A recent excavation on the eastern escarpment near Prince Henry Drive hit unexpected basalt floaters at 4 metres — right where the borelog showed weathered clay. The contractor lost three days resetting the hammer. That scenario plays out across Toowoomba because the Tertiary volcanics draped over the Clarence-Moreton Basin create sharp lateral velocity contrasts that conventional drilling misses. Seismic tomography maps those transitions. The P-wave refraction survey picks the competent basalt interface and the S-wave reflection section reveals shear zones within the weathered profile, giving the excavation team a continuous velocity model before the first bucket goes in. For sites along the range front, we pair the seismic lines with MASW profiling to capture the Vs30 profile for AS 1170.4 site classification, and with deep excavation monitoring when the cut exceeds 6 metres in the Toowoomba basalt.
A 24-channel seismic refraction line across the Toowoomba range front typically resolves the bedrock surface within ±0.5 metres vertical accuracy, eliminating the blind spots between boreholes.
FAQ
What is the difference between seismic refraction and reflection for a Toowoomba basalt site?
Refraction maps velocity layering — ideal for finding the top of competent basalt under weathered clay. Reflection images impedance contrasts at sub-horizontal boundaries, which works better for detecting voids, mine workings in the Walloon Coal Measures, or fracture zones within the basalt flow. On most Toowoomba projects we run both: refraction for the excavation surface and reflection for hazards below it.
How many seismic lines does a typical residential subdivision need in Toowoomba?
For a standard 20-lot subdivision on the basalt plateau, three to four lines of 115 metres each, oriented parallel and perpendicular to the slope direction, usually provide enough coverage to map the bedrock surface and identify any paleochannels. We calibrate each line to at least one borehole to meet AS 1726 requirements.
Can seismic tomography detect old mine workings under Toowoomba?
Yes — the reflection component is particularly effective. Abandoned coal workings in the Walloon Coal Measures appear as low-velocity anomalies and disrupted reflectors on the S-wave section. We have mapped workings at depths between 20 and 45 metres across several sites west of the CBD. The data guides targeted drilling and grouting programs.
What does a seismic tomography survey cost in Toowoomba?
A typical program with three refraction lines, one reflection profile, and a MASW spread for site classification runs between AU$4,080 and AU$7,470, depending on line length, access conditions, and the number of calibration boreholes required. Sites with dense vegetation or steep slopes fall toward the upper end.
How does seismic tomography help with AS 2870 site classification?
The MASW component of the survey measures the shear-wave velocity profile (Vs30) directly, which feeds into the AS 1170.4 site class determination. Combined with the refraction data, it also maps the depth to rock — a key parameter for classifying sites as Class A, S, or P under AS 2870. This replaces the default conservative assumptions that often drive up foundation costs.
