The limestone bedrock in San Antonio can look solid and still transmit poor shear wave velocity. That disconnect costs projects time when the seismic site class comes back wrong. We run active and passive MASW surveys across Bexar County to nail down the real VS30 before foundation design locks in. The Balcones Escarpment runs right through the northwest side of the city, creating abrupt transitions between Glen Rose limestone and deep expansive clay. These transitions are exactly where a seismic refraction profile adds value alongside the MASW array, mapping the rockhead geometry while MASW captures the stiffness gradient. For downtown sites on the old river terrace deposits, the liquefaction screening starts right here with the VS30 data you already need for IBC classification.
A single 24-channel line gives us a Vs profile to 30 meters. We deliver the processed dispersion curves and the site class letter within five business days. No waiting on lab curing. No guesswork on the geophone spacing.
A site class D vs. site class C decision changes the seismic base shear by a factor of nearly two in San Antonio. You need the real Vs before the structural model starts.
Methodology and scope
Local ground factors
The Glen Rose limestone in northwest San Antonio has solution cavities and pinnacled rock that scatter Rayleigh waves badly. A poor dispersion curve from a short array will alias those scatterers as a soft layer and drop the VS30 into site class D where the rock actually qualifies for class C. That single letter mistake can add tens of thousands in unnecessary structural steel. Our processing workflow masks low-coherence time windows and runs both f-k and slant-stack transforms to separate the fundamental mode from higher-mode contamination. On the deep clay sites south of Highway 90, the risk is different: the passive source array must capture wavelengths long enough to reach 30 meters in material where Vs runs under 200 m/s. A short passive window here produces a truncated profile and an artificially high VS30, masking a real soft-soil amplification hazard that the structural engineer must account for. We run the passive array a full 30 minutes minimum on these sites.
Regulatory framework
ASCE 7-22 Seismic Site Classification (Vs30), IBC 2021 Section 1613, ASTM D7400 Standard Test Methods for Downhole Seismic Testing, ASTM D5777 Standard Guide for Seismic Refraction
Associated technical services
IBC Site Class Package
One or two MASW arrays, processed dispersion curves, Vs30 calculation, site class letter signed by our geophysicist. Meets City of San Antonio building review requirements.
MASW plus Liquefaction Screening
Vs30 profile plus cyclic stress ratio analysis for the river terrace deposits downtown and along the San Antonio River corridor. Delivers the liquefaction potential index the structural reviewer expects.
Typical parameters
Frequently asked questions
What does a MASW survey cost in San Antonio?
A standard MASW survey with two array spreads and a VS30 site class report runs between US$1,570 and US$3,000, depending on site access, pavement conditions, and whether we need extended passive recording for noisy urban locations.
How long does the field work take?
Two spreads on an open lot take about two hours. If the site is inside the Loop 410 corridor with heavy traffic noise, we budget three hours to capture clean passive data.
Can MASW work on paved sites?
Yes. We mount the geophones on pavement with adhesive base plates. The coupling is slightly different than soil, so we run a few extra active shots to confirm the dispersion picks. The passive array still works fine through asphalt.
What site class does San Antonio typically fall into?
It varies block by block. The limestone northwest of the Balcones Escarpment often hits site class C. The clay flats south and east of downtown frequently come back as site class D, and occasionally class E on deep alluvium near the river.
Do you provide the raw dispersion curves?
We deliver the full processed curves with the fundamental mode picked, the Vs profile to 30 meters, and the raw shot gathers if the structural engineer or geotechnical reviewer wants to run their own processing.