For any project in Coffs Harbour, compliance with AS 1726 (geotechnical site investigations) and AS/NZS 1170 (structural design actions) is standard. The HVSR microtremor survey (Nakamura method) offers a non-invasive way to determine the fundamental resonance frequency of the ground — a critical parameter in this city, where the transition from coastal Quaternary sands to the underlying Coffs Harbour Block (a complex of metamorphosed greywacke and slate) can create sharp impedance contrasts. Before designing foundations or seismic retrofits, we often recommend coupling the HVSR survey with a dilatometer test to capture lateral stress variations in these layered profiles.
A single HVSR station reveals the fundamental frequency of the ground, helping engineers avoid resonance between structure and soil during seismic loading.
Method and coverage
In practice, the soil response differs markedly between the Diggers Beach area (where deep dune sands overlie the bedrock) and the hillside suburbs like Korora, where residual soil profiles developed on phyllite and argillite dominate. The HVSR microtremor survey (Nakamura method) captures this variability by recording ambient vibrations with a three-component seismometer — no drilling, no heavy equipment. We deploy the sensor for 30 to 60 minutes per station, then process the horizontal-to-vertical spectral ratio to identify the site's fundamental frequency (f0). This is especially useful when planning cimentaciones-rellenos on fill materials, where the resonance peak can shift markedly compared to natural ground. The technique also helps estimate the average shear wave velocity to 30 m depth (VS30) when combined with a tomografia-sismica array.
Technical reference image — Coffs Harbour
Regional considerations
Coffs Harbour's urban expansion since the 1970s has pushed development onto steeper terrain and into former floodplains. Many early subdivisions on the city's hillsides lacked systematic geophysical surveys. The result: structures built on variable residual soils that may amplify seismic waves unpredictably during a moderate earthquake. The HVSR microtremor survey (Nakamura method) provides a rapid way to screen these legacy sites, identifying areas where the natural frequency falls within the range of common low-rise buildings (2–8 Hz). Ignoring this can lead to resonance, excessive drift, and non-ductile failure in unreinforced masonry walls common in older Coffs Harbour homes.
A rapid, cost-effective assessment for small residential sites or preliminary screening. One 3-component seismometer deployed for 45 minutes per station, processed to yield f0 and peak amplitude. Ideal for verifying site class per AS/NZS 1170.4.
02
Multi-Station HVSR Array
For larger subdivisions or commercial developments, we deploy up to 10 stations in a grid pattern. This produces a 2D map of resonance frequencies across the site, highlighting variations in soil thickness and stiffness that a single borehole could miss.
03
Combined HVSR + MASW Package
HVSR provides the resonance frequency, while MASW (multichannel analysis of surface waves) delivers a 1D shear wave velocity profile to 30 m. Together, they give a complete picture for seismic site classification and foundation design in accordance with AS 1726.
Standards that apply
AS 1726:2017 (Geotechnical site investigations), AS/NZS 1170.4:2007 (Structural design actions – Earthquake actions), SESOC 2013 (Seismic assessment of existing structures – New Zealand, often referenced in Australian practice)
Top questions
What is the main difference between HVSR and a standard borehole investigation?
HVSR is non-invasive, measuring ambient vibrations from a single surface sensor. It identifies the fundamental resonance frequency of the soil column, which a borehole cannot directly provide. Boreholes give physical samples and strength parameters, but HVSR fills a key gap: understanding how the entire soil profile will behave dynamically during an earthquake. We use both methods together for a complete picture.
How long does an HVSR survey take on a typical residential block in Coffs Harbour?
For a single-family home site, we deploy one station for about 45 minutes. Setup, recording, and pack-up take roughly 1.5 hours total. For a multi-station array covering a 1-hectare subdivision, expect 4–6 hours in the field. Data processing and reporting add another 2–3 working days.
Does the HVSR method work on steep slopes or in areas with thick vegetation?
Yes, it works well in those conditions because it relies on ground vibrations, not line-of-sight or ground contact. We have successfully deployed HVSR stations on the slopes of the Great Dividing Range behind Coffs Harbour, where access for drilling rigs is impossible. The sensor must be placed on stable ground (not on loose rock or tree roots), but otherwise vegetation and slope angle do not affect the measurement.
