Seismic Tomography (Refraction/Reflection) in Longueuil

NBCC 2020, reinforced by local amendments, demands clear subsurface imaging in a city like Longueuil where the contact between overburden and bedrock can shift abruptly over short distances. Seismic refraction and reflection tomography answer that need directly: they map the velocity structure of the ground, flagging weak zones, cavities, and the true depth to refusal long before a backhoe ever hits the site. We run these surveys throughout the South Shore, from Vieux-Longueuil to Saint-Hubert, and the data routinely saves projects from costly guesswork. A seismic refraction line can also be paired with a MASW spread on the same layout to pull Vs30 profiles without doubling the field time.

Reflection tomography in the Champlain Sea clays can pick the bedrock reflector within 0.5 m of the cored depth, even where the bedrock surface dips more than 20 degrees.

Methodology and scope

Longueuil sits on the St. Lawrence Lowlands, where glacial till and marine clays of the Champlain Sea blanket a Paleozoic sedimentary bedrock sequence dominated by shale and limestone. Overburden thickness rarely exceeds 30 meters in the central boroughs, but the bedrock surface is frequently irregular, with buried channels and differential weathering that create velocity inversions no borehole log can resolve alone. That is where tomographic processing makes the difference: instead of assuming flat layers, the algorithm iterates thousands of ray paths to reconstruct a 2D velocity cross-section that respects the real geometry. We trigger a sledgehammer or weight-drop source, occasionally a small explosive charge for deeper targets, and the geophone spread captures first arrivals as well as reflected phases when the impedance contrast is strong enough. The result is a continuous image of compressive-wave velocity, directly correlated to rock quality and excavatability. On tighter urban lots, CPT testing provides complementary point data where seismic access is limited by buried utilities.

Local considerations

In Longueuil, we see too many projects where the contractor bid based on a handful of boreholes that hit rock at, say, 8 meters, only to discover a 14-meter-deep trough between the holes once excavation starts. The extra cost of hammering or blasting that volume of rock—plus the delay—eats the contingency in weeks. Seismic tomography catches those troughs because it illuminates the entire section, not just isolated points. Another risk is misclassifying a weathered shale layer as competent bedrock: a velocity of 1,800 m/s excavates with a ripper, while 4,000 m/s requires a hydraulic breaker. The tomography line gives the contractor a rippability map they can actually use to schedule equipment, and that alone usually pays for the survey.

Technical parameters

Parameter	Typical value
P-wave velocity in intact local limestone	3,200–5,800 m/s
P-wave velocity in weathered shale	1,200–2,500 m/s
Typical geophone spacing (refraction)	2–5 m
Typical spread length	46–115 m
Depth of investigation (refraction)	Up to 30 m with weight-drop source
Sampling interval	0.125–0.250 ms
Stack count per shot	3–8
Data format delivered	SEG-2, SEG-Y, PDF cross-sections

Associated technical services

Seismic Refraction Tomography

2D P-wave velocity cross-sections for rippability assessment, bedrock profiling, and detecting buried valleys. We use 24- or 48-channel seismographs with 4.5 Hz geophones, processing with time-term inversion and iterative ray-tracing to handle lateral velocity gradients common in the St. Lawrence Lowlands.

Seismic Reflection Profiling

High-resolution common-midpoint (CMP) surveys for deeper targets or sites where a low-velocity layer overlies a faster one—a scenario refraction cannot resolve. We apply NMO correction, stacking, and post-stack migration to image bedrock stratigraphy and fault zones.

Combined MASW + Refraction

Single-spread acquisition that delivers both a P-wave velocity model and a Vs30 shear-wave velocity profile. The shear-wave data feeds site classification per NBCC Table 4.1.8.4.A, while the P-wave model handles excavatability—two deliverables from one field mobilization.

Frequently asked questions

How much does a seismic tomography survey cost in Longueuil?

For a typical refraction line in Longueuil, budgets run between CA$3,930 and CA$7,760 depending on the spread length, number of shots, and whether reflection processing is added. Urban sites with asphalt cutting for geophone planting push toward the upper end. We provide a fixed-price quote after reviewing the site plan and target depth.

How deep can refraction tomography see in the Longueuil area?

With a 115-meter spread and a weight-drop source, we typically map 25 to 30 meters into the subsurface—enough to reach competent bedrock across most of the city. If the overburden is particularly thick or the bedrock velocity is low, we extend the spread or use a small explosive source to push the investigation deeper.

Can you run a seismic line on a paved parking lot?

Yes, but it requires drilling small holes through the asphalt at each geophone station so the spike couples with the underlying soil. We carry a rotary hammer for that purpose. The asphalt is patched afterward with cold mix. It adds about 15 percent to the field time, but the data quality is comparable to a grassed site.

What is the difference between refraction and reflection tomography for a foundation job?

Refraction maps the velocity of the first-arriving wave, which works well when velocity increases with depth—the typical case in Longueuil where soft clay overlies limestone. Reflection images impedance contrasts directly, so it handles velocity inversions, like a loose sand lens under stiff clay, that refraction would miss. For most foundation investigations we start with refraction and add reflection if the geology demands it.