and Environmental Problems 1997. Copyright restrictions may apply.