The Anomalous Behavior Of Sh-Waves Across The Water Table

Most theoretical studies of seismic wave propagation in a porous medium do not predict a significant<br>increase in SH-wave velocity with increasing water saturation. Although that type of behavior is commonly<br>predicted for P-waves (and confirmed by countless observations), the expectation for SH-waves is a slight<br>decrease in propagation velocity with increasing water saturations. While published measurements of SH-wave<br>velocity in laboratory studies have been supportive of such a slight decrease in velocity, the data have been<br>biased towards high pressures (typical of oil reservoirs at large depths of burial). On the other hand, the few<br>published low pressure laboratory measurements have revealed significantly different results.<br>The authors’ in-situ measurements of seismic wave velocities in a shallow, coarse grained, unconfined<br>alluvial aquifer document a significant SH-wave velocity increase in the transition from the vadose zone to the<br>water table. In one vertical seismic profile (VSP), the P-wave velocity increases by a factor of 4.2 and the SHwave<br>velocity increases by a factor of 2.6. What is not clear at this point is the true nature of the increase. Is<br>the velocity increase an expression of the presence of water in the pores, or does water alter the rigidity of the<br>soil matrix?<br>In addition to the broad-band velocity increase, we have also observed changes in the attenuation of SHwaves<br>across the water table. After correcting for geometric spreading, the amplitude decay observed in the<br>vadose zone has been found to be larger than that observed below the water table. However, the variation in<br>amplitude decay as a function of frequency and the measurements of body wave dispersion were found to be<br>larger below the water table than above. That is, the water saturated soil behavior is consistent with a Voigt<br>solid, but the dry material appears to follow a different model.<br>The authors will discuss these observations in the context of the current debate on Poisson’s ratio and the<br>Vp/Vs ratio.