MASW & VS30 Shear Wave Velocity Testing in Longueuil

Longueuil sits at just 12 meters above the St. Lawrence River, on a complex sedimentary basin where the post-glacial Champlain Sea left deposits of sensitive marine clay. The 2013 revision to the National Building Code of Canada (NBCC) made seismic site classification mandatory for most structures, and for good reason—the soft Leda clay layers common across the South Shore can amplify ground motion significantly during an event. A MASW survey delivers the VS30 profile needed to assign Site Class C, D, or E under NBCC Table 4.1.8.4.A without the cost and disturbance of deep boreholes. Our crew runs the full acquisition and inversion workflow within a day on standard lots in Vieux-Longueuil and Greenfield Park, then processes the dispersion curves using an ISO 17025-accredited procedure that holds up under peer review. For deeper bedrock targets near the RCM of Longueuil’s industrial zones, we often pair the surface wave data with a targeted CPT test to calibrate the upper 20 meters against a direct measurement.

A MASW survey delivers the VS30 profile required under NBCC 2015 without drilling through sensitive Leda clay—saving time and reducing site disturbance.

Methodology and scope

The soil profile changes fast once you cross the tracks from LeMoyne into Saint-Hubert. LeMoyne sits on 18 to 25 meters of soft silty clay over till, which routinely produces VS30 values below 180 m/s and lands squarely in Site Class E. Saint-Hubert’s eastern sections, closer to the old sand pits, often hit 250 to 350 m/s and classify as Site D with a shallower impedance contrast. Our acquisition setup uses a 24-channel seismograph with 4.5 Hz geophones spaced at 2-meter intervals, firing a 10 kg sledgehammer source on an aluminum plate. We run both forward and reverse shots, stack five to seven impacts per spread, and extract the fundamental-mode Rayleigh wave dispersion curve from 5 Hz up to at least 30 Hz. The inversion is constrained against any available borehole logs or test pits to reduce the non-uniqueness of the shear wave velocity model, which is critical when the depth to engineering bedrock exceeds 30 meters in parts of the city. Every profile is checked against the NEHRP site classification criteria and cross-referenced with the NBCC site factors for Sa(0.2) and Sa(1.0) so the structural engineer gets a defensible design spectrum.

Local considerations

Longueuil’s winter freeze-thaw cycles and the high water table near the river create a specific challenge for surface wave work. Frozen crust in January through March introduces a high-velocity layer that distorts the dispersion curve at frequencies above 20 Hz, mimicking a stiffer near-surface than actually exists. We schedule MASW surveys between April and November whenever possible, or apply a frozen-layer correction derived from local experience on the South Shore. The bigger risk is misclassifying a site due to a buried soft layer that the fundamental mode alone cannot resolve. In zones where the clay is underlain by a dense till at 15 meters and then bedrock at 40 meters, the inversion can converge on a model that averages the two impedance contrasts and reports a VS30 that is 20–30 percent optimistic. We mitigate this by running a seismic refraction line on the same spread whenever the site geometry permits, using the P-wave first breaks to constrain the top of the till, and by integrating any available CPT tip resistance profiles to resolve the shear wave velocity ambiguity at the critical 10-to-30-meter depth range.

Technical parameters

Parameter	Typical value
Method	MASW (Multichannel Analysis of Surface Waves), Rayleigh wave dispersion
Geophone array	24-channel, 4.5 Hz vertical geophones, 2 m spacing
Energy source	10 kg sledgehammer on aluminum plate, 5–7 stacks per shot
Frequency range	5 Hz to ≥30 Hz fundamental mode
Maximum investigation depth	Typically 30–40 m with standard spread; 50 m with extended layout
Output parameter	VS30 (time-averaged shear wave velocity to 30 m), NBCC Site Class A–E
Applicable standard	NBCC 2015 Table 4.1.8.4.A, NEHRP site classification, ASTM D7400
Data delivery	Dispersion curve, 1D VS profile, VS30 calculation, site class letter

Associated technical services

VS30 Site Classification Survey

Full MASW acquisition and interpretation for one building lot or infrastructure alignment. Includes dispersion curve extraction, fundamental-mode inversion, VS30 calculation, and NBCC site class assignment with a signed professional engineer’s report.

Combined MASW + Seismic Refraction

Simultaneous P-wave and surface-wave acquisition on the same spread. The refraction tomography constrains the depth to high-velocity layers, reducing inversion uncertainty where a till or bedrock interface falls within the 30-meter column.

Site-Specific Ground Motion Amplification Study

One-dimensional equivalent-linear site response analysis using the measured VS profile. Outputs surface acceleration spectra, amplification factors, and design ground motion parameters for NBCC dynamic analysis requirements.

Frequently asked questions

How much does a MASW survey cost for a single-family lot in Longueuil?

For a standard residential lot in Longueuil, a complete MASW survey with VS30 calculation and an NBCC site class letter typically runs between CA$2,030 and CA$3,990. The range depends on access constraints, the need for extended spreads to reach deeper bedrock, and whether complementary seismic refraction is added to constrain the velocity model.

What seismic site class does Longueuil generally fall into under the NBCC?

Most of Longueuil falls into Site Class D or E depending on the thickness of the Champlain Sea clay. Neighborhoods like Vieux-Longueuil and LeMoyne with 18 to 25 meters of soft clay often classify as Site E (VS30 below 180 m/s). Areas around Saint-Hubert and Greenfield Park with shallower till or sandier deposits more commonly reach Site Class D (180 to 360 m/s). A site-specific MASW survey is the only reliable way to assign the class for permit submission.

Can you run a MASW survey in winter when the ground is frozen?

Winter surveys are possible but require caution. Frozen ground creates a high-velocity crust that biases the dispersion curve above 15–20 Hz. We apply a seasonal correction based on local South Shore experience and, where the budget allows, run a parallel CPT test to independently verify the shallow shear strength. That said, we strongly recommend scheduling MASW work between April and November to avoid the frozen-layer ambiguity.