ASCE 7 and the North Carolina Building Code require reliable subsurface characterization for any major structure in the Piedmont. In Raleigh, where saprolite overlies diabase and granite bedrock at variable depths, seismic tomography—both refraction and reflection—delivers continuous velocity profiles that standard borings alone cannot. We field crews have mapped transition zones beneath I-540 extensions and downtown parking decks, resolving rippability limits and fracture sets that directly affect foundation design. The method works equally well on greenfield sites and urban infill lots, provided we manage the ambient noise from I-40 traffic and the Norfolk Southern rail corridor. Combining this dataset with a CPT campaign often sharpens the geotechnical model where low-velocity zones suggest residual clay pockets.
Seismic velocity cross-sections resolve what borings extrapolate—in Raleigh’s variable saprolite, that difference often changes the foundation recommendation.
Process and scope
Site-specific factors
The most expensive mistake we see in Raleigh is mistaking a boulder field for bedrock on the tomogram. A single high-velocity anomaly floating in saprolite can mimic a shallow refusal if the survey line is too short or the source energy insufficient. Contractors who run a five-ton excavator expecting rippable soil hit a diabase float and assume refusal, then redesign footings at extra cost. We avoid this by shooting reciprocal spreads and running a tomographic inversion that recovers velocity gradients, not just first-break picks. Another common oversight is ignoring Poisson’s ratio: S-wave refraction, combined with MASW, separates saturated clay lenses from low-velocity rock, which matters enormously when the water table in the Raleigh Belt fluctuates seasonally.
Reference standards
ASTM D5777 – Standard Guide for Using the Seismic Refraction Method, ASTM D7128 – Standard Guide for Using the Seismic Reflection Method, ASCE 7-22 – Minimum Design Loads and Associated Criteria, 2021 International Building Code (IBC)
Complementary services
2D seismic refraction profiling
Standard 24- or 48-geophone spreads to map top-of-bedrock and rippability across residential and commercial lots.
High-resolution shallow reflection
Targets stratigraphic boundaries, paleochannels, and karst features in the top 100 feet with 1- to 2-foot vertical resolution.
Downhole and crosshole seismic
Borehole-based P- and S-wave velocity measurements for site-class determination per ASCE 7, Chapter 20.
MASW and S-wave refraction combo
Integrated surface-wave and body-wave processing to derive Vs30 profiles and identify low-velocity saturated zones.
Typical parameters
Common questions
What depth can seismic refraction reach in the Raleigh area?
With a 230-foot spread and a sledgehammer source, we typically resolve bedrock down to 80 or 100 feet. An accelerated weight drop can push that to 150 feet in competent rock. Where saprolite is thick—common east of the Raleigh Belt—the velocity contrast with fresh diabase remains strong enough to give a clear first arrival.
How much does a seismic tomography survey cost on a typical Raleigh lot?
A single 2D refraction line with 24 geophones generally falls between US$2,840 and US$5,030, depending on line length, source type, and whether we add S-wave or reflection acquisition. Multi-line programs and borehole seismic work are quoted per project.
Can seismic tomography replace soil borings for IBC site classification?
No—the IBC requires direct sampling for classification. Seismic tomography complements borings by providing continuous velocity data between and below sampling points. We often combine borings with downhole seismic and MASW to meet both the letter of the code and the geotechnical reality of Raleigh’s piedmont geology.