On a recent project along the Tamar River, a six-storey residential development required a seismic site classification before foundation design could proceed. The local council mandated a VS30 profile to confirm the site class under AS/NZS 1170. Rather than drilling multiple boreholes, the geotechnical team deployed a MASW survey. This non-invasive method records surface waves generated by a sledgehammer source, then inverts the dispersion curve to produce a shear wave velocity profile down to 30 metres. For Launceston, where much of the urban area sits on Quaternary alluvium and Tertiary basalt flows, the MASW / VS30 approach provides a rapid, cost-effective way to classify the ground without disturbing the site. The data feeds directly into the building code compliance process.
In Launceston, MASW surveys resolve the basalt-alluvium boundary that controls seismic site class and liquefaction potential.
Methodology and scope
The MASW method is governed by AS 1289 and the multi-channel analysis technique described in the NEHRP guidelines. In Launceston, we typically deploy 24 geophones spaced 2 to 5 metres apart, depending on target depth. The survey records Rayleigh waves across a 1-D linear array; the resulting dispersion curve is inverted to produce a 1-D shear wave velocity profile. For projects requiring deeper profiles or 2-D imaging, we extend the spread or combine the survey with active-source surface wave testing to resolve velocity contrasts at the basalt-alluvium interface common in the Launceston basin. The output includes VS30 (average velocity to 30 m depth) and the site class per AS 1170.4. We also report the time-averaged velocity to 30 m, which is the key parameter for seismic design.
Technical reference image — Launceston
Local considerations
The Launceston urban area straddles the Tamar River valley, where soft alluvial clays and silts overlie Tertiary basalt. This two-layer velocity contrast can amplify ground motion during an earthquake. Without a measured VS30 profile, engineers may overestimate or underestimate the site class. The 2012 Meeberrie earthquake (magnitude 5.1) was felt across northern Tasmania, reminding us that even low-to-moderate seismicity regions need proper characterisation. In our experience, a MASW / VS30 survey identifies the depth to the stiff basalt layer. If the basalt is shallower than 30 m, the VS30 value rises, potentially moving the site from Class D to Class C. That shift reduces base shear demands by up to 30%, saving on structural costs.
Linear array with 24 geophones, hammer source, dispersion curve inversion, and VS30 calculation. Output includes site class per AS 1170.4 and the full velocity profile.
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2-D MASW Profile
Multiple overlapping 1-D arrays assembled into a 2-D shear wave velocity cross-section. Ideal for mapping lateral variability at sites with fill wedges or basalt pinnacles.
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Passive Surface Wave (Microtremor) Array
For sites where active source energy is limited (e.g., high ambient noise), we deploy a circular array of geophones to record ambient vibrations and extract Rayleigh wave dispersion at low frequencies.
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Integrated MASW + Borehole Logging
Combine MASW surface data with downhole shear wave velocity from a borehole (AS 1289) for cross-validation. This hybrid approach provides the highest confidence in VS30 for critical infrastructure.
Applicable standards
AS/NZS 1170.4:2007 Structural design actions – Earthquake actions, AS 1289/D4428M-14 Standard test methods for crosshole seismic testing, NEHRP Recommended Seismic Provisions for New Buildings (FEMA P-1050), AS 1726:2017 Geotechnical site investigations
Frequently asked questions
What is VS30 and why is it important in Launceston?
VS30 is the time-averaged shear wave velocity in the top 30 metres of the ground. It is the primary parameter used to assign site class in AS/NZS 1170.4. In Launceston, where soft alluvium overlies basalt, the VS30 value determines whether a site falls into Class C (shallow basalt) or Class D (deep alluvium). This classification directly affects the seismic design forces.
How long does a MASW survey take in Launceston?
A standard 1-D MASW line with 24 geophones takes 2 to 4 hours in the field, including setup, data collection, and demobilisation. The subsequent data processing and inversion require one to two working days. We typically deliver the full report, including VS30 and site class, within five business days.
Can MASW be used on steep slopes or rocky terrain in Launceston?
Yes. MASW requires only ground contact for the geophones and a clear area for the source. On steep slopes near the Cataract Gorge or in the Tamar Valley, we use shorter geophone spacings and a lighter hammer source. The method works well on rock outcrops as long as the geophones can be planted firmly.
What is the typical cost range for a MASW / VS30 survey in Launceston?
For a single 1-D line with 24 geophones, the cost ranges between AU$2,340 and AU$4,300, depending on site access, array length, and reporting detail. A 2-D profile with multiple lines or a combined passive-active survey will be higher. Contact us for a fixed quote based on your site layout.
