Seismic Tomography (Refraction/Reflection) for Site Characterization in San Bernardino

During the preliminary grading for a commercial project off Hospitality Lane, the contractor hit unexpected groundwater in what the geological maps showed as dry alluvium. The site sat on a buried channel of the Santa Ana River, invisible from the surface. In a basin like San Bernardino, where Pleistocene alluvial fans interlock with fine-grained playa deposits from the old Lake Manix system, the subsurface is never as uniform as the flat terrain suggests. Seismic tomography—both refraction and reflection—gives us a continuous velocity profile that reveals these hidden transitions. By measuring P-wave and S-wave travel times from an array of geophones, we map bedrock depth, detect fracture zones, and assess rippability without a single borehole. For engineers working under the City of San Bernardino’s geologic hazard review, the method provides the lateral coverage that a standard test pit or discrete boring simply cannot offer. When the project sits within the liquefaction susceptibility zone of the Upper Santa Ana Valley, we often pair the tomography data with a site-specific liquefaction analysis to satisfy the California Geological Survey’s Seismic Hazard Mapping Act requirements.

A well-executed refraction survey in San Bernardino’s alluvial basin can distinguish compacted fanglomerate from weathered granite wash at 80 feet depth—a distinction that changes the foundation type from spread footings to drilled piers.

How we work

San Bernardino lies at 1,049 feet above sea level, pinned between the San Bernardino Mountains to the north and the badlands of the Reche Canyon to the south. The elevation change—over 5,000 feet within five miles—creates one of the steepest groundwater gradients in Southern California. This matters for seismic tomography because the velocity contrast between dry and saturated alluvium can mimic a bedrock signature if not interpreted carefully. Our field crew uses a 24-channel seismograph with 4.5 Hz vertical geophones spaced between 5 and 20 feet, depending on target depth. The source is typically a 20-pound sledgehammer on a strike plate for shallow refraction profiles under 100 feet, or an accelerated weight drop for deeper reflection work. We process the picks using generalized reciprocal method (GRM) traveltime inversion and supplement with seismic reflection processing that includes normal moveout correction and common midpoint stacking. Clients ask us to integrate these velocity models with CPT soundings so the cone tip resistance can be calibrated against the shear-wave velocity for liquefaction triggering correlations. For deep foundations near the Sierra Avenue fault strand, combining tomography with a pile design analysis helps the structural engineer anticipate lateral spreading demands that the code prescribes.

Site-specific factors

The geophone cable runs 575 feet across a graded pad in San Bernardino at 105°F in August. The PVC jacket softens just enough that the connector pins start wobbling, and a single loose channel means re-shooting the entire spread. Our crew carries spare cables and keeps the seismograph under a reflective tarp—lessons learned from summers where asphalt temperatures exceed 140°F. Beyond the equipment headaches, the biggest interpretative risk is misidentifying a groundwater-refraction crossover as bedrock. In the San Bernardino basin, the water table can rise seasonally within 10 feet of the surface, creating a high-velocity layer that masks deeper structure. We run reciprocal shots from both ends of the spread to verify the velocity model and cross-check against any available water well logs from the San Bernardino Valley Municipal Water District’s database. When the tomography profile crosses a suspected trace of the San Jacinto fault zone, we tighten the geophone spacing to 5 feet and extend the recording time to capture low-frequency S-wave arrivals that help delineate the fault gouge.

Technical parameters

Parameter	Typical value
Seismic source for shallow refraction (depth ≤100 ft)	Sledgehammer (20 lb) on aluminum strike plate, stacked 5–10 blows
Seismic source for deep reflection (depth >100 ft)	Accelerated weight drop or Betsy seisgun (shotshell) in rural areas
Geophone array	24-channel, 4.5 Hz vertical geophones, 5–20 ft spacing
Recording instrument	24-bit seismograph, 0.25 ms sample interval
P-wave velocity range in San Bernardino basin sediments	1,200–5,500 ft/s in dry alluvium; 5,500–8,000 ft/s in saturated or cemented deposits
Typical depth of investigation (refraction)	30–150 ft with 230–575 ft spread length
Data processing method	GRM traveltime inversion; reflection CMP stacking with NMO correction

Associated technical services

Seismic Refraction Profiling

P-wave and S-wave traveltime inversion using 24-channel arrays. We deliver layered velocity models, depth-to-bedrock contours, and rippability assessments per Caterpillar D8/D10 specifications for mass grading plans.

Seismic Reflection Surveys

Common midpoint (CMP) acquisition with accelerated weight drop source for imaging stratigraphy below 100 feet. Suitable for mapping buried channels, fault offsets, and groundwater barriers in the San Bernardino basin.

Vs30 and Site Class Determination

Shear-wave velocity profiling to compute the time-averaged Vs30 for ASCE 7 site classification. Required for structural design in Seismic Design Category D and E zones within the city.

Integrated Geophysical & Geotechnical Correlation

Cross-calibration of seismic velocity models with borehole SPT N-values and CPT tip resistance for liquefaction triggering analysis and settlement calculations under the IBC.

Reference standards

ASTM D5777-18 (Standard Guide for Using the Seismic Refraction Method for Subsurface Investigation), ASCE 7-22 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Seismic provisions), 2022 California Building Code, Chapter 16 (Structural Design, incorporating IBC seismic hazard criteria), California Geological Survey Seismic Hazard Mapping Act (PRC 2690–2699.6), liquefaction and ground shaking zones, ASTM D7400-19 (Standard Test Methods for Downhole Seismic Testing, applicable for cross-verification)

Questions and answers

What is the approximate cost of a seismic refraction survey for a typical commercial lot in San Bernardino?

For a standard commercial project requiring 3 to 5 refraction lines with a total spread length of 600–1,200 feet, the field acquisition, processing, and reporting usually falls between US$2.870 and US$5.810. The final number depends on the number of spreads, the source type (sledgehammer versus weight drop), and whether S-wave acquisition is needed for Vs30 site classification. Sites with difficult access—steep terrain near the foothills or active construction zones—may require additional layout time that affects the mobilization cost. We provide a fixed-price proposal after reviewing the site plan and the depth of investigation your geotechnical engineer requires.

How deep can seismic refraction and reflection see in the San Bernardino basin sediments?

Refraction with a 575-foot spread and a sledgehammer source typically resolves to 100–150 feet in the dry alluvium and fanglomerate deposits common across San Bernardino. Reflection surveys using an accelerated weight drop can image to 300–400 feet, enough to map the contact between Quaternary basin fill and the underlying crystalline basement of the Perris Block. The actual depth penetration depends on the velocity structure and ambient noise—working near the BNSF rail yard or the I-215 freeway requires nighttime acquisition to reduce traffic vibration.

Can seismic tomography detect faults on my property before we build?

Seismic reflection and high-resolution refraction can identify velocity discontinuities, offset reflectors, and disturbed zones consistent with faulting. In San Bernardino, where the San Jacinto and San Andreas fault systems create a complex network of splays, the method helps map the approximate trace and width of fault zones. However, seismic tomography alone does not satisfy the Alquist-Priolo Act requirements for fault setback determinations—that requires exploratory trenches logged by a California Professional Geologist. We coordinate with the trenching contractor so the tomography data guides the trench location, reducing the number of linear feet excavated.