Bunbury sits on a mix of Quaternary alluvial sands and estuarine clays, with the water table often within 2–3 m of the surface. This shallow, soft profile amplifies ground motion during an earthquake, making resonance frequency data critical for design. An HVSR microtremor survey (Nakamura method) measures ambient vibration to identify the fundamental frequency of the soil column. We pair this with a microzonificación sísmica to map how different suburbs respond to shaking, and with cimentaciones sismicas to ensure foundations are tuned to the actual site period. The result is a cost‑effective way to avoid resonant coupling between structure and ground.
In Bunbury, resonance frequencies below 3 Hz are common in estuarine zones, which can amplify long‑period shaking in medium‑rise buildings.
Methodology and scope
- Fundamental resonance frequency (f₀) – typically 1–6 Hz for Bunbury’s soft soils.
- Amplification factor (A₀) – indicates ground motion amplification.
- Vs₃₀ estimate – derived from f₀ and thickness of soil cover.
- Site class per AS 1170.4 – A, B, C, D or E.
- Depth to seismic bedrock – estimated from the frequency‑thickness relationship.
Local considerations
In Bunbury, many older subdivisions were built before modern seismic codes. We often see two‑storey houses on soft clays where the building period matches the soil’s natural frequency — a classic resonance risk. An HVSR microtremor survey catches this mismatch early. Without it, engineers may design for generic site classes and miss local amplification. Combining the survey with a losa-de-cimentacion analysis helps select a stiff foundation that shifts the building period away from the soil’s peak. The cost of the survey is small compared to the retrofit required after a moderate earthquake.
Applicable standards
AS 1726 – Geotechnical site investigations, AS/NZS 1170.4 – Structural design actions (earthquake), SESAME Guidelines (2004) – H/V processing standards, NEHRP Site Classification (BSSC 2003)
Associated technical services
Single‑Station HVSR Survey
Quick reconnaissance of resonance frequency at one location. Ideal for small residential lots or preliminary screening before detailed borehole investigations.
Multi‑Station HVSR Array
Six or more stations across a site to generate a resonance‑frequency contour map. Used for subdivisions, schools, and commercial buildings where spatial variation in soil stiffness is expected.
Typical parameters
Frequently asked questions
How much does an HVSR microtremor survey cost in Bunbury?
A standard single‑station survey in Bunbury ranges from AU$2,260 to AU$3,380, depending on access and number of stations. Multi‑station arrays for larger sites are quoted per station with a volume discount.
What is the difference between HVSR and MASW?
HVSR measures ambient vibration to find the soil's fundamental resonance frequency (f₀). MASW uses an active source (sledgehammer) to generate a shear‑wave velocity (Vs) profile with depth. HVSR is faster and cheaper for site‑class screening; MASW gives quantitative Vs layers. We often combine both for projects requiring a Vs₃₀ value per AS 1170.4.
Do I need an HVSR survey for a single‑storey house in Bunbury?
Not always, but it is recommended if the house is on soft estuarine clays (e.g. near the Leschenault Estuary) or if the builder wants to optimise the slab design. The survey costs less than a weekend of extra concrete and can prevent long‑term cracking from differential settlement during shaking.
How long does the fieldwork take?
Each station takes about 40 minutes — 30 minutes of recording plus setup and retrieval. A typical 6‑station survey for a suburban block is completed in one morning. No traffic management is required, and the equipment is completely non‑invasive.