Step 01
Prepare the borehole
A dedicated borehole is cased with PVC and the annulus fully grouted, so the casing is coupled to the ground rather than floating in slurry.
Downhole Seismic Test
Seismic design needs stiffness at very small strain, which no laboratory sample can give you reliably. We measure wave velocity in the ground itself, down the borehole, layer by layer.
Downhole Seismic Test: what it actually tells you
A borehole is cased and grouted, then a triaxial geophone is clamped at successive depths. A shear source at the surface, usually a weighted plank struck from both ends, sends a polarised shear wave down to the receiver, and the travel time to each depth gives the shear wave velocity of each layer.
Shear wave velocity gives you small strain shear modulus directly, which is the stiffness that governs foundation response to vibration and to earthquake shaking. Compression wave velocity, recorded from a vertical strike, gives Poisson ratio when paired with the shear velocity.
The headline number is Vs30, the time averaged shear wave velocity over the top 30 metres. It is what building codes use to place your site in a seismic class, and in Pakistan that classification drives the design response spectrum. Karachi and much of Sindh sit on ground where this classification genuinely changes the design forces.
At a glance
- Depth rangeTypically 30 m, deeper on request
- Reading interval1.0 m
- BoreholeCased and fully grouted, prepared in advance
- OutputVs and Vp profiles, Vs30, site class
How we run it
The sequence we follow on site and in the laboratory, and the point of each stage.
Step 02
Set the source
A shear source is set at the surface at a fixed offset, weighted down, and struck from each end so the shear arrival can be identified by polarity reversal.
Step 03
Clamp and record
A triaxial geophone is clamped against the casing wall at one metre intervals, and records are taken at each depth with repeat strikes stacked to improve the signal.
Step 04
Pick arrivals
Shear arrivals are picked on the reversed traces where the polarity flips, which is far more reliable than picking a first break by amplitude alone.
Step 05
Compute the profile
Travel times are corrected for the source offset and converted to interval velocities, then to shear modulus, Poisson ratio and the Vs30 average.
What you receive
- Shear wave velocity profile with depth
- Compression wave velocity profile
- Vs30 and the resulting seismic site class
- Small strain shear modulus for each layer
- Poisson ratio and dynamic elastic moduli
When this test is the right one
- Structures designed to a seismic code
- Dynamic machine and turbine foundations
- High rise buildings where site response matters
- Liquefaction assessment alongside SPT data
- Any project where the design response spectrum must be justified
Methods and standards
Every result is reported with its method reference and the calibration status of the instrument used.
ASTM D7400 downhole seismic testingASTM D4428 crosshole seismic testingEurocode 8 ground type classificationIBC and UBC seismic site class definitions
Common questions
What engineers most often ask us about downhole seismic test.
Correlations from N value to shear wave velocity carry a very wide scatter, easily enough to move a site across a class boundary and change the design base shear. Where the classification actually affects the design, measure it.
Yes. It must be cased with PVC and the annulus grouted with a cement bentonite mix of similar stiffness to the ground. A hole left open or filled with slurry gives poor coupling and unreliable arrivals.
Surface wave methods such as MASW give a Vs profile without drilling, and are useful for screening. Downhole remains the more direct measurement, and is what most reviewers expect where the classification is critical.
Often ordered alongside
Most investigations combine several of these. We scope them together so you are not paying to mobilise twice.
Borehole Drilling
The starting point of nearly every investigation.
Read more
Electrical Resistivity Test
A non intrusive way to read the ground between boreholes, and the measurement earthing engineers and pipeline designers...
Read more
Soil & Rock Testing
The laboratory is where samples become design parameters.
Read moreNeed downhole seismic test on your project?
Send us the location, the structure and your programme. You get a scoped quotation with the standard, the number of tests and the turnaround written into it.