The geophone spread rolls out across the lot off Concord Avenue, twenty-four 4.5 Hz receivers planted in a straight line, and the sledgehammer hits the aluminum plate three times before we move the source offset. That is the rhythm of an active-source MASW survey in Concord: methodical, repeatable, and tuned to the stiff alluvial soils that dominate the valley floor. The data runs through a 24-channel seismograph, and by the time we strike the last shot point, the field crew has captured enough Rayleigh-wave dispersion to build a shear-wave velocity profile down to 30 meters. In this part of Contra Costa County, where the Great Valley Sequence meets younger basin fill, getting VS30 right is what separates a defensible seismic design from a costly overestimate. We run these surveys alongside CPT soundings when the stratigraphy looks transitional, because a cone trace paired with a dispersion curve gives both stiffness and layer boundaries in one afternoon.
A well-picked dispersion curve on Concord's Pleistocene alluvium can move a project from Site Class D to C, directly reducing the seismic design category and foundation cost.
How we work
The soil contrast between the Todos Santos Plaza area and the newer subdivisions east of Clayton Road tells the story of Concord's subsurface. Near downtown, you hit stiff Pleistocene alluvium—sands and gravels that generate dispersion curves with a clean fundamental mode, easy to pick and yielding VS30 values commonly in the 360-500 m/s range, which puts those sites in ASCE 7 Site Class C. Move two miles east toward the foothills of Mount Diablo, and the profile shifts: thinner soils over weathered Franciscan bedrock, higher velocities, and occasionally a velocity reversal that complicates the inversion. Our experience across both settings means we recognize when a dispersion image shows a higher-mode contamination from that shallow bedrock reflector, and we adjust the picking accordingly. The active-source method works well here because Concord's traffic noise is manageable during mid-morning windows, and the 30-meter target depth is reachable with a standard 8-kg sledgehammer source on most sites—no need for a vibroseis truck unless we are working next to Highway 4, where ambient noise demands a heavier energy input.
Relevant standards
ASCE/SEI 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Chapter 20 (Site Classification Procedure), ASTM D4428/D4428M-14 Standard Test Methods for Crosshole Seismic Testing (surface-wave adaptation for MASW), 2024 International Building Code (IBC) Section 1613, referencing ASCE 7 site class determination, California Building Code (CBC) Title 24, Part 2, adopting ASCE 7 with state-specific amendments
Common questions
What does a MASW survey in Concord actually measure, and how is VS30 calculated from that?
A MASW survey measures the phase velocity of Rayleigh waves across a range of frequencies, producing a dispersion curve that shows how shear-wave velocity varies with depth. We deploy a 24-geophone linear array with 1-2 meter spacing, strike a sledgehammer source at multiple offsets, and record the surface wave train. The dispersion curve is picked from the frequency-wavenumber spectrum, then inverted into a 1D shear-wave velocity profile. VS30 is the travel-time-weighted average shear-wave velocity in the upper 30 meters, calculated per ASCE 7-22 Section 20.4. If bedrock is shallower than 30 meters—common near the Mount Diablo foothills—VS30 is computed over the soil column to refusal and the rock velocity is assumed constant below that depth.
How much does a MASW/VS30 survey cost for a typical single-family residential lot in Concord?
For a standard residential lot in Concord requiring one or two MASW arrays to achieve 30-meter depth, the survey typically ranges from US$1,660 to US$3,100 depending on site access, number of spreads needed, and whether we deploy a heavier source due to traffic noise near arterials like Willow Pass Road. The price includes field acquisition, dispersion analysis, inversion, and a stamped report with the VS30 value and NEHRP site class. Lots with heavy vegetation, steep slopes, or proximity to 24-hour noise sources may require additional mobilization time.
Can MASW replace a boring and SPT for foundation design in Concord?
No, and we tell every client this up front. MASW gives you shear-wave velocity and seismic site class, which are essential for determining seismic design forces, but it does not tell you soil type, groundwater level, bearing capacity, or settlement potential. For a complete geotechnical investigation in Concord, the MASW survey should be paired with at least one boring with SPT sampling and laboratory classification tests. The two datasets together—velocity profile plus stratigraphy and strength—give you everything needed for both seismic design and foundation sizing.
How long does a MASW survey take on site, and what access do you need?
A single active-source MASW spread on a typical Concord lot takes about 60 to 90 minutes of field time, assuming clear access and no major noise interruptions. We need a linear strip roughly 40 to 50 meters long for the geophone array plus room at each end for source offsets. The ground can be grass, asphalt, or compacted soil—we plant geophones with spike bases on soil or use base plates on hard surfaces. Data processing and inversion are done in the office and typically delivered within three to five business days. If the site is adjacent to BART tracks or the Highway 4 corridor, we schedule testing during low-noise windows and may use an accelerated weight drop instead of the handheld hammer.
