A 24-channel seismograph deployed on a linear array with 4.5 Hz geophones is how we start every MASW survey in San Diego. The setup captures Rayleigh wave dispersion across frequencies from 5 to 30 Hz, which we then invert to obtain a 1D shear wave velocity profile down to 30 meters. San Diego’s varied terrain—from the hard Mesozoic granitic rocks of the Peninsular Ranges to the deep alluvial sediments in Mission Valley—demands site-specific VS30 measurements rather than default proxy values. We run active-source arrays using a 16-lb sledgehammer with a steel plate, then stack multiple shots per spread to improve the signal-to-noise ratio in urban corridors where traffic hum from I-5 and I-15 bleeds into the records. The raw shot gathers are inspected before processing to reject traces with excessive cultural noise. For deeper profiling beyond 30 meters, we often combine active MASW with passive microtremor recordings using the seismic refraction method as a complementary tool when the bedrock interface is shallow. In loose coastal and bay-fill deposits near the San Diego Bay, the liquefaction assessment relies directly on the VS30 value we deliver, making the precision of the dispersion curve inversion critical for the site class determination.
VS30 is not a layer velocity—it is the time-averaged shear wave velocity of the top 30 meters, and San Diego’s geology can shift that value by 400 m/s across a single project site.
Service characteristics in San Diego
Local geotechnical conditions in San Diego
San Diego’s post-war expansion pushed residential and commercial development into Mission Valley and the Silver Strand, areas built on deep sedimentary basins that were poorly characterized before modern seismic codes existed. A site classified generically as ‘Site Class D by default’ when it actually falls into Site Class E carries a hidden penalty: the design spectral accelerations from ASCE 7 increase sharply, and the structural system may require costly detailing without the engineer ever knowing the true ground conditions. We have seen San Diego projects where a single MASW survey line revealed a thin high-velocity cap over soft bay mud, producing a VS30 that was 30% lower than the value estimated from nearby borings alone. The risk compounds in the Midway District and areas around Lindbergh Field, where artificial fill overlies Holocene estuarine deposits. Missing the low-velocity zone at 10 to 20 meters depth—exactly where our dispersion curve is most sensitive—results in an unconservative site classification that undermines the entire lateral force-resisting system design.
Our services
Our San Diego MASW surveys deliver the VS30 value required for IBC seismic site classification, plus the full 1D shear wave velocity profile that structural engineers use for site-specific ground motion analysis.
VS30 Site Classification Survey
Single or dual-line active MASW survey with 24-channel acquisition and fundamental-mode dispersion inversion, providing a calibrated VS30 in compliance with ASCE 7-22 and the California Building Code. Deliverables include raw shot gathers, dispersion curves, 1D velocity profiles, and a site classification letter signed by our geophysical team. Typical turnaround in San Diego is five to seven business days from field acquisition.
Deep Passive-Array MASW
Combined active MASW and passive microtremor array (MAM) for sites where the 30-meter VS30 average requires velocity control deeper than the active-source penetration, such as deep alluvial basins in Mission Valley or the Otay River floodplain. We deploy circular arrays with 12 to 24 geophones and record ambient noise for 20 to 30 minutes, then jointly invert the active and passive dispersion data for a unified velocity model.
Frequently asked questions
What does a MASW survey in San Diego cost?
Active MASW surveys in San Diego County typically range from US$1,450 for a single-line residential lot to US$3,230 for a two-line commercial site with passive array integration. The final quote depends on site access, line length, and whether we need traffic control or private property permits. We provide a fixed-price proposal after reviewing the project address and any available geotechnical data.
How is VS30 calculated from MASW data?
We extract the fundamental-mode Rayleigh wave dispersion curve from the shot gathers using the phase-shift method, then invert it iteratively to obtain a 1D shear wave velocity model. VS30 is the time-averaged velocity from the surface to 30 meters depth—the total depth divided by the sum of each layer’s travel time. The inversion is constrained by any available borehole stratigraphy to reduce non-uniqueness.
Why can’t I just use the default Site Class D for my San Diego project?
The IBC allows default Site Class D only if there is no site-specific data and the soil properties are unknown. In San Diego, defaulting to Class D can be unconservative in areas like Mission Bay or the Silver Strand where bay mud produces VS30 below 180 m/s (Site Class E), or overconservative on granitic outcrops east of I-15 where VS30 exceeds 760 m/s (Site Class B). A MASW survey eliminates that uncertainty for a fraction of the structural cost implications.
How long does a MASW field survey take?
A single active MASW line with 24 geophones and multiple shot points takes about 45 to 90 minutes in the field, including setup, calibration, and breakdown. Adding a passive microtremor array extends the recording time by 30 minutes. We typically complete data processing and deliver the VS30 report within five to seven business days after the survey.
Do I need a MASW survey if I already have SPT boreholes?
SPT N-values provide point measurements of penetration resistance, but converting N-values to shear wave velocity involves empirical correlations with significant scatter. MASW directly measures VS across a continuous profile, which is the preferred method under ASCE 7-22 Section 20.4 for Site Class determination. We often run MASW lines between existing boreholes to calibrate the velocity profile and reduce uncertainty in the seismic design parameters.