Non-Invasive Seismic Surface Technique for Mapping and Monitoring Soils in the Vadose Zone.

It is well-known that seismic/acoustic velocity is sensitive to soil conditions such as moisture, water potential, and state of stress. The relationship between the seismic velocity and these soil parameters can be understood by using the concept of the effective stress. In this talk, a newly developed seismic surface wave technique, the multi-channel analysis of surface wave (MASW) method, is described. The MASW takes advantage of one of surface waves, i.e. Rayleigh wave, and uses spectrum analysis along with inversion algorithm to non-invasively measure seismic shear wave velocity as a function of depth in the vadose zone. With the method spatial and temporal variations of near surface soil (up to 2 meters deep) can be mapped and monitored. Two applications of the MASW are presented: (1) a long-term survey was conducted for studying and monitoring the seasonal and weather effects on the subsurface soil; (2) a two-dimensional shear wave velocity image was obtained and from the contrast of the image, the presence, depth, and extent of a fragipan were identified.