The Cretaceous Selma Group Chalk underlies much of Montgomery, but near the Alabama River the overburden is highly variable — loose alluvial silts and soft clays extending 30 to 50 feet deep. A boring log alone won't capture the shear wave velocity contrast at the soil-rock interface. That's where an MASW survey adds real value. We shoot a linear array of 24 geophones, record surface waves generated by a 20-lb sledgehammer, and invert the dispersion curve to extract a 1D VS profile. The result is a direct VS30 value that feeds into your Site Class determination under IBC Section 1613 and ASCE 7 Chapter 20. In Montgomery County, where the water table can sit within 10 feet of grade in the floodplain, the saturated soft soils depress VS30 significantly — pushing many sites from Class C into Class D. For deeper investigation you can pair the survey with SPT drilling to calibrate the velocity profile against blow count data, or run a CPT sounding when continuous stratigraphy is needed through the chalk contact.
A one-letter difference in IBC Site Class can change your seismic base shear by 30%. The VS30 number that drives that letter comes from data — not from a default assumption.
How we work
Local considerations
Here's an observation from too many Montgomery projects: the geotechnical report lists Site Class D based on SPT N-values in the upper 100 feet, but nobody checked the chalk refusal depth. If the Selma Chalk is shallower than 50 feet, the VS30 can jump enough to push the site into Class C — and the structural engineer just designed for 20% higher seismic loads than necessary. The opposite scenario is worse: assuming Class C when the soft alluvium is thicker than expected, leading to an unconservative design. An MASW survey eliminates the guesswork by measuring VS directly across the full 100-foot column. The IBC permits site classification by measured shear wave velocity as a preferred method; it removes the uncertainty inherent in correlation-based approaches that rely on blow counts or soil type alone.
Video overview
Relevant standards
IBC 2021 Section 1613 (Earthquake Loads), ASCE 7-22 Chapter 20 (Site Classification Procedure), ASTM D7400-19 (Standard Test Methods for Downhole Seismic Testing — surface wave methods referenced)
Associated technical services
Standard VS30 Site Classification
Single-line MASW survey with 24 geophones, 5-shot stack per spread. Delivers 1D VS profile, VS30 value, and IBC Site Class determination. Suitable for building permit submittals on sites under 2 acres.
2D Shear Wave Velocity Cross-Section
Multiple MASW spreads rolled along a survey line to construct a continuous 2D VS cross-section. Identifies lateral velocity contrasts, buried channels, and irregular bedrock topography across the site.
Combined MASW + Seismic Refraction Package
Co-located MASW and P-wave refraction lines providing both VS and VP profiles. Enables calculation of Poisson's ratio and dynamic elastic moduli for settlement analysis and soil-structure interaction modeling.
Typical parameters
Quick answers
What does a Montgomery MASW survey cost?
For a standard single-line VS30 survey on a typical Montgomery County lot, the cost ranges from US$1,920 to US$2,840. The price depends on site access, line length needed to reach 100-foot investigation depth, and whether vegetation clearing is required. We provide a firm quote after reviewing your site plan.
How long does the field work take on site?
A single MASW line takes about two hours of field time — that includes laying out the 24-geophone spread, shooting multiple source positions, and verifying data quality on the seismograph before demobilizing. We can typically complete two to three lines in a single day.
Can you run MASW on a paved parking lot?
Yes, but it requires a different coupling approach. On asphalt or concrete we bolt the geophones to pavement spikes drilled into the surface, and we use a thicker strike plate to avoid fracturing the pavement. The data quality is generally excellent because the rigid surface layer transmits high-frequency energy efficiently.
How does the IBC use VS30 for site classification?
IBC Table 1613.2.2 defines Site Class A through F based on the average shear wave velocity in the upper 100 feet. VS30 above 5,000 ft/s is Class A (hard rock); 2,500 to 5,000 is Class B; 1,200 to 2,500 is Class C; 600 to 1,200 is Class D; and below 600 is Class E. Montgomery sites with thick alluvium often fall into Class D, but sites underlain by shallow Selma Chalk can reach Class C.
What's the difference between MASW and a downhole seismic test?
MASW is non-invasive — we never drill a borehole. It measures surface waves and inverts them to get a 1D VS profile. A downhole test requires a cased borehole and measures direct arrival times from a surface source to a downhole receiver. MASW is faster and cheaper for site classification; downhole gives higher vertical resolution and is preferred when you need detailed VS at specific depths for soil-structure interaction models.