MASW & VS30 Shear Wave Velocity Testing in Madison, WI

We roll out a 24-channel seismograph with 4.5 Hz geophones spaced across the site—usually a 46- or 69-meter spread depending on how deep we need to profile. The sledgehammer source hits an aluminum plate, and the surface waves travel through layers that tell a story about Madison’s glacial past. Between the Yahara River lakes and the drumlin fields west of town, we’ve mapped VS30 values from soft lacustrine silts below 180 m/s to dense till above 400 m/s.

Getting the array straight and the geophone coupling right matters here. Madison’s topsoil sits over a mix of outwash sand, clayey lake sediment, and weathered sandstone—each layer shifts the dispersion curve differently. We process the shot gathers in the field, pick the fundamental mode, and invert to a 1D shear-wave velocity profile that feeds directly into the seismic microzonation dialogue with the structural engineer. The whole workflow aligns with IBC Chapter 16 and ASCE 7-22 site classification requirements.

A VS30 number without understanding the impedance contrast beneath it can misclassify a site—we’ve caught that on more than one Madison office building.

Our service areas

Our approach and scope

Madison’s freeze-thaw cycle changes the picture. From November through March the upper 12 to 24 inches can be frozen stiff, pushing the apparent VS30 artificially high. We schedule surveys after thaw or pair them with a CPT test to ground-truth the velocity profile below the frost line—the cone data removes any doubt about what’s ice and what’s dense till.

What we see most around the isthmus and the Beltline corridor is a sharp impedance contrast where outwash sands overlie the Tunnel City sandstone or the Eau Claire shale. That contrast shows up as a velocity jump around 15 to 30 meters depth, and it’s critical for site classification because the IBC’s 30-meter average can mask a soft layer sitting right under a stiff cap. Our processing includes Rayleigh-wave ellipticity checks and, when the geology gets layered, we run a joint inversion with a refraction-first-break model for tighter constraints on the Vs30 number that goes onto the permit drawings.

MASW & VS30 Shear Wave Velocity Testing in Madison, WI — Technical reference — Madison

Local geotechnical context

Madison sits at 863 feet above sea level on a landscape shaped by the last glaciation, and the 2018 Sun Prairie explosion reminded everyone that shallow bedrock isn’t uniform across Dane County. A site classified as Site Class D when it should be E changes the seismic design coefficients—and the foundation cost—by a wide margin. We cross-check every MASW line with available boring logs or run a short SPT drilling to verify where refusal depth sits relative to the Vs30 average. The Wisconsin Geological and Natural History Survey’s county-level Quaternary maps are a good starting point, but the local variability between the Horicon Marsh deposits and the Yahara valley fill means no two sites share the same velocity structure, and guessing the site class from a map alone has caused expensive redesigns on more than one Madison project.

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Email: contact@geotechnicalengineering1.org

Reference standards

ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, IBC 2021 Chapter 16 Structural Design, Section 1613 Earthquake Loads, ASTM D7400 Standard Test Methods for Downhole Seismic Testing (reference for Vs interpretation), FHWA NHI-05-037 Geotechnical Site Characterization (reference for dispersion analysis)

Reference parameters

Parameter	Typical value
Line length	46 m or 69 m typical
Geophones	24-channel, 4.5 Hz vertical
Source	8 kg sledgehammer on aluminum strike plate
Sampling interval	0.5 ms
Recording duration	2.0 s
Inversion method	Fundamental-mode dispersion curve inversion
Output	1D Vs profile, Vs30, site class per ASCE 7

Frequently asked questions

What does a MASW survey cost for a typical Madison commercial lot?

For a standard 46-meter or 69-meter spread with one source location, our Madison pricing runs between US$1,720 and US$3,140 depending on site access, line length, and whether we combine it with refraction or a CPT tie-in. We provide a fixed-price quote after reviewing the parcel and existing geotechnical data.

How does the IBC use Vs30 to determine site class?

IBC Table 1613.2.3 references ASCE 7-22 Table 20.3-1, which assigns Site Class A through F based on the time-averaged shear-wave velocity in the upper 30 meters. Class C runs 360 to 760 m/s, Class D 180 to 360 m/s, and Class E below 180 m/s, which we frequently encounter in Madison’s lakebed clay deposits.

Can you run a MASW line on a tight urban lot in downtown Madison?

Yes, we use a 23-meter or shorter spread when space is limited, although the investigation depth reduces to roughly half the array length. For deeper profiles on small lots we often suggest adding a CPT test or downhole Vs measurement from an existing borehole.

How do frozen ground conditions affect the Vs30 result?

Frozen soil can increase the apparent shear-wave velocity by 30 to 50 percent in the top meter, which may push a Site Class E into D. We either schedule the survey after spring thaw or apply a frost correction using shallow CPT data so the final report reflects the thawed condition required by the building code.

What deliverables do we get with a Madison MASW report?

Each report includes the field setup geometry, raw shot gathers, the picked dispersion curve with uncertainty bounds, the inverted 1D Vs profile, the calculated Vs30 value, and the corresponding IBC/ASCE 7 site class. We also include a site plan showing the array location and any notes on nearby utilities or obstructions.

Location and service area

We serve projects in Madison and surrounding areas.

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