The high plains around Cheyenne present a geotechnical profile that engineers do not find in the basin fill of Denver or the alluvial valleys further west. At 6,062 feet elevation, the near-surface geology is dominated by the Ogallala and Arikaree formations—Tertiary silts, fine sands, and weakly cemented sandstone—overlain by thin colluvial veneer on the slopes descending toward Crow Creek. These materials generate shear wave velocity profiles that often plot near the boundary between IBC Site Class C and D, a gray zone where the difference in seismic base shear can exceed 30 percent. When a site sits on 15 to 30 feet of stiff silt over weathered bedrock, borehole refusal can occur before reaching the 100-foot depth required for a full VS30 calculation.
We deploy active-source MASW with a 24-channel seismograph and 4.5 Hz geophones to capture the fundamental-mode Rayleigh wave dispersion curve, then invert it to a 1D shear wave velocity profile. In Cheyenne’s dry, low-plasticity silts, the method works reliably down to 100 to 130 feet, well past the 30-meter target. For deeper bedrock mapping on larger parcels—like the data center sites developing east of town—we combine the MASW line with a seismic refraction spread to constrain the P-wave bedrock interface before running the inversion.
At 6,000 feet on the Cheyenne Ridge, a VS30 of 1,200 ft/s can push a site from Class D into Class C—cutting the design base shear by 20 percent with one field test.
Our approach and scope
Cheyenne’s growth since the Union Pacific Railroad established its roundhouse here in the 1860s has pushed development from the original downtown grid onto the undulating terrain of the High Plains. The city’s adopted IBC with Wyoming-specific amendments requires Site Class determination for all structures assigned to Seismic Design Category C and above, which includes most essential facilities, schools, and multi-story commercial buildings in Laramie County. Our field procedure follows the guidelines of the Kansas Geological Survey MASW method and the ASTM D7400 standard for downhole seismic testing, adapted for surface acquisition.
We lay out a 115-foot linear array with 2-foot geophone spacing, use a 10-pound sledge on an aluminum strike plate for the active source, and stack three to five shots per spread to improve signal-to-noise ratio against Cheyenne’s persistent afternoon wind. Post-processing extracts the dispersion image, picks the fundamental mode between 5 and 45 Hz, and runs a least-squares inversion to produce a layered shear wave velocity model. The
liquefaction screening protocols in ASCE 7-22 reference this same velocity data, so the MASW result serves double duty when the water table is shallow near the creek corridors.
Local ground factors
Cheyenne’s seismic hazard is moderate but not negligible—the USGS 2023 NSHM places the city within the Western Cheyenne Belt of diffuse seismicity, with a 2% probability of exceedance in 50 years at a peak ground acceleration around 0.10 to 0.12 g. The risk is not the PGA itself; it is misclassifying the site and under-designing the lateral system. A site on 25 feet of stiff Arikaree silt may produce a VS30 of 1,100 ft/s and map to Site Class D under the default assumption, but a MASW profile that captures the velocity jump into the underlying sandstone can lift the weighted average above the 1,200 ft/s threshold for Class C. That single reclassification can reduce the seismic base shear coefficient Cs by 15 to 25 percent, directly shrinking foundation sizes, shear wall lengths, and anchor bolt diameters. We have seen this exact scenario on two medical office projects near the Cheyenne Regional Medical Center campus, where the geophysical data saved the structural team from over-designing for a code-default Site Class D.
Quick answers
How much does a MASW survey for site classification cost in Cheyenne?
A single-line active-source MASW survey for VS30 determination in the Cheyenne area typically runs between US$1,830 and US$3,510, depending on the number of spreads required and whether the site is open ground or has existing pavement that needs saw-cutting for geophone coupling. The price includes field acquisition, dispersion analysis, inversion modeling, and the signed site classification letter. Multi-line surveys for 2D cross-sections fall at the upper end of that range.
What depth does the MASW method reach in Cheyenne's soils?
With the standard 115-foot array and a 10-pound sledge source, we consistently resolve shear wave velocity to 100–130 feet (30–40 meters) in the Ogallala and Arikaree silts that dominate the Cheyenne Ridge. The active-source method works well here because the near-surface materials are dry and stiff, transmitting high-frequency Rayleigh waves efficiently. When deeper bedrock mapping is needed beyond 40 meters, we augment the survey with passive-source recordings using ambient noise and a longer triangular array.
How long does it take to get the site classification letter after the field survey?
Field acquisition for a single MASW line takes about two hours on site. We run initial dispersion processing in the field to verify data quality before demobilizing. The full inversion, VS30 calculation, and signed report are delivered in three to four business days. For projects on a tight deadline—like a foundation permit that is waiting on the Site Class letter—we can expedite to next-day delivery.
