Comparison Of Array Microtremor Survey Methods For Estimation Of Dispersion Curves In Launceston, Australia

Abstract

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. Introduction We compare microtremor observations in Launceston recorded from 2 passive seismic microtremor survey methods; the spatially averaged coherency method (SPAC), and the frequencywavenumber method (FK). The bedrock at Launceston is composed of hard, dense, elastic but fractured and weathered dolerite from the Jurassic period. The bedrock is overlain by poorly consolidated sediments which can be compressible, water saturated, plastic and low density. A gravity survey completed by Leaman (1994) outlines the presence of the Tamar valley, filled with soft sediments from the Tertiary and Quaternary periods. Interpretation of gravity data indicates that the Tamar valley has a width of 700m to 1000m, with a maximum depth of 250m (Figure 1, top right). Alluvial sediments (silts, gravels, fills) were deposited during the Quaternary period at the bottom of the valley and other marshy areas near sea level. We use centered hexagonal arrays of seven Mark L28 – 4.5Hz geophones to record SPAC and FK data at seven separate sites in the city. We present observations from two sites; site GUN located over assumed layered earth on the north bank of the North Esk River, and site DBL located inside the Tamar valley (hexagons, Figure 1, left). An earlier comparison of the river-edge sites GUN and KPK is given in Claprood and Asten (2007, 2008). The use of hexagonal arrays has proven to be sufficient to approximate spatial stationarity (Asten et al., 2004) required for SPAC analysis. The averaged coherencies are calculated for 4 inter-stations separations shown in Figure 1, bottom right.