Comparison of array microtremor survey methods for estimation of dispersion curves in launceston, Australia

The geology of Launceston (Tasmania, Australia) is characterized by the presence of soft Tertiary and Quaternary sediments filling the Tamar rift valley, overlying a hard dolerite bedrock of Jurassic age. We compare data interpreted from two microtremor (passive seismic) array processing methods at Launceston; the spatially averaged coherency (SPAC) and the frequency-wavenumber (FK) methods. We present array observations at two selected sites; GUNN Company (site GUN) located at a site with expected shallow bedrock interface, and the Detention Basin of Launceston (site DBL) inside the Tamar valley with expected deep bedrock interface. Through interpretation of SPAC data, we get a precise estimation of the shear wave velocity profile (or slowness profile) with depth, from which we derive the dispersion curve. Using FK, we estimate the shear-wave velocity and direction of propagation of the wave of most energy on the wavenumber spectra at different frequencies, also leading to the computation of the dispersion curve. At both sites GUN and DBL, we observe that SPAC analysis enables interpretation over a wider range of frequencies, while the FK method seems to overestimate the velocity at low frequency. SPAC is thus preferred for the evaluation of shear wave slowness profiles. However, FK processing is useful for evaluating the spatial distribution of the waves direction of arrival.