HVSR Microtremor Survey (Nakamura Method) in Cardiff

HVSR uses ambient noise recorded by a single seismometer to measure the fundamental resonance frequency of the soil column via the H/V spectral ratio. MASW (Multichannel Analysis of Surface Waves) requires an active source (sledgehammer or weight drop) and an array of geophones to generate a shear-wave velocity (Vs) profile with depth. HVSR is faster and cheaper for first-pass site classification, while MASW provides a deeper, more detailed Vs profile. In Cardiff, we often use HVSR as a screening tool before committing to MASW on complex sites.

For a single building plot (e.g. a detached house or small block of flats), one station placed centrally is usually sufficient. For larger developments (e.g. a housing estate or commercial park), we recommend at least 3 to 5 stations to capture lateral variability in the soil column. The City of Cardiff Council’s planning guidance may require a minimum of two stations for sites over 0.5 hectares, especially in areas underlain by alluvium.

The water table affects the shear-wave velocity of the soil, which in turn influences the fundamental frequency recorded by the HVSR survey. Saturated loose sands and soft clays typically have lower Vs values (below 200 m/s), shifting the site period toward longer values (lower frequency). The H/V spectral ratio curve will show a clear peak if there is a strong impedance contrast between the saturated layer and the underlying bedrock. Our report explicitly notes the groundwater conditions and their effect on the interpreted site class.

For a standard single-station survey in the Cardiff area, prices generally range between £1,010 and £1,990, including data acquisition, processing, and a basic interpretation report. Multi-station surveys or those requiring integration with other geophysical methods will be quoted on a project-specific basis. Contact us for a fixed-price quotation tailored to your site size and complexity.

HVSR alone is not sufficient for a full liquefaction assessment, as it does not directly measure penetration resistance or shear-wave velocity at depth. However, it provides the fundamental frequency and amplification factor, which are inputs for cyclic stress ratio (CSR) calculations. For a proper liquefaction evaluation in Cardiff’s alluvial deposits, we recommend combining HVSR with CPT or SPT data. The resonant period identified by HVSR helps identify strata most prone to excess pore pressure buildup during shaking.