In Thunder Bay, the contrast between shallow bedrock on the Canadian Shield and the deep glacio-lacustrine deposits near the Kaministiqua River delta creates vastly different seismic site responses within a single project footprint. A uniform foundation design assumption fails here. We see this regularly when reviewing borehole logs from the Intercity area versus the flat-lying clay plains of Fort William. The MASW technique resolves these transitions by measuring shear wave velocity (Vs) profiles to 30 meters depth, delivering the VS30 value required by the National Building Code of Canada (NBCC) for seismic site classification. Rather than relying on proxy correlations from SPT blow counts that can misclassify sensitive Leda-type clays, the direct measurement of stiffness via surface waves provides the engineer with defensible Site Class C, D, or E boundaries. For projects where bedrock depth varies sharply, combining this with a seismic refraction survey helps map the top of competent rock while MASW characterizes the overburden stiffness profile.
In Thunder Bay, two boreholes 50 meters apart can yield different VS30 values if one hits a bedrock high and the other stays in basin fill — MASW maps the lateral variation that point measurements miss.
Common questions
What is the difference between MASW and seismic refraction for VS30?
Seismic refraction measures P-wave velocity of compressional waves and maps the top of bedrock very well in Thunder Bay's Shield terrain. MASW measures S-wave (shear) velocity, which is the direct input for NBCC site classification. Refraction can miss a velocity inversion — where a stiff layer overlies soft clay — while MASW resolves it through the dispersion of Rayleigh waves. For VS30, MASW is the preferred method; refraction is complementary for bedrock depth mapping.
How much does a MASW survey cost in Thunder Bay?
A typical MASW site classification with 2–4 spreads and a report falls in the CA$2,350 to CA$4,500 range, depending on the number of test locations, depth to bedrock, and whether calibration boreholes are required. Sites on deep clay south of the Neebing River tend toward the upper end due to longer spreads and more involved inversion work.
Can you do MASW on asphalt or concrete pavement?
Yes, with modifications. On asphalt, we use a coupling plate and high-frequency geophones (14 Hz or higher) to capture the stiff near-surface. The pavement layer appears as a high-velocity cap in the dispersion image. For VS30 classification, we must account for this stiff layer in the inversion — otherwise the VS30 can be overestimated. We typically drill a small core through the pavement to confirm its thickness for the starting model.
How does the NBCC 2020 use the VS30 value from MASW?
The NBCC 2020 assigns a Seismic Site Class from A (hard rock) to E (soft soil) based on the time-averaged shear wave velocity in the upper 30 meters. VS30 below 180 m/s is Site Class E, 180–360 m/s is Class D, 360–760 m/s is Class C, and above 760 m/s is Class B or A. This classification feeds directly into the short-period and long-period site coefficients that scale the design spectral acceleration for structural analysis.
