Canberra's subsurface isn't uniform. Deep Ordovician sediments butt against Silurian volcanics across short distances. A site in Belconnen can behave nothing like one in Tuggeranong. That contrast makes generic site classification dangerous. The seismic refraction method helps map bedrock depth, but for shear wave velocity profiling we deploy MASW. It captures Vs profiles down to 30 metres without boreholes. The data feeds directly into AS 1170.4 site class determination. For sites with highly variable fill over weathered rock, integrating MASW with CPT testing yields a continuous stiffness profile that neither method achieves alone.
VS30 alone is not enough. The velocity gradient in the top 10 metres often controls Canberra's site amplification more than the 30-metre average.
Q&A
How does MASW compare to a downhole seismic test for Canberra's rock conditions?
MASW measures average Vs over a receiver spread and works well even where drilling is difficult, such as in the fractured Canberra Formation. Downhole testing gives a point measurement at the borehole location and requires a cased hole. MASW provides lateral coverage that a single downhole test cannot. On sites with shallow, steeply dipping rock, combining both methods is advisable: MASW for spatial coverage and downhole for depth-specific verification.
What does a typical MASW survey cost in Canberra?
For a standard single-array MASW survey producing one VS30 profile, the fee ranges from AU$2,930 to AU$4,940 depending on array length, site access conditions, and whether a traffic control plan is required. Multi-line or grid surveys for microzonation are priced per linear metre after the first line.
How long does it take to get results from a MASW survey?
Fieldwork for a single array typically completes in half a day. Processing the dispersion curves, running the inversion, and interpreting the Vs profile takes an additional three to five working days. The final report with AS 1170.4 site classification is delivered within one week of field acquisition, provided weather conditions during the survey were acceptable.
