The Barnett Shale sits roughly 2,500 feet beneath Fort Worth, but the real story for any structure here lies in the upper 100 feet. The Eagle Ford and Woodbine groups transition into expansive clays and alluvial terrace deposits across the Trinity River basin. We run seismic microzonation campaigns that map shear wave velocity profiles across entire project footprints, not just a single borehole. For sites near the West Fork or Clear Fork, where soft sediments amplify long-period motion, this data feeds directly into site class determination per ASCE 7-22 Chapter 20. A MASW survey can efficiently cover the lateral variability, and when we need refusal-layer confirmation, SPT drilling ties the geophysical model to N-values and sample recovery.
A site class E boundary running diagonally through your building pad means differential ground motion — we map that before the structural engineer draws the first beam.
Technical details of the service in Fort Worth
Demonstration video
Risks and considerations in Fort Worth
A lot of Fort Worth construction sits on the Eagle Ford Group, which weathers to a stiff clay that feels like rock on a backhoe but still classifies as site class D. The problem arises when that weathered profile is only 15 feet thick over a deeper, softer terrace deposit — you get an impedance contrast that amplifies short-period shaking. We have seen Vs30 values shift from 600 ft/s to 400 ft/s within 80 lateral feet on the east side of downtown. Skip the microzonation, and you might design the entire building for the wrong site class. The IBC explicitly requires site-specific ground motion analysis for site class F, but even for class D and E structures, the 20% design acceleration penalty under ASCE 7 Table 11.4-1 pays for the investigation several times over in foundation concrete alone.
Our services
Our seismic microzonation work in Fort Worth splits into two service tiers, depending on whether you need regulatory compliance or a full performance-based design input.
Code-Compliant Vs30 Mapping
Multi-line MASW and MAM arrays gridded across your site to produce a contoured Vs30 map. We deliver the site class boundary polygons, average shear wave velocity, and the design spectral accelerations SDS and SD1 per IBC and ASCE 7. This is the minimum requirement for site-specific ground motion studies under Section 11.4.8.
Site Response and Amplification Analysis
One-dimensional equivalent-linear or nonlinear site response using DEEPSOIL or STRATA. We model the full soil column from bedrock to surface, incorporating modulus reduction and damping curves calibrated to local Eagle Ford clay and terrace sand units. The output is a site-specific response spectrum and acceleration time histories for structural analysis.
Frequently asked questions
When does the Fort Worth building department require a site-specific seismic study?
The City of Fort Worth adopts the IBC with local amendments. Under IBC 2024 Section 1613.1, a site-specific ground motion analysis is required for structures assigned to Seismic Design Category D, E, or F when the site class is F (liquefiable soils, peats, very high plasticity clays). It is also triggered when the mapped spectral accelerations exceed the code limits for the chosen site class. Practically, this applies to essential facilities, tall buildings above 240 feet, and structures on soft soil profiles near the river corridors.
How many MASW lines do I need for my site?
It depends on site area and geological variability. For a typical 2-acre commercial lot, we recommend 5 to 7 lines with at least one crossing the long axis. For larger industrial sites, we use a grid pattern with 50-foot spacing and tie lines every 200 feet. The goal is to capture lateral changes in the top 100 feet — the Eagle Ford weathering profile can shift from stiff to soft within a single city block. We finalize the array layout after reviewing any existing boring logs and the USGS surficial geology map for the specific Fort Worth quadrangle.
What is the difference between MASW and downhole seismic?
MASW measures surface waves — Rayleigh waves — to build a 1D shear wave velocity profile without drilling. It is non-invasive and covers lateral variability quickly. Downhole seismic requires a cased borehole and measures direct arrival times from a surface source to a downhole geophone. Downhole gives you higher resolution at depth, but only at one point. For seismic microzonation in Fort Worth, we combine both: MASW for the Vs30 map across the site, and downhole at one or two key locations to tie the surface wave model to a direct measurement through the weathered Eagle Ford and into the Woodbine sandstone.
How much does a seismic microzonation study cost in Fort Worth?
A seismic microzonation study typically ranges from US$4,040 to US$18,730 depending on site size, number of array spreads, and whether a downhole calibration is required. A basic code-compliance package for a single-building lot with 5 MASW lines starts at the lower end. A full campaign for a multi-acre development with passive arrays, downhole seismic, and nonlinear site response analysis falls at the upper end. We provide a fixed-fee proposal after reviewing the site plan and any existing geotechnical data.
Do you need access inside existing buildings?
No. All surface wave methods — active MASW and passive MAM — are deployed on open ground. We can work on paved surfaces with proper geophone coupling using plaster or sand bags. For active shots, we need a clear zone of about 200 feet to lay out the geophone spread. If the site is heavily obstructed, we adapt with shorter spreads and higher-frequency geophones, though the maximum investigation depth decreases accordingly. The processing compensates for near-field noise from traffic or machinery, which is common on Fort Worth job sites near I-35W.