Granby sits at 140 meters elevation on the Yamaska River, right at the edge of the Appalachian foothills. The 1988 Saguenay earthquake—5.9 magnitude, felt clear across the Eastern Townships—changed how engineers think about seismic risk in this region. Now every new commercial building, school, and mid-rise residential project in Granby needs a proper site class per NBCC 2020, and that means a measured VS30. Not assumed from tables, not borrowed from a site across town. We run the MASW survey directly on your lot, processing the Rayleigh wave dispersion curve to get a shear wave velocity profile that holds up under plan review. For lots with complex stratigraphy we often pair it with a few SPT drillings to ground-truth the geophone array data against actual blow counts. The result is a seismic site class—C, D, or E depending on the clays and tills under your footing—that satisfies the municipality and your structural engineer on the first submittal.
A measured VS30 from a MASW survey replaces conservative default assumptions with site-specific data—and in Granby’s Champlain clays, that often saves a full site class jump.
Methodology applied in Granby Quebec
Risks and considerations in Granby Quebec
We use a Geometrics Geode 24-channel seismograph with 4.5 Hz vertical geophones on a takeout cable—light enough to deploy on a muddy lot without rutting the subgrade, sensitive enough to pick up Rayleigh wave dispersion down to 5-6 Hz. The main risk in Granby isn't the equipment; it's background vibration. Rue Principale traffic, nearby compressors, even the wind shaking the cable if we don't pin it down properly. We run multiple stacked shots per station and check the amplitude spectra before starting the inversion. If the dispersion image looks smeared, we re-shoot. The other local factor is frost: from December through March, the top 1.2 meters can be frozen solid, which creates a misleading high-velocity cap. We note the ground conditions in the report and, when possible, schedule MASW surveys between April and November to avoid the frost effect altogether.
Our services
Beyond the MASW survey itself, we provide the supporting geotechnical services that an Eastern Townships project typically requires once the seismic site class is established.
MASW Seismic Site Classification
Full MASW survey with 24- or 48-channel array, dispersion curve processing, 1D VS profile inversion, and VS30 calculation mapped to NBCC 2020 site class (A through E). Includes a signed report with raw shot gathers and the dispersion image for peer review.
Combined Geotechnical & Seismic Package
For projects needing both bearing capacity and seismic classification, we bundle the MASW survey with test pits or SPT drillings. The shear wave velocity profile also feeds directly into liquefaction triggering analysis using the Boulanger & Idriss (2014) CPT-based or SPT-based procedures per NBCC 2020 Commentary L.
Quick answers
What does a MASW survey cost in Granby?
How long does it take to get the VS30 results?
Field work typically takes two to three hours for a single 48-meter line. The processing and inversion—picking the fundamental mode dispersion curve, running the iterative inversion, and verifying against any borehole data you have—adds another two to three days in the office. You'll usually have the draft report within a week of the survey. We can fast-track to 48 hours if the foundation permit is held up.
Can you run MASW inside an existing building or on a small urban lot in Granby?
Yes, within limits. A 24-channel array with 1-meter geophone spacing needs about 25 meters of clear ground. On tight lots—think downtown Granby infill—we can sometimes use a shorter array and accept a reduced depth of investigation, or we run the line diagonally. Indoors we have to switch to a lighter source and watch for slab reflections, which complicates the dispersion image. The report flags any data quality limitations so the structural engineer can apply judgment.