Spatial Autocorrelation (SPAC) Method for Subsurface Shear Wave Velocity Profiling: A Non-invasive Approach to Site Characterization

Abstract

Abstract The Spatial Autocorrelation (SPAC) method is considered a robust method for site characterization and seismic hazard assessment, being a nondestructive but credible method for determining subsurface shear wave velocity profiles. This research uses the SPAC method to obtain the 1D profile that focuses mainly on theory and the direct fitting of spatial autocorrelation coefficients, which leads to a higher accuracy with the employment of Rayleigh wave modes for both fundamental and higher modes. The field setup consists of seven vertical-component geophones arranged in the nested triangular configuration with radii of 9.24m, 4.62m, 2.31m, and 1.15m. The comprehensive dispersion curve was obtained using the SPAC method at frequencies from 7 to 50 Hz, showing a slight depth increase. At shallower depths, the results were from 200 m/s to 400 m/s, indicating the presence of loose sediments and clay-rich deposits. In contrast, deeper layers revealed values of up to 1000 m/s, possibly related to more compacted formations. The dispersion curves derived from different radii display a stable trend of the phase velocity with depth, and the Bessel function fitting process disperses a high correlation. The research findings demonstrate the SPAC technique’s efficiency in recording the correct velocity profiles and presenting clear and reliable results for other geophysical data processing methods, such as envelope detection, which significantly increases the reliability of the results. The non-invasive feature of SPAC is an advantage in that it reduces the logistic problems associated with active-source surveys, and, therefore, it is very useful in long-term monitoring projects and in geologic settings that are different from each other.

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