Joint acquisition and processing of seismic reflections and surface waves in a sensitive clay deposit in the outaouais region (Québec), Canada

Abstract

The joint acquisition and processing of vertically polarized shear (SV) wave seismic reflections and surface waves during a seismic survey were carried out in Buckingham (Québec), near Ottawa, Canada, to characterize a thick (20–40 m) sensitive clay deposit. At the study site, the outcropping clay unit overlays a 20–50 m thick layer of sand and gravel and the bedrock depth reaches more than 90 m along the survey line. The seismic reflection survey using common-mid-point (CMP) inversion of SV-wave reflections allowed the localization of the clay-sand and sand-bedrock interfaces as well as the measurement of SV-wave velocities down to the bedrock contact. Velocity variations at depths less than 10 m could not be assessed due to the early reflections hidden by seismic arrivals such as surface waves. However, multi-channel analysis of surface waves (MASW) provided the variations in S-wave velocity from the surface down to a depth of 12 m at each CMP location. The joint acquisition and processing of SV reflections and Rayleigh waves provided a more complete and accurate 2D SV velocity model than both methods taken separately. To test the accuracy of the proposed approach, a multi-offset seismic piezocone penetration test (SCPTu) was performed along the survey line from the surface down to a depth of 25 m. The vertical variations in seismic velocities in sensitive clay as inferred from the SV seismic reflection survey and MASW are comparable to the SCPTu S-wave profile.