Gold Coast sits on a mix of coastal sands, alluvial deposits, and buried volcanic bedrock channels that vary sharply within a few hundred meters. Many times we see projects where standard boreholes miss a dipping basalt layer or a paleochannel filled with soft sediments, leading to foundation settlements that could have been avoided. Seismic tomography resolves these lateral transitions by measuring P-wave and S-wave velocities along multiple receiver lines. When combined with MASW-Vs30 for shear wave profiling, the velocity model becomes a reliable tool for classifying site subsoil class under AS 1170.4. The method works particularly well on the Nerang River corridor and along the coastal strip, where the water table is shallow and refraction boundaries are sharp.
Refraction tomography resolves velocity inversions that the conventional intercept-time method misses entirely, a common issue in Gold Coast's layered alluvium.
Methodology and scope
Local considerations
A 15-story residential tower on the Broadbeach foreshore required seismic tomography to characterize the top 40 meters before pile design. The first refraction line showed an unexpected low-velocity zone at 12 m depth, not detected in three previous boreholes. That zone was a buried channel of loose sand with shell fragments — a liquefaction hazard under earthquake loading. Without the velocity model, the foundation would have relied on end-bearing piles that stopped short of the competent layer. The client avoided a redesign by adding precarga-sobrecarga to densify the channel before piling. The lesson holds for any Gold Coast site underlain by the Bundamba Group or Quaternary alluvium: seismic velocities reveal hidden variability that point samples miss.
Applicable standards
AS 1726-2017 – Geotechnical site investigations, AS 1170.4-2007 – Earthquake actions (site class determination via Vs30), AS 1289 – Standard guide for seismic refraction, AS 4678-2002 – Earth retaining structures (where tomography informs bedrock profile)
Associated technical services
P-Wave Refraction Tomography
Multi-channel refraction survey with tomographic inversion for bedrock depth, rippability assessment, and excavation classification. Typical profile length 120 m, depth range 10–30 m.
S-Wave Reflection Imaging
Shear-wave reflection profiling using horizontal geophones and a transverse source. Ideal for detecting faults, paleochannels, and stratigraphic pinch-outs below the water table.
Combined Refraction/Reflection for Deep Profiles
Simultaneous acquisition of P-wave refraction and S-wave reflection on the same spread. Delivers both velocity structure and structural geometry down to 50 m depth.
Typical parameters
Frequently asked questions
What is the difference between seismic refraction and reflection tomography?
Refraction tomography uses first-arrival travel times to build a velocity model of the subsurface, ideal for mapping layer boundaries and velocity gradients. Reflection tomography processes later arrivals (reflected waves) to image sub-horizontal interfaces and faults. Refraction works well in Gold Coast for depths up to 30 m; reflection is preferred for deeper targets or where velocity inversions exist.
How much does a seismic tomography survey cost in Gold Coast?
A standard 120 m refraction profile with tomographic inversion typically costs between AU$4,370 and AU$7,640, depending on site access, source type, and number of receiver lines. Reflection surveys with CMP processing are at the higher end of that range. These figures include field acquisition, data processing, and a written report with velocity cross-sections.
What site conditions in Gold Coast make seismic tomography necessary?
Coastal sites with sand over basalt, infilled river channels along the Nerang River, and projects near the Gold Coast hinterland where weathered rock grades into fresh rock within short distances. Tomography is also required for site class determination under AS 1170.4 when Vs30 cannot be estimated from boreholes alone.