The thick blanket of sand and clay overlying karst-prone limestone across Leon County masks a subsurface that does not respond uniformly to long-period energy. Tallahassee’s position near the edge of the Gulf Coastal Plain, where the Hawthorn Group transitions into the Floridan aquifer system, creates sharp impedance contrasts that influence ground motion amplification. A standard boring log alone cannot capture how shear waves propagate through these interbedded Miocene sediments. We run active-source MASW surveys using a 24-channel seismograph and 4.5 Hz geophones, processing dispersion curves to extract a Vs profile down to 30 meters. The resulting VS30 value feeds directly into the site classification framework of ASCE 7-22 Chapter 20, and when liquefaction potential must be ruled out for saturated sands near the Cody Scarp, the same velocity data constrains cyclic resistance estimates without extra drilling.
VS30 is not a formation property — it is a 30-meter average that can mask a soft interlayer. We report the full profile, not just the number.
Our approach and scope
Local ground factors
Around Tallahassee, we often see foundation reports that assign Site Class D based solely on SPT N-values in the upper 20 feet, ignoring a stiff caprock layer at 40 to 50 feet that would raise VS30 and drop the design spectral response. The reverse situation is more dangerous: a dense sand layer in the top 10 meters can inflate the average, hiding a thick, soft clay zone between 15 and 25 meters that amplifies mid-period energy. This matters because the Florida Building Code, which adopts the IBC with state-specific amendments, ties the seismic design category directly to the site class. Misclassifying a Site D as a Site C can under-design lateral force-resisting elements by 20 to 30 percent. On large-footprint projects near the Thomasville Road corridor, lateral variability in Hawthorn stratigraphy means a single MASW line is insufficient — we recommend at least three spread locations to capture the range of VS30 values across the building pad, and we integrate the results with CPT soundings to verify the stratigraphic layering inferred from the velocity model.
Reference standards
ASCE/SEI 7-22 (Minimum Design Loads, Chapter 20 – Site Classification), IBC 2024 (Florida Building Code adoption, Site Class definitions), ASTM D7400-19 (Standard Test Methods for Downhole Seismic Testing — cited for general Vs principles; MASW follows same performance criteria), NEHRP Recommended Seismic Provisions (site amplification factors Fa, Fv)
Additional services
VS30 Site Classification Package
A single-report deliverable combining active MASW profiling with a NEHRP site class letter. Includes the raw dispersion curves, the inverted Vs profile, and the VS30 calculation per the ASCE 7 travel-time average. We carry out the design spectral acceleration parameters for both short-period (S_DS) and 1-second (S_D1) based on the mapped MCE_R values for Leon County.
Downhole Seismic Verification
For critical structures where the MASW VS30 falls within 15 percent of a site class boundary, we mobilize a downhole tool in an existing SPT borehole to measure direct arrival velocities. The downhole data serves as ground truth for the surface-wave inversion and satisfies the higher scrutiny required by some Tallahassee structural peer reviewers.
Typical parameters
Questions and answers
How much does a MASW / VS30 survey cost in Tallahassee?
The fee for a complete active MASW survey with a NEHRP site class report typically runs between US$1,770 and US$3,250. The range depends on the number of spread locations requested (most sites need two or three), the total linear footage, and whether the site has dense vegetation requiring extra clearing. That figure covers field acquisition, dispersion analysis, inversion, and the signed engineering letter.
Can MASW determine the IBC site class when bedrock is deeper than 30 meters?
Yes, and that is the standard scenario in Tallahassee, where the Floridan aquifer limestone often lies below 30 meters. The IBC definition of VS30 is based on the average shear wave velocity in the upper 30 meters regardless of whether rock is encountered. MASW measures the actual velocity of the overburden, so a site with 30 meters of sand and clay will yield a VS30 representative of that material, classifying it as D or E as appropriate.
What surface conditions prevent a reliable MASW survey?
Asphalt or concrete pavement does not block the survey — we couple geophones to hard surfaces with base plates and high-viscosity gel. The bigger obstacles are buried utility lines running parallel to the spread (they create waveguide effects), heavy traffic vibration on adjacent roads, and extremely soft, saturated organic soils near Lake Jackson that attenuate high-frequency Rayleigh waves. In those cases we shift to a longer receiver spacing and lower source frequency, or supplement with a downhole test.
How does the Florida Building Code use VS30 for seismic design?
The Florida Building Code adopts the IBC site classification system, which maps VS30 ranges to Site Classes A through F. For Tallahassee, where mapped MCE_R spectral accelerations are low, the site class primarily influences the amplification factors Fa and Fv. A soft Site E profile can double the short-period design acceleration relative to a Site B reference, making the VS30 determination a direct input to the lateral force calculations required under ASCE 7.