MASW & VS30 Shear Wave Testing in Juneau, AK

We deploy a 24-channel seismograph with 4.5 Hz geophones spaced across the terrain—a spread that picks up surface waves down to 100 feet below grade. In Juneau, the setup changes with every job: tight spacing on Gastineau Channel fill, wider arrays up against the Mendenhall Glacier outwash. The field crew has worked saturated organics along Egan Drive and steep colluvium in Douglas. Juneau's seismic demands are real—the Queen Charlotte-Fairweather fault sits just offshore. So we don't guess at shear wave velocity. We measure it directly with active-source MASW and passive microtremor arrays when site noise allows. A single line typically yields a 1D VS30 profile in under two hours, giving you the IBC site class before the excavator shows up. For deeper bedrock mapping, we link the seismic refraction line to the same spread and run both methods without moving the cable. Clients in Juneau keep coming back because we deliver a report that the structural engineer can use same day.

Our Juneau crew can run a full MASW line and deliver VS30 with IBC site class within one working day—before the foundation subcontractor mobilizes.

How we work

ASCE 7-22 and Chapter 16 of the IBC set the framework for seismic site classification, but in Juneau the real challenge is glacial rebound and marine silt. VS30 alone doesn't capture the sharp impedance contrast at the clay-bedrock interface that we see below the Mendenhall Valley. That's why our processing workflow includes dispersion curve inversion with multiple modes and a misfit threshold under five percent. We correlate shear wave profiles with nearby borehole data wherever possible. When a project sits on steep terrain above Thane Road, we combine the VS30 result with a slope stability analysis to evaluate the effect of dynamic amplification on cut faces. The MASW line can be oriented parallel or perpendicular to the slope strike, depending on the failure mechanism being investigated. Every dataset is reviewed by a licensed geotechnical engineer familiar with Southeast Alaska soil conditions. We provide the raw dispersion curves and the inversion picks so the design team can verify assumptions without waiting for an addendum.

Site-specific factors

The Mendenhall Valley and Douglas Island sit on different geological stories. The Valley floor is deep glacial outwash and marine clay—VS30 values routinely below 250 m/s push you into Site Class D or E. Douglas has shallow bedrock and stiffer till, often Class C. A developer who assumes both sides of the bridge behave the same will overdesign the foundation on Douglas or under-design in the Valley. We saw a retail building in the Lemon Creek area where the design assumed Class C based on a 30-year-old regional map. Our MASW line returned VS30 of 210 m/s—Class D. The structural revision added 15 percent to the lateral system cost, but it was caught before steel was ordered. Juneau's microclimates affect scheduling, too: a saturated surficial layer after a week of rain can attenuate high-frequency surface waves. We note the moisture condition in the field log and adjust the source energy to compensate. Leaving out the VS30 measurement when you have 30 feet of soft silt over rock is not a corner worth cutting.

Technical data

Parameter	Typical value
Source type	8 kg sledgehammer or weight drop
Geophone frequency	4.5 Hz (vertical component)
Array length	46 m to 92 m (site-dependent)
Receiver spacing	1 m to 2 m
Depth of investigation	Up to 30 m in glacial till
Sampling rate	0.5 ms
Number of channels	24
Dispersion analysis	Active MASW + passive MAM

Associated technical services

MASW Survey with VS30 & Site Class

Active-source MASW using a 24-channel array. Includes dispersion curve, 1D shear wave velocity profile, VS30 calculation, and a stamped IBC site class determination. Typical turnaround is three business days for a standard single-line survey.

Combined MASW + Seismic Refraction

One cable layout, two methods. We process MASW for VS30 and seismic refraction for rippability and bedrock depth. Ideal for sites with uncertain overburden thickness near Downtown Juneau or Douglas.

Relevant standards

ASCE 7-22 Minimum Design Loads for Buildings and Other Structures, IBC 2024 Chapter 16 – Structural Design (Seismic Site Class), ASTM D7400 Standard Test Methods for Downhole Seismic Testing (cross-reference for borehole correlation), FHWA-NHI-16-072 Geotechnical Site Characterization (MASW guidelines)

Quick answers

What does a MASW survey cost for a typical single-family lot in Juneau?

For a standard single-line MASW survey on a residential lot within the Juneau road system, the fee usually falls between US$1,560 and US$3,570. The exact figure depends on array length, site access, and whether we need to pair passive MAM recording to reach deeper bedrock. We provide a fixed-price quote after reviewing the parcel and any existing soil logs you have.

How long does it take to get the VS30 report after the field work?

We process the dispersion curves and run the inversion the same evening whenever field conditions allow. For a single-line survey without complex passive array processing, you receive the draft VS30 profile and IBC site class letter within 48 to 72 hours. Rush turnaround is available if the foundation contractor is already waiting on the classification.

Can MASW work on a sloping site like the ones above Thane Road?

Yes, and we do it regularly. On slopes steeper than 15 degrees we adjust the geophone coupling with longer spikes and orient the array along the contour to minimize topographic artifacts in the dispersion image. The inversion software includes a topographic correction module. For very steep cuts where slope stability governs, we usually recommend pairing the MASW line with a refraction survey to get both the velocity model and the bedrock profile in one mobilization.