The Mississippi River alluvium under St. Louis is deeper than most engineers expect. Reaching 30 to 50 meters of soft clay and loose sand near the floodplain. That changes your site class. And your foundation costs. We run MASW surveys here weekly. The contrast between the stiff glacial till on the bluffs in South City and the compressible river deposits downtown couldn't be sharper. A standard CPT test tells you tip resistance, but it won't give you a full VS30 profile without a seismic cone. That's where the surface wave method shines. We also pair it with SPT drilling when the geologist needs a physical sample to confirm the stratigraphy at depth. This isn't raw exploration. It's site classification done right the first time.
A measured VS30 of 220 m/s puts you in Site Class E. The USGS proxy map won't tell you that. Only a field survey will.
Process and scope
Local ground factors
St. Louis grew along the river. The earliest industrial fill was ash, cinders, and demolition debris dumped into the old sloughs. You find this stuff under Laclede's Landing and parts of the Near North Riverfront. It's loose. It's variable. And it has a very low shear wave velocity. If your structural engineer designs for Site Class C on paper but the ground is actually Class E, the seismic base shear jumps. We've had projects where the lateral force demand increased by 40% after the MASW survey came back. Retrofit is expensive. Litigation is worse. The liquefaction potential in the sand lenses below the fill is a separate concern. You need the VS profile to run a simplified procedure. Seed and Idriss remain the reference. The math doesn't change. The input velocity does.
Reference standards
ASCE 7-22 Chapter 20: Site Classification Procedure, IBC 2021 Section 1613: Earthquake Loads — Site Ground Motion, ASTM D7400: Standard Test Methods for Downhole Seismic Testing (referenced for cross-check), ASTM D4428/D4428M: Crosshole Seismic Testing (applicable methodology reference), FHWA-NHI-16-072: Geotechnical Engineering Circular No. 5 (GEC 5) — Geotechnical Site Characterization
Associated technical services
1D MASW for Site Class
Single array MASW survey to compute VS30. We follow IBC and ASCE 7 Chapter 20 guidelines. You receive a dispersion curve, shear wave velocity profile, and final site class letter within three business days.
2D MASW Cross-Sections
Multiple shot gathers along a line to map lateral velocity changes. Essential near the riverfront where channel fill creates sharp transitions. We overlay the velocity section on your boring logs.
Combined Seismic & Geotechnical Package
MASW plus downhole seismic in the same borehole. This cross-checks the surface wave results with a direct interval velocity measurement. Recommended for Risk Category III and IV structures.
Typical parameters
Questions and answers
How much does a MASW survey cost in St. Louis?
A single 1D MASW survey for VS30 determination typically ranges from US$1,750 to US$3,280 in the St. Louis metro area. The price depends on site access, array length needed, and whether we're working on pavement or soft ground. Multiple arrays or 2D cross-sections increase the scope. We provide a fixed-price quote after a site walk.
How long does the field work take?
A single array setup takes about 45 minutes on site. With mobilization and line layout, most jobs wrap up in half a day. Processing and reporting take two to three business days. Rush delivery is available if you're up against a permit deadline.
Can you run MASW on asphalt or concrete?
Yes. We couple the geophones to pavement using a gypsum paste or quick-setting plaster. The high-velocity surface layer shows up clearly in the dispersion curve. We account for it in the inversion. No drilling required. The method works on parking lots, roadways, and compacted gravel pads.
What's the difference between ReMi and MASW?
Both use surface waves. ReMi relies on passive ambient noise and works well in high-vibration urban settings. MASW uses an active source and gives better control over the frequency range. In St. Louis we often record passive data alongside the active shot. Processing both datasets extends the depth of investigation and cross-validates the dispersion picks.