In short: seismic refraction measures how fast sound travels through the ground. Sound moves faster in harder material, so the arrival times recorded by a line of geophones show where loose soil ends, where rock begins and where the ground is weak. Tomographic inversion turns those times into a velocity section along each profile, which engineers use to set the depth of slip surfaces, soil nails, anchors and foundations.
How a seismic refraction survey works
A line of 24 geophones is laid along the profile, typically 3–5 m apart. A sledgehammer is struck on a steel plate at several shot points along and beyond the line, and the seismograph records the waves at every geophone. Each shot is repeated and stacked to lift the signal above traffic and wind noise. Waves that refract along faster layers reach distant geophones first; the pattern of first-arrival times reveals each layer's depth and velocity.
What the velocities tell you
P-wave velocity rises as the ground gets harder and less fractured. The ranges below are typical guides only; every site is calibrated against geology and, where available, boreholes.
| Typical P-wave velocity | What it usually means | Why it matters |
|---|---|---|
| Below about 800 m/s | Dry soil, fill, loose colluvium | Material most likely to slide; thickness sets nail length and excavation |
| About 800–1,500 m/s | Compact or saturated soil, highly weathered rock | Often the zone of shallow slides after rain; water table indicator |
| About 1,500–2,500 m/s | Weathered or fractured rock | Possible slip zones; rock that can often be ripped |
| Above about 2,500 m/s | Fresh, competent rock | Anchor bond zone, foundation level, tunnel rock |
Where we use it
- Landslides and slopes: thickness of unstable cover and depth of possible slip surfaces. See landslide and slope stability.
- Roads and railways in hills: cut-slope design, depth to rock along the alignment, rippability for excavation.
- Tunnels and portals: rock quality and weak zones at portals and shallow sections.
- Dams, weirs and barrages: depth to bedrock along the axis and at abutments.
- Bridges, substations and buildings: foundation level between boreholes.
Why combine it with drone LiDAR
Every geophone and shot point is surveyed by DGPS, and the ground elevations along the profile come from our drone LiDAR terrain model. The tomogram therefore sits exactly on the true ground surface and plots directly on the LiDAR cross-sections, so surface shape and subsurface layers are read together in one ground model. That is how we work on LiDAR and slope projects.
Specifications we work to
| Item | Typical specification |
|---|---|
| Seismograph | 24-channel engineering seismograph, SEG-2 output |
| Geophones | Vertical, 10–14 Hz, at 3–5 m spacing |
| Source | 8 kg sledgehammer on a steel plate, stacked shots |
| Shots | About 9 per spread: off-ends, ends and interior |
| Positioning | DGPS for every geophone and shot; elevations from LiDAR |
| Processing | First-break picking, delay-time starting model, tomographic inversion; RMS misfit target under 5% |
| Quality checks | Daily line and geophone tests, reciprocal-time check, repeat of noisy shots, second-person review of picks |
| Guidance | IS 15681 and ASTM D5777 |
What you receive
- Field data (SEG-2) and observer logs
- Profile location map (KMZ, DWG) in your coordinate system
- First-break picks and time–distance plots
- Velocity tomogram for each profile, with ray coverage and RMS error
- Interpreted section: soil, weathered and fresh rock, weak or saturated zones
- Depth to competent strata along each profile, and a short report
Recent work
On a ~66 km road corridor in Assam we shot 66 seismic refraction profiles, about one per kilometre, alongside drone LiDAR of the full corridor and geological mapping at 7 critical locations. Read the case study.