A six-story mixed-use project near Santa Ana Boulevard required depth-to-bedrock confirmation after erratic CPT refusal. The site sat within the Los Angeles Basin, where Quaternary alluvium can mask irregular bedrock topography. The geotechnical team ordered a seismic refraction tomography line to map the velocity contrast between the young sediments and the underlying competent formation. By deploying 48 geophones along a 115-meter spread with a 5-meter spacing, the crew captured refracted arrivals that resolved the overburden thickness to within 0.4 meters. The resulting P-wave velocity model showed a clear 1,800 m/s transition at 14 meters depth, matching the top of the Fernando Formation. In Santa Ana, where the groundwater table sits shallow and the basin fill includes interbedded sands and clays, seismic tomography provides continuous coverage that boreholes alone cannot achieve. When the stratigraphy is complex or the project footprint is large, combining MASW with refraction tomography delivers both Vs and Vp profiles for a more complete site classification.
A velocity cross-section from seismic tomography can reveal a buried paleochannel or a fault step that a regular borehole grid would miss entirely.
