Compliance with the National Building Code of Canada (NBCC 2020) requires accurate subsurface velocity models for Site Class determination, especially where the Fraser River meets the upland till in New Westminster. Our seismic tomography surveys combine P-wave and S-wave refraction with high-resolution reflection profiling to map stratigraphy, detect buried channels, and identify the top of competent bedrock. The city’s variable geology — from soft deltaic silts in Queensborough to dense glacial till on the downtown slopes — demands a method that resolves lateral transitions without excessive borehole interpolation. We acquire data with 24-channel and 48-channel seismographs, applying tomographic inversion algorithms that honor the curved raypaths typical in velocity-gradient media. The resulting cross-sections give geotechnical engineers the compressional and shear-wave velocity profiles needed for seismic site classification per Table 4.1.8.4.A of NBCC. When the target is deeper than 30 meters, we combine surface arrays with downhole receivers and integrate the results with CPT testing for a calibrated ground model.
Seismic velocity is a direct input to NBCC Site Class. Guessing it from blow counts alone introduces unnecessary conservatism — or worse, misses a Class E site.
