Direct estimation of V using spatial autocorrelation and centreless circular array coefficient curves obtained from microtremor array data

The average S-wave velocity (V) in the upper 30 m (V) is a proxy for seismic wave amplification. Microtremor array exploration is one of the available methods for site characterization, but the recorded data require complicated processing that can lead to different estimations of V depending on the analyst and processing software. We propose a method of estimating V by using derivatives obtained in the early stages of microtremor array data processing. Statistical analysis with 2376 virtually generated subsurface V structure models revealed that the frequencies at which the spatial-autocorrelation (SPAC) coefficients and centreless circular array (CCA) coefficients take specific values strongly correlate with V, which we used to develop formulas for estimating V30. Numerical validations using actual V profiles at 616 sites in Japan showed that the proposed method could estimate V with a root-mean-square deviation (RMSD) of 57–80 m/s with SPAC coefficients and 56m/s with CCA coefficients. Our proposed methods were applicable for 98–100 per cent of theV profiles when we limited our estimation to sites with V < 760 m/s. The results indicated that SPAC coefficients from arrays with radii of 8–20 m can be used for V estimation and are less affected by incoherent noise. In contrast, CCA coefficients are much more sensitive to incoherent noise, which resulted in the overestimation of V30. The estimated V values from the recorded microtremor array data were in good agreement with the reference values from the actual V profiles. The proposed method allows for robust and efficient V estimation without relying on the analyst’s skills or software.