Too many projects in Portland push forward with a generic Site Class D assumption because someone decided it was cheaper than testing. That shortcut backfires when the geotech report lands on the structural engineer’s desk and the real shear wave velocity profile demands a totally different seismic design category. We see it in the West Hills, along the Willamette silt benches, and out on the Columbia floodplain — the subsurface here is anything but uniform. A proper MASW survey gives you the Vs30 number that the city’s building department actually wants, not a default that triggers unnecessary overstrength factors. Our field crew runs both active-source and passive-source arrays depending on site access, following the IBC 2021 and ASCE 7-22 framework for site classification. For deeper profiles we often pair it with a CPT test to tie the small-strain stiffness to the stratigraphy, or a downhole seismic cross-check when the client needs redundancy for a peer review.
Guessing Site Class D in Portland costs more in rebar and foundation overdesign than running the MASW line in the first place.
Service characteristics in Portland Oregon
Demonstration video
Local geotechnical conditions in Portland Oregon
Portland’s geology is a patchwork of Missoula Flood deposits, Columbia River Basalt, and Boring Lava domes — meaning two lots half a mile apart can have Vs30 values differing by 300 m/s. The BES building department and neighboring jurisdictions now routinely flag projects that rely on assumed site classes without measured shear wave data. Overestimating the soil stiffness puts the structure at risk of underestimated base shear; underestimating it inflates the seismic design category and the construction budget. On the east side, thick alluvial silts over gravel create strong impedance contrasts that trap seismic energy and amplify ground motion in the 0.5–2 Hz range — exactly where mid-rise buildings have their fundamental period. Without a measured Vs30, the design acceleration parameters from the USGS hazard tool remain unadjusted for site effects, and that gap creates liability that no E&O policy is eager to cover.
Our services
Our Portland-area testing program covers the full workflow from field acquisition to interpreted Vs30 reporting. Every survey includes raw shot gathers, dispersion images, and the inversion model in a format structural engineers can drop into their calculations.
Active-Source MASW
A 24-channel linear array with sledgehammer source, ideal for pavement and compacted subgrade. Resolves Vs to 15–20 meter depth. One-day turnaround on dispersion processing.
Passive-Source MASW (Refraction Microtremor)
Uses ambient traffic and microtremor noise with a circular or L-shaped array. Penetrates to 30+ meters in urban Portland where active sources lack energy. SPAC and FK processing included.
Vs30 Site Classification Package
Combined active and passive survey with a signed report assigning Site Class per IBC 2021. Includes time-averaged Vs30, fundamental site period, and comparison to generic USGS basin amplification factors.
Common questions
What does a MASW test in Portland typically cost for a single-family lot?
For a standard residential parcel in the Portland metro area, a combined active and passive MASW survey with a signed Vs30 report generally runs between US$1,890 and US$3,160. The spread depends on how many array setups are needed to clear the site, whether passive recording time gets extended for deeper penetration, and how much traffic noise interferes with data quality on the day of testing.
How long does the MASW field work take on a typical Portland site?
Most single-array active-source surveys finish in 90 minutes. When we add a passive array for deeper coverage — which is typical in the Willamette Valley alluvium — plan on three hours of field time. Processing and the final report are usually ready within three business days.
Does the City of Portland accept MASW for IBC site classification?
Yes, the Bureau of Development Services routinely accepts MASW-derived Vs30 for assigning Site Class under IBC 2021 Section 1613, provided the report comes from a qualified testing firm and follows the methodology consistent with ASCE 7-22. We include the raw dispersion images, inversion parameters, and interpreted Vs profile so the reviewer can verify every step.
Can MASW replace a borehole on my project?
MASW provides the shear wave velocity profile needed for seismic site classification, but it does not recover soil samples for index testing or visual classification. On most Portland projects, we recommend pairing MASW with at least one SPT boring to calibrate the Vs profile to actual stratigraphy, especially when silt layers from the Missoula Floods create subtle velocity inversions that the surface-wave inversion alone may smooth out.
What is the difference between active and passive MASW, and which one do I need?
Active MASW uses a sledgehammer impact to generate high-frequency surface waves and resolves the upper 15–20 meters well. Passive MASW records ambient noise — cars on I-5, MAX trains, wind — and extracts lower frequencies that sample deeper, often down to 30 meters or more. For a complete Vs30 measurement per ASCE 7, we almost always combine both on Portland sites because the average shear wave velocity calculation integrates the top 30 meters, and you need the passive component to constrain the deeper layers that heavily influence the final Vs30 value.