MASW and VS30 Shear Wave Velocity Testing in Fort Worth

The Eagle Ford Shale under Fort Worth isn’t uniform. Weathering depth varies from 5 to 30 feet across Tarrant County, and that variability directly impacts site classification. A flat lot on the west side can classify as Site C while a site three miles east hits Site D once the shale drops deeper. We run MASW surveys here every week, and the spread in VS30 values across Fort Worth is wider than most engineers expect. The seismic refraction method works well in areas where the water table is shallow and the shale is highly fractured. Our crew uses 24-channel seismographs with 4.5 Hz geophones, and we process with both active and passive source data to get a reliable shear wave velocity profile down to 30 meters without guessing at the last few meters.

A measured VS30 of 650 ft/s versus an assumed 1,200 ft/s changes the seismic design category — and that decision gets locked in on day one of the foundation design.

Technical details of the service in Fort Worth

Fort Worth grew fast. Barns became warehouses in the 1940s, then warehouses became mid-rise offices by the 2000s. The problem is that a lot of those early structures were built on spread footings sized for Site C assumptions, and when we run MASW on the adjacent lot for the new addition, we often measure VS30 values that push the site into Site D. That changes the seismic base shear by 30 to 50 percent under ASCE 7-16. The city’s geotechnical review process has tightened significantly since the 2015 code adoption, and they now expect a measured VS30 rather than a correlated value wherever possible. We run the survey, pick dispersion curves, invert with Occam or genetic algorithm routines, and deliver a VS profile and site class letter within three business days. No shortcuts. The data stands up to peer review because the processing is transparent and repeatable. For deep soft clay pockets in the Trinity River floodplain, combining the MASW with an SPT drilling program gives you both the dynamic soil properties and the blow count correlation for liquefaction screening.

MASW and VS30 Shear Wave Velocity Testing in Fort Worth

Parameter	Typical value
Survey method	MASW active + passive (Rayleigh wave)
Maximum depth of investigation	30 m standard; optional 40 m with extended spread
Seismograph channels	24-channel vertical stack
Geophone frequency	4.5 Hz vertical component
Source type	10 kg sledgehammer on aluminum plate; passive from ambient noise
Data inversion	Fundamental mode dispersion, iterative least-squares or genetic algorithm
Applicable standards	ASTM D4428, NEHRP site classification, ASCE 7-16 Chapter 20
Reporting	VS profile, VS30 value, NEHRP site class letter, signed by PE

Risks and considerations in Fort Worth

We see the same thing on job sites near the Trinity River: the contractor assumes bedrock is shallow because they hit refusal with a backhoe at 8 feet, but the MASW shows a shear wave velocity of 600 ft/s at 15 feet and doesn’t cross 1,000 ft/s until 45 feet. That’s Site E, not Site C. The difference in seismic design forces between those two classifications can push a steel frame from an ordinary moment frame into a special moment frame, and that’s a six-figure cost swing on a mid-rise project. Another common trap is running a survey too close to a busy road. Traffic noise below 10 Hz contaminates the passive source data, and if the geophysicist doesn’t recognize it in the f-k spectrum, the inverted VS30 can be off by 200 ft/s. We’ve corrected enough of those surveys to know that field technique matters as much as processing software.

Need a geotechnical assessment?

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Email: contact@geotechnical-engineering1.org

Applicable standards: ASTM D4428, ASCE 7-16 Chapter 20 Site Classification, NEHRP Recommended Seismic Provisions, IBC 2021 Section 1613

Our services

Every MASW survey we run in Fort Worth is processed in-house by the same geophysicist who ran the field acquisition. No subcontracting. No black-box processing. The two service packages below cover the vast majority of project needs.

Standard MASW Package

One survey line, active and passive source acquisition, dispersion analysis, 1D VS profile inversion to 30 meters, VS30 calculation, and NEHRP site class letter. Typical turnaround is three business days. Suitable for most single-lot commercial and residential projects.

Full Geophysical Characterization

Two or more MASW lines plus parallel seismic refraction lines for P-wave velocity and rippability. Includes cross-plot of VS and VP, Poisson’s ratio profile, and correlation with any available boring logs. Used for hospital, school, and mid-rise projects where peer review is anticipated.

Quick answers

When does the City of Fort Worth require a measured VS30 instead of a correlated value?

The city follows IBC 2021 and ASCE 7-16. For structures in Seismic Design Category C or higher, or for sites within 5 miles of a known fault, the building official may request a site-specific geophysical survey. In practice, most mid-rise commercial projects in Fort Worth now include a measured VS30 as standard due diligence.

How long does a MASW survey take on site?

A single-line MASW survey with active and passive acquisition typically takes 60 to 90 minutes of field time, assuming good access and no major traffic noise interference. Larger multi-line surveys may take half a day. Processing and reporting happen back at the lab afterward.

What is the typical cost range for a MASW survey in Fort Worth?

Can MASW data be used for liquefaction analysis?

Yes. VS30 is a key input for simplified liquefaction triggering procedures per Youd and Idriss (2001). When combined with SPT blow counts or CPT tip resistance from a subsurface investigation, the shear wave velocity profile improves the reliability of the liquefaction potential index calculation.

Is a PE stamp included with the MASW report?

Every report we issue includes a Texas-licensed Professional Engineer’s stamp and signature. The report states the site class per ASCE 7-16 and is formatted for direct submission to the building department or structural engineer of record.