Detection Of Shallow Water Table Fluctuation Using The Spectral Analysis Of Surface Waves (Sasw) Technique

The spectral analysis of surface waves (SASW) technique uses measurements of surface wave<br>dispersion to construct shear velocity versus depth profiles. Although the method is commonly used in<br>geotechnical investigations, applications to environmental studies have been limited. We have evaluated<br>the potential of SASW to monitor water table fluctuation at shallow depths (< 2 m) at two sites located<br>near the Rio Grande northwest of El Paso, Texas. The shallow depth of the water table and presence of<br>low velocity zones make use of the seismic refraction method difficult at these sites. The<br>inhomogeneity of sediment grain size and seasonal variations in sediment moisture and salinity also<br>make use of electrical techniques problematic. The first study site was located on an earth-fill levee ~20<br>m west of the Rio Grande. An irrigation ditch was located immediately west of the levee. We<br>successfully detected water infiltration from the ditch into the levee by comparing shear velocity profiles<br>from surveys taken before and after irrigation. The second site was located on homogeneous soils of a<br>historical channel of the Rio Grande (~10 m east of the present Rio Grande). Seasonal fluctuations of<br>the water table related to changes in river level were detected at this site and confirmed with borehole<br>information. Our results suggest the SASW method is effective for detecting the depth and fluctuation of<br>the water table and provides additional valuable information (e.g. shear velocity, shear modulus) when<br>coupled with other types of seismic surveys.