Seismic Tomography Surveys in Stockton, CA

Out here in the northern San Joaquin Valley, the subsurface doesn't broadcast its intentions. Layers of alluvium, lenses of organics, and the deep Stockton Arch bedrock make every site a puzzle. We've seen projects stall because the standard borehole grid missed a buried channel or a sharp lateral contrast in stiffness. Seismic refraction alone often loses resolution past the water table or when velocity inversions appear. That's why the crew relies on full seismic tomography. We integrate first-arrival traveltime picks with multi-offset reflection processing. The result is a continuous velocity cross-section, not just a low-res 2-layer model. For a warehouse pad near the Port of Stockton, we combined the tomographic profile with MASW to confirm Vs30 values before grading, and the comparison with existing SPT drilling logs tightened the interpretation of the alluvial fill thickness.

A tomogram is not a single velocity. It's 1,200+ ray paths inverted until the model matches the real first arrivals within 2 milliseconds.

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How we work

Stockton's expansion eastward toward the Sierra foothills and westward into Delta-influenced deposits has produced a fragmented geotechnical picture. Historic reports from the 1960s often treated the area as a simple two-layer system: loose sands over stiff Older Alluvium. Modern digital tomography reveals a more complex stratigraphy. The 24- or 48-channel spread deployed by the field team captures subtle velocity gradients within the unsaturated zone above the Corcoran Clay equivalent. On a recent school site near Weston Ranch, we processed over 1,200 ray paths to image a paleochannel at 45 feet depth—something a conventional 10-foot test pit couldn't reach. The inversion algorithm runs 15 to 20 iterations until the RMS misfit drops below the noise floor. For deeper targets, we couple the tomographic line with a seismic refraction walkaway survey to extend the velocity model to 100 feet or beyond. The final deliverables include ray coverage density plots, something the local structural reviewers expect when the design calls for mat foundations on variable fill.

Seismic Tomography Surveys in Stockton, CA — Technical reference — Stockton

Local geotechnical context

The crew arrives with a 48-channel Geometrics Geode system and a spread of 4.5 Hz geophones. On a Stockton summer morning, the temperature can swing from 55°F to 95°F by noon. That thermal drift messes with trigger timing. The field lead checks the uphole geophone before every shot and runs a reciprocal check every 12 shots. Urban noise is the real enemy. A fire truck rumbling down Pacific Avenue or a Union Pacific freight train two blocks away injects 15–30 Hz surface-wave noise that masks the refracted arrival. The crew stacks 5 to 8 impacts per shot point and applies a bandpass filter in the field to verify the first-break pick. Skipping this quality control yields a tomogram that looks smooth but misplaces the bedrock surface by 8 feet. When the profile runs parallel to a canal levee, the saturated fine sands attenuate the signal fast. The team switches to a lower-frequency geophone and increases the stack count to recover the weak P-wave arrival from the deeper refractor.

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Regulatory framework

ASTM D5777-18 Standard Guide for Using the Seismic Refraction Method, ASCE 7-22 Chapter 20 Site Classification Procedure, IBC 2024 Section 1613 Earthquake Loads, ASTM D7128-18 Standard Guide for Seismic Reflection

Technical parameters

Parameter	Typical value
Typical spread length for urban sites	72–115 m (24–48 ch)
Geophone frequency range	4.5–14 Hz vertical component
Energy source for >60 m depth	8–12 lb sledgehammer or accelerated weight
Inversion algorithm	Non-linear traveltime tomography
RMS misfit target	≤ 2.0 ms for refraction, ≤ 5 ms for reflection
Max investigation depth (reflection)	Up to 200 ft with 24-fold stack
Deliverable format	Velocity grid, ray coverage map, SEG-Y files

Frequently asked questions

What depth can refraction tomography reach at a typical Stockton site?

With a 115-meter spread and a 12-pound hammer, the reliable investigation depth is 60 to 80 feet in the saturated alluvium common north of the Calaveras River. The first-arrival energy from the Older Alluvium refractor arrives clearly above the noise. For deeper imaging of the Mehrten Formation contact, we extend the line to 230 meters and use an accelerated weight drop source, reaching depths of 150 to 200 feet.

How much does a seismic tomography survey cost for a one-acre lot in Stockton?

A full survey with a single 115-meter refraction line plus one high-resolution reflection profile typically runs between US$2,440 and US$4,750. The final number depends on site access, the number of shot points needed, and whether we file a subsurface utility clearance ticket with USA North 811 before the field day.

Can the tomogram be used directly for the ASCE 7 site class determination?

Partially. The P-wave velocity model from tomography identifies material boundaries and depth to rock. To determine the shear-wave velocity (Vs30) required for ASCE 7-22 site class, the tomographic line is always paired with a MASW or downhole survey. We deliver a combined report that maps both Vp and Vs across the profile, with the Vs30 value calculated per Chapter 20 guidelines.

Location and service area

We serve projects in Stockton and surrounding areas.

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