The glacial and colluvial overburden that blankets much of Greater Vancouver masks an unpredictable bedrock surface—depth to refusal can swing 15 metres across a single city lot. In the Fraser River delta, soft post-glacial silts add another complication: low-velocity layers that distort conventional seismic profiles. Our approach to seismic tomography cuts through that noise. We deploy 48- to 120-channel spread geometries with both P-wave refraction and S-wave reflection acquisition so the final velocity model captures the sharp impedance contrasts typical of the Georgia Basin. Because Vancouver sits within a active subduction zone, the NBCC requires a site-specific shear-wave velocity profile for Site Class determination on projects taller than three storeys. We process data with iterative travel-time tomography following ASTM D5777, delivering a Vs30 profile in the same report.
A 4.5-times spread-length rule is the minimum for Vancouver—anything shorter misses the bedrock lows that control excavation cost.
