Characterisation of an urban site by ambient noise HVSR Method : resonance frequencies and site amplifications

The resonance frequencies and site amplifications of a proposed development site on the Penrith campus of the University of Western Sydney have been studied by means of a non"invasive ambient noise Horizontal to Vertical Spectral Ratio (HVSR) method. Unlike other non"invasive surface wave techniques (such as the SASW, MASW, CSW etc) which require an array of connected sensors, the HVSR technique is a single station method requiring no such connection. Moreover, the HVSR technique is a passive method in relying only on the ambient microtremors caused by natural events (tides, large"scale meteorological conditions) and cultural activities (industrial machinery, cars and trains) as excitation sources. This paper presents one part of a study which aims to characterise the site effects and layered soil stratigraphy of a development site at the Penrith campus. In this study, measurements of the ground responses in the vertical and two orthogonal horizontal directions were carried out using a lightweight battery operated, wireless 3"component digital tomograph. The estimated HVSR curves, the resonance frequencies and site amplifications, and interpretations of the results obtained for the site are discussed. The experience from this study indicates that this could be a low"cost and attractive technique having a great potential in near surface geotechnical characterisation of urban sites. Further investigation is currently being undertaken to study its efficiency and accuracy in establishing the S"wave velocity profiles and soil stratigraphy of the site by inverse analysis of the HVSR curves.