Statistical validity control on SPAC microtremor observations recorded with a restricted number of sensors

Maxime Claprood, Michael Asten

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    The interpretation of an observed spatially averaged coherency (SPAC) spectrum assumes spatial and temporal stationarity of the microtremor wave field. This hypothesis gains in importance when recording coherency spectra (COHs) with a limited number of sensors. SPAC observations were recorded at three separate sites in Launceston (Australia) using a pair of sensors, triangular arrays, and hexagonal arrays to study the effect of the number of sensors, length of time series, and frequency interval for evaluating the shear-wave velocity (SWV) profile. The imaginary component of the observed complex COH is separated into roughened and smoothed parts. The root mean square of the roughened imaginary COH (rms (Im)) is an expression of the statistical noise in the observed coherency, while the behavior of the smoothed imaginary COH gives some indication of the distribution of the microtremor wave field. The mean square of residuals (MSR) between observed and theoretical COHs is an indication of the confidence level on the SWV profile interpreted. MSR values evaluated at pairs of sensors of different orientations give some indication of the azimuth distribution of the microtremor wave field and provide some guidelines on the field procedure when using a limited number of sensors to record SPAC observations.
    Original languageEnglish
    Pages (from-to)776 - 791
    Number of pages16
    JournalBulletin of the Seismological Society of America
    Issue number2
    Publication statusPublished - 2010

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