The MASW array in Kelowna starts with a 24-channel seismograph connected to a spread of low-frequency geophones — typically 4.5 Hz vertical units — deployed across 46 to 92 meters of asphalt, gravel, or exposed glacial till. We couple each geophone with a spike-and-putty assembly because the surficial silty clay overlying Okanagan Centre drift transmits high-frequency noise poorly; without firm coupling the dispersion curve degrades before it reaches the 30-meter depth target. Once the active shot is stacked — usually six to eight blows of a 10-kg sledgehammer on an aluminum plate — the field crew checks the raw shot gather for coherent Rayleigh-wave energy while the GPS records 49.8879, -119.4959, tying the line to the lot geometry. In the lab, the team runs phase-velocity inversion with a laterally constrained model, which is critical here because the valley-fill stratigraphy often contains interbedded sand and silt lenses that can fool a 1D assumption. For deeper bedrock confirmation on slopes above Knox Mountain, we sometimes pair the survey with a seismic refraction profile to resolve the top-of-rock interface where Vs contrast exceeds 500 m/s.
A 20 m/s variation in Vs30 can shift the NBCC site class boundary and alter the design base shear by tens of thousands of dollars in foundation concrete.
Method and coverage
When we run a line along the base of Dilworth Mountain or across the alluvial bench near Mission Creek, the first thing we check is whether the dispersion image shows a persistent low-velocity zone between 8 and 15 meters — that signature typically corresponds to the soft, normally consolidated silts of the Okanagan Centre Formation, and it controls the Vs30 calculation more than the upper gravel crust. Our processing workflow applies a dispersion-curve picking algorithm with spectral weighting, followed by an iterative damped least-squares inversion that generates a 1D shear-wave velocity profile down to 35 meters, which we then average using the travel-time method per NBCC 2020 Section 4.1.8.4. The resulting Vs30 value, usually clustering between 210 and 380 m/s in the central Kelowna basin, determines whether the site falls into Class C, D, or E — a distinction that impacts the design spectral acceleration by as much as 40%. We run background noise monitoring before every shot because traffic on Highway 97 and the occasional floatplane departure from Okanagan Lake can contaminate traces above 5 Hz; bad records are flagged and re-acquired before the crew demobilizes.
Regional considerations
The most common mistake we see in Kelowna is a structural design team assuming Site Class C based on a desktop geological map without running a Vs30 measurement — only to discover during excavation that 12 meters of soft lake-bottom clay push the site into Class D or even low-end Class E territory under the Mission Creek fan. When that happens after the foundation drawings are sealed, the structural engineer must recalculate the design spectral acceleration Sa(T) using the higher site coefficient F(T), which often means thicker shear walls, larger footing dimensions, or a switch from shallow footings to a deep foundation system. The NBCC 2020 is explicit about this in Commentary L: site classification derived from shear-wave velocity takes precedence over SPT N-value correlations, and in the Okanagan Valley, where Vs30 can drop from 400 m/s to 190 m/s within a single city block, skipping the survey is a liability that no geotechnical report should carry. Municipal building officials in Kelowna now routinely ask for the Vs30 measurement report before issuing a foundation permit for Part 3 buildings.
Common questions
How long does a MASW survey take on a typical Kelowna lot?
Most single-line surveys are completed in 2 to 3 hours, including geophone layout, noise monitoring, shooting, and preliminary dispersion-curve quality control. The final processed report with Vs30 and NBCC site class is delivered within 3 business days.
What depth does MASW reach compared to borehole methods?
With a 92-meter spread and 4.5 Hz geophones, the effective investigation depth reaches 35 to 40 meters in the silty Okanagan soils, which exceeds the 30-meter averaging interval required by NBCC 2020. For projects needing deeper bedrock confirmation, we combine MASW with seismic refraction.
Does the City of Kelowna require Vs30 testing for building permits?
For Part 3 buildings (assembly, institutional, high-hazard, and residential over 600 m²), the City of Kelowna building department follows NBCC 2020 and expects site classification based on direct Vs measurement or conservative default assumptions. Providing a measured Vs30 avoids the penalty of a default Class E assumption.
How much does a MASW survey cost in the Kelowna area?
A standard single-line MASW survey with Vs30 calculation and NBCC site classification report ranges from CA$2,490 to CA$4,720, depending on array length, site access conditions, and whether additional lines or downhole verification are required.
What is the difference between Vs30 from MASW and a site class from SPT blow counts?
The NBCC 2020 hierarchy gives preference to shear-wave velocity measurements over SPT N-value correlations. MASW provides a direct Vs profile and an averaged Vs30 with lower uncertainty, whereas SPT-based classification relies on empirical correlations that can be conservative in the interbedded silts and sands typical of the Okanagan valley fill.
