A contractor called us last spring about a site up on Tacoma’s hilltop with a planned four-story medical office. The geotech report was clean, but the excavator hit a buried basalt ridge at 9 feet. The project lost two weeks. That’s the kind of surprise Tacoma geology throws at you. With 47.25°N latitude and the shadow of Mount Rainier on clear days, the city sits on the complex Vashon glacial till overlying Tertiary volcanic and sedimentary rocks. Seismic tomography—both refraction and reflection—gives us a continuous velocity cross-section before anyone puts a bucket in the ground. When the drill rig can’t access a slope or the budget demands broader coverage, we run a seismic spread across the parcel and deliver a 2D velocity model that maps the top of competent rock, low-velocity zones, and weathered horizons. In a port city where the Commencement Bay shoreline was historically filled, combining seismic refraction with targeted boreholes clarifies where natural deposits transition to anthropogenic fill, which matters a lot for foundation design in Tacoma’s seismic setting.
A P-wave velocity jump from 800 m/s in till to 2,800 m/s in basalt defines the excavation line in Tacoma’s hill districts.
Service characteristics in Tacoma
Demonstration video
Critical ground factors in Tacoma
The most common mistake we see in Tacoma is relying on a single borehole to characterize a site with lateral variability. A geologist logs stiff till at the drill point, but 30 meters away the till thins over a buried channel of the Puyallup Formation—and nobody knows until the excavator finds running sand. Seismic tomography catches that lens because the velocity drops from 1,500 m/s to 700 m/s across the array. Another classic error is assuming that refusal in a hollow-stem auger means bedrock. We have seen refusal on a 0.5-meter boulder resting in till, and the real rock was another 5 meters down. The refraction profile shows a continuous high-velocity layer, not a point anomaly. In Tacoma’s Seismic Design Category D, misidentifying the site class because of a thin stiff layer over soft clay can shift the short-period acceleration coefficient and lead to an under-designed lateral system. The IBC requires shear-wave velocity data for Site Class determination; a P-wave refraction survey plus a few downhole or MASW points provides the Vs30 value and the stratigraphic control to justify it.
Our services
Every Tacoma survey starts with a site walk to check access constraints—narrow alleys in the Stadium District, buried utilities near the port, vibration limits next to old masonry. We then design the seismic line orientation to cross known or suspected geologic boundaries, because a line running parallel to a buried channel margin gives ambiguous data. The deliverables include a velocity cross-section with interpreted horizons, a rippability chart for the contractor, and the layer velocities needed for site class determination under IBC.
Seismic Refraction Tomography Survey
A 24- or 48-channel spread laid perpendicular to strike of Tacoma’s glacial features. We deliver a 2D P-wave velocity model with annotated top-of-till and top-of-bedrock surfaces, ray-coverage map, and Vs30 estimate when combined with MASW or downhole calibration. Typical turnaround: 5 working days from field completion.
Combined Refraction + Reflection for Deep Bedrock Mapping
When Tacoma projects require bedrock depth below 30 meters—such as deep utilities or bridge piers near the Hylebos Waterway—we acquire a dual-mode line. The refraction data resolves the shallow till and fill; the high-fold reflection stack images the deeper unconformity. Processing includes velocity analysis, NMO correction, and post-stack depth conversion tied to any available borehole control.
Quick answers
What is the typical cost of a seismic refraction survey for a Tacoma residential lot?
For a single-family or small commercial parcel in Tacoma, a 115-meter refraction line with 24 geophones generally runs between US$2,710 and US$5,240. The range depends on site access, surface conditions (asphalt vs. grass), and whether we need traffic control if the line crosses a street. A full report with velocity cross-sections and IBC site class input is included.
Can seismic tomography distinguish between Vashon till and the Lawton Clay?
Yes, often. The Vashon till in Tacoma typically shows P-wave velocities of 1,200–1,600 m/s when dense, while the underlying Lawton Clay sits in the 900–1,100 m/s range because of its higher silt and moisture content. The contrast is subtle and requires good signal-to-noise ratio, but with tight geophone spacing and careful first-break picking, the interface can be traced across a site. We usually ground-truth with at least one borehole or CPT sounding to confirm the contact.
How long does a seismic tomography survey take on a Tacoma site?
Field work for a standard 115-meter refraction line takes one day with a two-person crew—assuming typical Tacoma conditions like sidewalk closures in the North End or vibration monitoring near unreinforced masonry. Processing and interpretation add three to four business days. A combined refraction-reflection survey with a longer spread may need two field days plus a week of processing.