MASW Testing & VS30 Shear Wave Velocity in Wellington

A 24-channel Geode seismograph connected to a spread of 4.5 Hz low-frequency geophones is the standard acquisition system we deploy across Wellington's suburban ridges and reclaimed flatlands. The array captures surface-wave dispersion across a frequency band that resolves shear wave velocity from the near-surface weathered greywacke down to the 30-metre reference depth required by NZS 1170.5:2004. Processing uses a solid frequency-wavenumber transform that handles the steep velocity gradients common at the interface between Port Nicholson reclamation fill and the underlying Wellington Alluvium. For sites on the Thorndon side where wind noise contaminates passive records, we shift to an active-source setup with a 10 kg sledgehammer and stacked impacts, ensuring the dispersion curve remains coherent enough to invert into a reliable one-dimensional VS profile without resorting to assumptions that would mask thin low-velocity layers.

A VS30 value derived from surface waves without a near-surface borehole tie is just a smooth model—it can miss the 3-metre peat lens that doubles the site period.

Our approach and scope

Across the Wellington fault zone, we often encounter velocity inversions where Holocene harbour sediments underlie stiffer, dessicated crust—a stratigraphic trap that conventional refraction fails to detect. In those cases, the Rayleigh wave fundamental mode picks up the drop at 8 to 12 metres clearly, and we combine the MASW dispersion curve with a-priori borehole information from a nearby CPT test to constrain the inversion. The output is a VS30 value that feeds directly into the site subsoil class determination, whether a site falls into Class C (shallow soil) on Tinakori Hill slopes or Class D (deep soil) along the Hutt Road corridor. All field work follows the NZGS guidelines for seismic site classification, with the seismograph triggering an internal performance test before each spread, and we log the gain settings and geophone coupling quality for every shot gather.

Local ground factors

The most common misstep we see on Wellington sites is rushing a VS30 estimate from a single-pass surface wave spread without checking for lateral velocity variation across the building footprint. A line run parallel to the old shoreline along Waterloo Quay can return a VS30 of 220 m/s, while a perpendicular spread just 40 metres inland, where the reclamation thins over weathered basement, jumps to 380 m/s—pushing the site from Class D to Class C and cutting the design base shear by nearly 30 percent in a high-seismicity zone. Another frequent error is using active-source dispersion alone on deep soil sites where the low-frequency content below 8 Hz is insufficient to constrain velocities below 25 metres; the resulting inversion overestimates stiffness at depth and underestimates the fundamental site period, which directly impacts the spectral shape in the structural analysis.

Typical values

Parameter	Typical value
Survey array length	46–69 m (24-channel, 2 m spacing typical)
Geophone frequency	4.5 Hz vertical-component, single-spike coupling
Source type	10 kg sledgehammer on aluminium plate (active); ambient noise (passive)
Maximum VS30 depth	30 m per NZS 1170.5:2004, extendable to 45 m
Dispersion processing	Frequency-wavenumber (f-k) transform, fundamental mode picking
Inversion algorithm	Damped least-squares, layered-earth parameterisation
Deliverable	1D VS profile, VS30 value, NZS 1170.5 site subsoil class letter

Complementary services

VS30 Determination for NZS 1170.5 Site Classification

Single or multiple MASW spreads processed to return a VS30 value and site class letter (A through E). Includes raw shot gathers, dispersion images, and a signed report stating the site subsoil class per the NZGS module.

1D Shear Wave Velocity Profiling for Site Response

Joint inversion of MASW dispersion data with borehole stratigraphy or CPT tip resistance logs. Outputs a layered VS model to 30–45 m depth suitable for input into equivalent-linear or non-linear ground response software such as DEEPSOIL or Strata.

Common questions

How much does a MASW survey for VS30 cost on a standard Wellington residential section?

For a typical 400–600 m² section with 2 to 3 spreads, the cost ranges from NZ$2,780 to NZ$5,900 depending on access, the number of lines, and whether passive (ambient) recording is required to supplement the active-source data on deeper soil profiles.

Can MASW work on steep Wellington hillside sites where the terrain is over 20 degrees?

Yes, but with modifications. On slopes exceeding 15 degrees we shorten the array to 12 channels at 1.5-metre spacing to maintain straight-line geometry, and we apply a topographic correction to the dispersion analysis. The geophones must be planted perpendicular to the slope face with careful coupling into the colluvium, and we avoid days with saturated ground after heavy Wellington rain to prevent poor sensor coupling.

How does VS30 from MASW compare to a downhole seismic test in a borehole?

MASW measures the average shear wave velocity over the array length, while a downhole test measures it point-location at the borehole. On laterally uniform Wellington alluvium the two methods typically agree within 10–15%. On sites with sharp lateral changes—like the boundary between reclamation and natural ground near the waterfront—MASW spatially averages while the downhole test captures only the local column, so we recommend running two orthogonal MASW spreads to identify any anisotropy before relying on a single VS30 value.