Within a high-resolution shallow reflection survey program in Saskatchewan, Canada, S-waves were produced using a single seismo-electric blasting cap and were found to be distinguishable from surface wave phases. The local glacial deposits have average velocities of 450 m/s. V P /V S ratios average 3.6 in these sequences, but they vary laterally, according to the velocity analyses done in two boreholes drilled along the seismic line. Vertical resolution for S-wave reflections are 0.75 m [in the vertical seismic profiling (VSP) data] and 1.5 m (in the CDP data). Yet, the S-wave CDP results are still better than corresponding P-wave data, which had a vertical resolution of 2.6 m. S-wave anisotropy is inferred in the glacial deposits on the basis of particle motion analysis and interpretations of S-wave splitting. However, the amount of observed splitting is small (~2-6 ms over 5-10 m) and could go undetected for seismic surveys with larger sampling intervals. VSPs indicate that S-wave reflectivity is caused by both distinct and subtle lithologic changes (e.g., clay/sand contacts or changes in clay percentage within a particular till unit) and changes in bulk porosity. Migrated S-wave sections from line 1 and line 2 image reflections from sand layers within the tills as well as the first "bedrock" sequence (known as the Judith River Formation). Shear wave images are not only feasible in unconsolidated materials, but provide additional information about structural relationships within these till units.
ASJC Scopus subject areas
- Geochemistry and Petrology