ASCE 7-22 Chapter 20 requires a shear wave velocity profile for Site Class determination on any project using the IBC in Texas. In Brownsville, the deep Holocene alluvium of the Rio Grande delta creates a soft soil profile that rarely reaches the 1,000 m/s seismic bedrock within the top 30 meters. Our MASW testing provides direct Vs30 measurement for Site Class E or F boundary determination, eliminating the default conservative assumptions that inflate foundation costs. The active-source multichannel array method we deploy captures Rayleigh wave dispersion across frequencies from 5 to 35 Hz, sampling the upper 30 meters of the deltaic clays and silts that dominate the local subsurface. For sites near the Resaca channels or within the 100-year floodplain, liquefaction analysis becomes critical once the Vs profile confirms the soft soil susceptibility. We also integrate seismic microzonation data from regional studies to refine the site response spectrum for the specific block location in Cameron County.
Vs30 is not an intrinsic property of the soil. It is a parameter defined by the 30-meter average shear wave velocity as specified in ASCE 7-22 Section 20.4.1.
Technical details of the service in Brownsville Texas
Risks and considerations in Brownsville Texas
Brownsville grew from Fort Texas in 1846 into a sprawling low-rise city built entirely on floodplain deposits. The subsurface is dominated by the Beaumont Formation, Pleistocene to Holocene clays and silts with interbedded sand lenses deposited by the meandering Rio Grande. These soils exhibit shear wave velocities typically between 150 and 250 m/s in the upper 30 meters. The seismic hazard is real: the city sits 250 km from the Gulf of Mexico subduction interface and 100 km from the active Mexicali seismic zone. A Vs30 below 180 m/s triggers Site Class E with a significant amplification factor. Below 150 m/s, IBC requires a site-specific ground motion hazard analysis. The soft clay also carries cyclic degradation risk under long-duration shaking. A single triaxial cyclic test on an undisturbed Shelby tube sample quantifies the modulus reduction curve for site response modeling. For projects with deep excavations near the historic downtown, excavation monitoring with inclinometers and piezometers provides real-time deformation control.
Our services
The MASW survey delivers the Vs30 number required by the building official. But the velocity profile also feeds into other design tasks. These four services package the geophysical data into actionable engineering parameters for the Brownsville subsurface.
MASW Active Source Survey
24- or 48-channel array with sledgehammer source. Dispersion curve extraction, inversion to 1D Vs profile, and Vs30 calculation per ASCE 7-22.
ReMi Passive Source Survey
Ambient noise recording with 2D array geometry for deeper Vs profiling beyond 30 m when bedrock depth exceeds active-source penetration.
Site-Specific Response Analysis
1D equivalent linear or nonlinear ground response analysis using DEEPSOIL or SHAKE. Input motions scaled to the USGS 2018 hazard for Cameron County.
Vs30 Map & Report
Signed and sealed report with dispersion curves, inversion parameters, Vs profile plot, and Site Class letter for submission to the City of Brownsville Building Department.
Questions and answers
What does a MASW test cost for a standard lot in Brownsville?
How does the City of Brownsville use the Vs30 value in plan review?
The Building Department enforces the IBC 2024 edition. They require a Site Class letter (A through F) based on the Vs30 from a field geophysical survey. A boring log with SPT N-values alone is not sufficient to distinguish between Site Class D and E in the deltaic clay. The Vs30 number goes into the structural design basis for the seismic force-resisting system.
Can MASW work on small residential lots with limited space?
A 24-channel array with 1-meter spacing requires 23 meters of clear ground. On tight urban lots under 25 meters, we switch to a downhole seismic test in a single borehole or combine a short active-source line with a passive-source ReMi array using wireless nodes. Both methods yield a valid Vs30 for the building permit.
What is the difference between Vs30 from MASW and from a downhole test?
MASW measures Rayleigh wave dispersion averaged over the array length and inverted to a 1D Vs profile. A downhole test measures the direct arrival of P- and S-waves between a surface source and a borehole geophone at discrete depth intervals. MASW gives a continuous velocity profile but loses resolution in sharp velocity contrasts. Downhole resolves thin layers better but requires a cased borehole. For the thick clay sequences in Brownsville, the two methods agree within 10% on Vs30.