We recently supervised a residential subdivision on the eastern slopes of Coffs Harbour where the contractor hit unexpected bedrock at 3.5 m instead of the predicted 8 m. That kind of surprise costs time and money. Seismic tomography (refraction/reflection) gives you a continuous velocity剖面 of the subsurface without drilling a grid of holes. For projects in Coffs Harbour, where the underlying geology ranges from meta-sediments to Quaternary alluvium along the creeks, this method provides reliable depth-to-bedrock data and identifies potential weak zones before you break ground. It is particularly useful when you need to plan cut-and-fill volumes or assess excavation feasibility across a large site.
Seismic tomography in Coffs Harbour reduces the risk of unexpected bedrock or weak zones that can delay excavation and add cost.
Method and coverage
A common mistake we see in Coffs Harbour is relying solely on boreholes to characterize a site with variable geology. Boreholes give point data; they can miss a buried channel or a fault line. Seismic tomography fills that gap by imaging the ground in 2D or 3D. The method uses an array of geophones and an energy source (sledgehammer, accelerated weight drop, or small explosive charge) to record the travel time of P-waves and S-waves. For deeper profiles we combine refraction and reflection processing, which improves resolution at intermediate depths. Before mobilizing the crew we always review existing borehole logs and conduct a preliminary site walkover to select the optimal line orientation. In Coffs Harbour's steep terrain we often couple this survey with microtremors HVSR to capture the fundamental site period at locations where access limits seismic line length.
Technical reference image — Coffs Harbour
Regional considerations
AS 1726-2017 requires site investigations to characterize subsurface conditions with adequate spatial coverage. In Coffs Harbour, where the local geology includes the Coffs Harbour Block — a complex of meta-sedimentary rocks with variable weathering profiles — a few boreholes may not capture the lateral variability. Seismic tomography (refraction/reflection) provides continuous coverage along survey lines, reducing the risk of missing a buried fault or a soft zone that could affect foundation performance. We have seen projects where a single seismic line revealed a 6 m deep weathered trough that five boreholes had missed entirely.
P-wave and S-wave refraction surveys with up to 48 channels. Ideal for bedrock mapping, rippability assessment, and excavation classification in Coffs Harbour's variable terrain.
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Reflection Tomography (2D/3D)
High-resolution reflection surveys for imaging dipping strata, fault zones, and buried channels. Recommended when refraction alone cannot resolve velocity inversions or deep interfaces.
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MASW + Refraction Combo
Combined Multichannel Analysis of Surface Waves with refraction picking to derive both P-wave and S-wave velocity profiles. Useful for site class determination per AS 1170.4 and liquefaction screening.
Standards that apply
AS 1726-2017 Geotechnical site investigations, AS 1289/D4428M-14 Standard test methods for crosshole seismic testing, AS/NZS 1170.0-2002 Structural design actions – general principles
Top questions
How deep can seismic tomography reach in Coffs Harbour?
Refraction surveys typically reach 5 to 50 m depth depending on energy source and line length. Reflection surveys can image up to 150 m in favorable geology. In Coffs Harbour's meta-sediments we usually achieve 20–40 m with a 48-geophone spread and weight drop.
What is the difference between refraction and reflection tomography?
Refraction uses first-arrival travel times of waves that travel along the interface between layers, best for mapping high-velocity layers (like bedrock). Reflection uses reflected wave amplitudes to image interfaces with sharp velocity contrasts, including faults and dipping layers. We often run both simultaneously to cross-validate results.
How much does seismic tomography cost in Coffs Harbour?
A typical refraction survey for a 200 m line with 24 geophones costs between AU$3,810 and AU$8,120 depending on access, line length, and processing requirements. Reflection adds about 20–30 percent for deeper imaging. The quote includes mobilization, field acquisition, processing, and a report with interpreted cross-sections.
When should I choose seismic tomography over boreholes?
Seismic tomography is ideal for large sites where you need to map lateral variability (bedrock depth, fault zones, buried channels) without drilling dozens of holes. It complements boreholes — we recommend at least one borehole per seismic line to calibrate velocity-to-geology relationships. For small lots (<1000 m²) with shallow foundations, boreholes alone are usually sufficient.
