We run MASW surveys in Mackay using a 24-channel seismograph paired with 4.5 Hz geophones spaced at 2-meter intervals along a 94-meter linear array. The source is a 10 kg sledgehammer striking an aluminum plate, generating Rayleigh waves that we invert to produce a 1D shear wave velocity profile down to 30 meters. Mackay’s alluvial plains and coastal sands require careful sensor coupling to avoid signal attenuation in loose topsoil. The resulting VS30 value feeds directly into ASCE 7 site class assignments and the seismic loading provisions of AS/NZS 1170.4. Before deploying the array, we cross-reference with nearby asentamientos data to anticipate differential settlement risks in soft layers.
MASW gives us a continuous velocity profile without drilling — critical for Mackay’s floodplain projects where boreholes alone miss lateral variability.
Methodology and scope
Mackay sits on the Pioneer River floodplain, with Quaternary alluvium overlying Tertiary basalt. The contrast between soft Holocene clays and stiff residual soils creates sharp velocity gradients that MASW resolves well. We process data using the f-k transform and pick dispersion curves manually — no black-box inversion. For projects on the northern beaches or near the Goonyella rail corridor, we combine MASW with resistividad-electrica-sev to distinguish saturated sands from clay-rich layers. Typical outputs include:
VS30 values between 180–360 m/s for site class D (stiff soil)
1D velocity profiles with depth increments of 0.5 m
Poisson's ratio estimates for dynamic foundation checks
The whole survey takes half a day, including setup and data download.
Technical reference image — Mackay
Local considerations
Mackay is in Seismic Zone 1 per AS 1170.4, but the 2015 Bowen earthquake (M5.2) reminded engineers that local site effects matter. Soft alluvium can amplify ground motion by a factor of 2 to 3 compared to rock sites. Without a measured VS30 profile, you risk underestimating spectral accelerations, especially for structures with natural periods between 0.3–0.8 s. The MASW survey provides the shear wave velocity data required by AS 1726 for site classification. We also flag liquefaction potential when VS30 drops below 180 m/s in saturated sands — a real concern near the Mackay Marina precinct.
Single 94 m array with 24 geophones, f-k dispersion analysis, and 1D VS profile to 30 m depth. Includes VS30 calculation and site class assignment. Suitable for residential subdivisions, small commercial buildings, and bridge approach fills.
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Multi-Array MASW (2D Section)
Three or more parallel arrays to build a 2D shear wave velocity cross-section. Ideal for linear infrastructure — rail embankments, levee upgrades, and pipeline corridors in the Mackay Regional Council area. Includes contour plots and layer boundaries.
Applicable standards
AS 1289/D4428M-14, AS 1726:2017 – Geotechnical Site Investigations, AS/NZS 1170.4:2007 (Amdt 2) – Earthquake Actions, NEHRP Recommended Provisions (FEMA P-1050)
Frequently asked questions
What is the difference between MASW and a borehole shear wave test?
MASW measures surface wave dispersion to infer shear wave velocity without drilling. A borehole test (e.g., downhole or crosshole) requires a cased hole and gives point measurements. MASW covers a larger volume — up to 30 m depth over a 94 m line — while borehole methods are more precise for thin layers. We recommend MASW for initial site classification and borehole methods for detailed layer boundaries.
How does Mackay's geology affect VS30 values?
Mackay's alluvial plain produces VS30 values typically in the 200–320 m/s range, placing most sites in Site Class D (stiff soil) per ASCE 7. Areas underlain by Tertiary basalt near the Eungella Range can exceed 500 m/s (Site Class B). The transition from soft Holocene clays to weathered basalt is often abrupt, and MASW captures this lateral change better than a single borehole.
What is the typical turnaround time for a MASW survey in Mackay?
Fieldwork takes half a day for a standard 1D array. Data processing and inversion require another 2 to 3 working days. We deliver a PDF report with dispersion curves, inverted VS30 profile, and site class classification. For multi-array 2D sections, add 1 to 2 days for processing.
How much does a MASW survey cost in Mackay?
The cost for a standard 1D MASW survey in Mackay ranges between AU$2,950 and AU$4,940, depending on site access, number of arrays, and reporting detail. Multi-array 2D sections or surveys in remote locations (e.g., Sarina or the Pioneer Valley) may incur additional mobilization fees. Contact us for a site-specific quote.
