When we run surface wave surveys in Boise, we are not just chasing a number. The local geology under this city is a patchwork of ancient Lake Bonneville sediments, fluvial deposits from the Boise River, and shallow basalt bedrock that can fool you if you only look at borings. ASCE 7-22 Chapter 20 requires a measured VS30 for Site Class determination, and the IBC enforces it. But out here, a generic assumption can overestimate the short-period site coefficient by 20 percent or more. We have seen that on projects near the airport, where stiff gravels sit over softer silts, and the seismic response is not what a default Site Class D would predict. The MASW method lets us map the shear wave velocity profile continuously, without the access problems of a drill rig in tight residential lots or the basalt float that stops SPT refusal short. We often pair the surface data with a few targeted SPT borings to ground-truth the stratigraphy, but the velocity profile is what drives the site classification.
A measured VS30 on Boise's alluvial terraces can shift the site class from D to C, reducing base shear demands by over 15 percent and cutting foundation costs on commercial projects.
Our approach and scope
Local ground factors
A four-story mixed-use building on Fairview Avenue, 2019. The geotechnical report assumed Site Class D based on SPT N-values in the upper 20 feet. The structural engineer designed for a short-period acceleration SDS of 0.28g. During plan review, the city asked for a site-specific VS30. We ran a 70-meter MASW line in the parking lot behind the existing strip mall. The basalt was at 18 feet, not 50, and the profile averaged 420 m/s. That pushed the site to Class C, dropping the design SDS by 18 percent. The lateral system got lighter, the footing sizes shrank, and the owner saved close to six figures in concrete and rebar. The flip side happens too: a warehouse in Meridian where the soils looked stiff but the velocity came back at 195 m/s, triggering a seismic amplification factor that the original design did not account for. In Boise, skipping the VS30 measurement is not a code violation, but it is an engineering bet you do not need to take.
Relevant standards
ASTM D4428 / D7400, ASCE 7-22 Chapter 20 - Site Classification Procedure, IBC 2021 Section 1613, NEHRP Recommended Provisions (FEMA P-2082)
Other technical services
VS30 Site Classification Survey
MASW targeting 30 m depth for IBC/ASCE 7 Site Class determination. Includes 1D VS profile, calculated VS30, and NEHRP site class letter. Typical for commercial pads, schools, and multifamily projects in Boise.
Deep VS Profiling for Seismic Hazard Analysis
Extended-receiver MASW with passive-source supplementation (HVSR) for depths to 100 m or more. Used for seismic base isolation design and site-specific ground motion studies when the Boise foothills fault proximity is a concern.
2D Shear Wave Velocity Cross-Sections
Multiple MASW lines gridded or parallel to map lateral velocity variations. We run these for large industrial sites in the Treasure Valley where the Boise River paleochannels create abrupt stiffness contrasts that affect differential motion across the footprint.
Typical parameters
Common questions
How long does a standard MASW survey take on a Boise site?
A single-line VS30 survey on a typical lot in Boise, with a 24-channel spread and good access, takes us about two to three hours in the field. Processing and reporting add another two to three days. Sites with heavy truck traffic on Federal Way or near the connector, where we have to wait for quiet windows, can stretch the field time to half a day.
What is the cost range for a MASW survey in Boise?
For a single-line VS30 survey on a standard commercial lot, the cost in Boise typically falls between US$1,850 and US$3,490, depending on the array length, number of spreads, and whether we need to supplement with passive-source recordings. Sites with basalt at very shallow depth that require multiple offset shots to isolate the fundamental mode can be at the higher end.
Do I need VS30 if my borings already say Site Class D?
Not by code, necessarily, but we have seen plenty of Boise sites where SPT-based classification is conservative by one class. The basalt is shallow under much of the Bench and North End, and the river gravels west of downtown are stiffer than they look. A measured VS30 can often justify moving from D to C, which reduces the seismic load path and can slim the structural frame. The cost of the survey is usually recovered in the concrete and steel savings.
Can you run MASW on a site with basalt float at the surface?
We can, but we have to work around the scatter. The high-velocity basalt blocks near the surface create a heterogeneous medium that breaks the plane-layer assumption in standard MASW inversion. We handle this by shooting multiple source offsets, stacking longer, and using joint inversion with refraction tomography when the contrast is extreme. On the Boise foothills, we often combine MASW with a short seismic refraction line to constrain the top-of-basalt depth before running the velocity inversion.
How does the Boise River water table affect the velocity measurements?
Not as much as you would think for the VS30 calculation, because the saturated silt and sand velocity does not drop dramatically compared to the same material above the water table. What does change is the Poisson ratio and the Vp/Vs ratio, which can help us identify the saturated zone in the inversion. In spring, when the river is high and the groundwater in the Boise Bench is within 5 feet of the surface, we see a clear Vp jump in the refraction picks that correlates with the saturation front.