The sharp velocity contrast between Franciscan bedrock and Bay Mud across San Francisco creates profiling challenges that conventional borings alone can't resolve. Our lab runs seismic tomography surveys—both refraction and reflection—to map those transitions without punching through sensitive estuarine clays. We deploy 24- to 48-channel arrays across sites from the Marina to Hunters Point, capturing P-wave and S-wave arrivals that reveal fill thickness, rockhead geometry and shear-wave velocities needed for Site Class determination under ASCE 7. When a single borelog misses a buried channel, the continuous velocity profile from a MASW survey fills the gap, and where you need cone resistance at depth we pair tomography with CPT soundings to tie geophysical response to measured soil behavior. San Francisco's high seismic hazard demands subsurface models you can stake a foundation on. We deliver exactly that.
Continuous velocity profiles catch buried channels and abrupt bedrock steps that isolated borings miss entirely.
Scope of work in San Francisco
Critical ground factors in San Francisco
Ground conditions in the Marina District and South of Market differ more than their half-mile separation suggests. The Marina sits on loose to medium-dense hydraulic fill and soft Bay Mud that amplifies long-period motion; SoMa rests on dense sand and shallow Franciscan bedrock that transmits high-frequency energy. Seismic tomography gives you the shear-wave velocity profile both areas demand for Site Class determination, but the resolution targets are different—shallow fill mapping versus bedrock surface delineation. Skipping the velocity measurement and relying on blow-count correlations alone introduces classification uncertainty that can shift your design spectrum. A mischaracterized Site Class in San Francisco means rework on the structural design, and that rework costs far more than a morning of geophone deployment.
Our services
Our seismic tomography work in San Francisco is structured around practical site investigation goals—bedrock mapping, fill thickness determination and shear-wave velocity measurement for seismic design.
Refraction Microtremor & Velocity Profiling
Twenty-four- and 48-channel refraction surveys for Vs30 and Vs100 profiling on urban lots. We invert travel-time curves for layer velocities and thicknesses, delivering the data you need for ASCE 7 Site Class determination.
High-Resolution Reflection Sections
Common-midpoint reflection profiling across steep bedrock relief and suspected fault traces. Stacked sections resolve strata geometry where refraction alone loses resolution on velocity inversions.
Combined Geophysics-Boring Integration
We tie velocity boundaries to logged stratigraphy from nearby borings, calibrating geophysical horizons to actual soil descriptions and standard penetration resistance for a defensible ground model.
Frequently asked questions
What depth can seismic refraction reach on a typical San Francisco lot?
With a 230-ft spread and a weight-drop source, we routinely map to 80-100 ft below grade on dense sand and Franciscan bedrock. Fill over soft Bay Mud limits penetration somewhat because the low-velocity layer attenuates energy, but reflection processing often recovers the deeper structure.
Can you work on busy streets like Market or Van Ness?
Yes. We schedule spreads during lower-traffic windows and use high-stacked impacts to overcome ambient noise. The crew sets up lane closures or flagging per SFMTA requirements, and we keep each spread active for 45-90 minutes depending on channel count.
How much does a seismic tomography survey cost in San Francisco?
Most urban refraction surveys fall between US$2,530 and US$5,540, depending on spread length, channel count and whether reflection processing is included. Larger sites with multiple spread setups run toward the upper end.
What's the difference between refraction and reflection for my project?
Refraction gives you layer velocities and thicknesses for Site Class—it is the standard for Vs30. Reflection images stratigraphic boundaries and is better when you have velocity inversions, like stiff clay under loose fill, where refraction assumptions break down.
How soon do I get the final velocity profile?
Preliminary first-break picks and a velocity stack are usually in your inbox three business days after field acquisition. The fully interpreted profile with CAD layers and a written summary follows within five business days.