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Seismic Tomography in Sydney: Refraction & Reflection Surveys for Subsurface Imaging

Rigorous testing. Clear reporting.

Sydney sits on a complex geological framework where Triassic Hawkesbury Sandstone overlies older Narrabeen Group sediments, often capped by Quaternary alluvium in the Parramatta River valley and Botany Sands district. AS 1726:2017 sets the standard for geotechnical site investigation, and seismic tomography is the only method that delivers continuous P-wave velocity profiles across these transitions. We deploy 24-channel seismographs with 4.5 Hz geophones spaced at 2 m intervals, generating velocity cross-sections that map the bedrock interface with sub-metre resolution. This data directly informs foundation design parameters, particularly where variable depth to rock affects pile termination levels. For large infrastructure projects, combining seismic tomography with calicatas exploratorias provides a dual validation of subsurface conditions.

Illustrative image of Seismic tomography (refraction/reflection) in Sydney

Seismic tomography in Sydney resolves the bedrock interface with sub-metre accuracy, critical for pile design in variable Hawkesbury Sandstone.

Our service areas

Scope of work

Sydney experiences an average of 20 to 30 felt earthquakes per year, with the 1989 Newcastle earthquake (magnitude 5.6) serving as a stark reminder of local seismic hazard. Seismic tomography in Sydney must account for the high velocity contrast between weathered sandstone (Vp 500–1500 m/s) and competent rock (Vp 2500–4000 m/s). We apply both refraction and reflection processing: refraction tomography resolves velocity inversions in shallow alluvial layers, while reflection imaging detects sub-vertical joints and bedding planes that control groundwater flow. Field acquisition follows AS 1289 for refraction and AS 1289 for reflection. The inversion algorithm uses a non-linear least-squares approach with a 5% RMS error threshold. This methodology is particularly effective when paired with MASW vs30 for shear-wave velocity profiles and microtremores hvsr for site resonance frequency.

Technical reference — Sydney

Area-specific notes

The coastal humidity and frequent rainfall in Sydney degrade geophone-ground coupling, introducing noise that masks first-arrival picks for refraction tomography. We mitigate this by using spike-base geophones and performing quality checks on each trace before stacking. Another risk is the presence of artificial fill in reclaimed areas like Barangaroo and the former Sydney Olympic Park site, where heterogeneous materials create scattering that degrades reflection images. In these settings, we pre-process data with band-pass filtering and deconvolution to improve signal-to-noise ratio before inversion.

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Standards used

AS 1726:2017 Geotechnical Site Investigations, AS 1289 Standard Guide for Refraction Seismic Methods, AS 1289 Standard Guide for Reflection Seismic Methods

Technical parameters

Parameter	Typical value
Number of channels	24
Geophone natural frequency	4.5 Hz
Shot spacing	2 m (refraction) / 1 m (reflection)
Record length	1024 ms
Velocity resolution	±5% P-wave velocity
Depth of investigation	Up to 30 m (refraction) / 50 m (reflection)

Quick answers

How does seismic tomography differ from standard borehole drilling in Sydney?

Seismic tomography provides continuous 2D velocity profiles across the entire survey line, whereas boreholes give point data at specific locations. In Sydney's variable Hawkesbury Sandstone, tomography reveals lateral changes in rock quality that a grid of boreholes might miss. We often use tomography to optimise borehole placement, reducing drilling costs by 20-30%.

What is the typical cost range for a seismic tomography survey in Sydney?

For a standard 200-metre survey line with 24-channel acquisition, costs range from AU$4.180 to AU$7.350. This includes field work, processing, and an interpreted report. Larger projects with multiple lines or deeper targets fall at the upper end of that range.

Can seismic tomography detect voids or cavities in Sydney's sandstone?

Yes, reflection tomography is particularly effective for detecting air-filled or water-filled voids as small as 1 m in diameter when the void has a strong acoustic impedance contrast with the surrounding sandstone. We have successfully identified solution cavities and old mine workings in the Sydney Basin using this method.

Location and service area

We serve projects across Sydney and its metropolitan area.

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