P- and S-wave characterization of near-surface reflectivity from glacial tills using vertical seismic profiles

Fundamental reflectivity properties are established within the glacial depo sits of central Saskatchewan, Canada. Multicomponent vertical seismic profi le (VSP) data collected in three shallow boreholes are used to obtain detai led acoustic property information within the first 80 m of the near-surface strata. The integration of both P- and S-wave VSP data, in conjunction wit h other borehole geophysics, provided a unique opportunity to obtain in-sit u seismic reflection response properties in layered clay and sand tills. P- and S-wave interval velocity profiles, in conjunction with P- and S-wave V SP reflectivities are analyzed to provide insight into seismic wavefield be havior within similar to 80 m of the surface. In general, shear wave energy identifies more reflective intervals than the P-wave energy because of bet ter vertical resolution for S-wave energy (0.75 m) compared to P-wave energ y (2.3 m) based on quarter wavelength criterion. For these saturated, uncon solidated glacial deposits, more details about the lithologic constituents and in-situ porosity are detectable from the S-wave reflectivity, but P-wav e reflections provide a good technique for mapping the bulk changes. The pr incipal cause of seismic reflectivity is the presence and/or amount of sand , and the degree of fluid-filled porosity within the investigated formation s.