Tempe’s expansion from an agricultural townsite on the south bank of the Salt River into a dense urban core brought with it a particular geotechnical challenge: the deep sequence of interbedded sands, gravels, and clays that form the basin fill. These deposits, laid down over millennia by the Salt and Gila river systems, exhibit vertical stiffness contrasts that can amplify seismic waves in ways that a simple borehole log cannot capture. A MASW survey measures shear wave velocity directly from the surface, providing the VS30 value that determines the seismic site class under ASCE 7. When the City of Tempe reviews a structural design, the site class derived from a VS30 measurement dictates the ground motion parameters that go into the lateral force-resisting system. Our team has run MASW lines across sites from the ASU Research Park to the neighborhoods north of Tempe Town Lake, where shallow groundwater and cemented caliche layers create velocity inversions that only a properly processed dispersion curve can resolve.
A measured VS30 of 420 m/s versus the default 260 m/s can reduce the design base shear coefficient by 15 to 20 percent on a mid-rise Tempe structure.
Scope of work in Tempe Arizona
Risks and considerations in Tempe Arizona
A four-story mixed-use building on a compacted fill pad near Rural Road was assigned Site Class E by the geotechnical consultant based on a single SPT blow count below 15 in the upper 10 m. The structural engineer had to design for SDS and SD1 values nearly 40% higher than what a Site Class C would have required, adding steel tonnage and delaying the permit. We ran a 92-meter MASW line across the pad and adjacent native soil. The dispersion curve revealed a stiff layer at 6 m—likely a buried Pleistocene gravel bar—that the borehole had missed because of its spacing. The measured VS30 came in at 470 m/s, solidly Class C. The City accepted the report, the structural design was revised to the lower seismic coefficients, and the project saved three weeks on the steel procurement timeline. Basin-fill stratigraphy in the Salt River Valley is laterally discontinuous; a single borehole is a point measurement, while MASW averages a continuous line. When the difference between site classes carries a six-figure structural cost, skipping the VS30 survey is a gamble that no owner in Tempe should take.
Our services
Our Tempe MASW group provides three levels of shear wave velocity characterization, each tailored to the project scale and the complexity of the basin stratigraphy. All reports include the VS30 calculation, the IBC site class determination, and the processed dispersion and inversion data in digital format.
Standard VS30 Survey
Single 46-meter or 92-meter MASW spread, one source position. Suitable for flat sites with uniform stratigraphy. Delivers 1D VS profile and VS30 value within 4 working days.
2D Cross-Section with Multiple Spreads
Roll-along acquisition with overlapping spreads to build a continuous 2D shear wave velocity cross-section. Recommended for sites with lateral heterogeneity, paleochannels, or buried caliche horizons.
MASW + Downhole Combination
Surface MASW calibrated with a downhole seismic survey in an existing borehole. Provides the highest-confidence VS30 when site class is contested or when the structural engineer requests a site-specific response spectrum per ASCE 7 Chapter 21.
Quick answers
How long does a MASW survey take on a typical Tempe commercial lot?
Field acquisition for a single-line MASW survey on a standard commercial lot (roughly 1 to 3 acres) takes between two and three hours, including array layout, source testing, and recording. We process the data in the office the same day or the following morning, and the signed report with the VS30 value and IBC site class is typically delivered within three to four working days. Larger 2D roll-along surveys or sites with heavy traffic that require lane closures may extend the field time by a day.
What is the cost of a MASW / VS30 survey in Tempe?
A standard single-line MASW survey for VS30 determination on a residential or small commercial site in Tempe ranges from US$1,690 to US$3,250, depending on the array length, number of source positions, and travel distance from our base. Larger 2D surveys or combination packages with downhole seismic or SPT drilling are quoted on a project-specific basis after reviewing the site plan and the geotechnical investigation scope.
Can MASW work on sites with asphalt or concrete pavement?
Yes, and we do it routinely in Tempe parking lots and along street rights-of-way. We plant the geophones on the pavement surface using a thin layer of high-viscosity coupling gel or quick-set plaster, which transmits the Rayleigh wave energy without the need to drill through the pavement. The high-velocity cap layer at the surface is correctly modeled in the inversion process. If the pavement is heavily cracked or delaminated, we prefer to remove small patches at the geophone locations to avoid spurious attenuation.
Does the City of Tempe accept a MASW-derived VS30 for site classification?
Yes. The City of Tempe Building Safety Division accepts shear wave velocity measurements obtained by MASW as a basis for assigning Site Class under ASCE 7-16, provided the survey is conducted and reported by a qualified geotechnical engineer or geophysicist. Our reports include the raw dispersion curves, the inversion parameters, and the calculated VS30, and we format them to match the submittal checklist that the City plan reviewers use. We have not had a MASW-based site class rejected in Tempe in the last decade.