Two microtremor array analysis methods, Spatial Autocorrelation (SPAC) and Centerless Circular Array (CCA) were applied to estimate the subsurface shear wave velocity (Vs) structure. The data for analysis were acquired by deploying the three types of array in the premises of the Department of Mines and Geology, Kathmandu Nepal.
The vertical component of microtremor records that are supposed to be dominated by Rayleigh waves are used in this study. Using the determined SPAC and CCA coefficients, the dispersion curves of Rayleigh-wave phase velocity were estimated. The dispersion curves of the three point (3P-array) and the 24 channel L-shaped array (L-array) for the SPAC method are independently determined and combined together for estimation of Vs structure. The frequency ranges of this combined dispersion curve from the SPAC method is from 2 Hz to 15 Hz and the CCA method 3.5 Hz to 13 Hz.
The Vs structure inverted from dispersion curve determined using both of the two SPAC and CCA methods shows the increment of Vs with depth. The result of the exploration up to 25 m depth from the both methods is summarized as a two layered structure. The surface layer has Vs 145 m/s to 195 m/s up to 13 m followed by higher Vs 227 m/s to 237 m/s. These are comparable with each other and consistent with the stratigraphic information given by the borehole log available near the observation site.
The CCA method explored same depth of the SPAC method even in the smaller radius array. Therefore, the CCA method seems available and more effective than the SPAC method in the urbanized area such as Kathmandu city.
Keywords: Microtremor, SPAC, CCA, Dispersion curve, Vs structure