Noise Field Characteristics of a Seismic Small Aperture Array Installation in Central Italy

Abstract

The realization of the first permanent seismic small-scale array in Central Italy is aimed to monitor the micro-seismicity at a local and regional range, including the epicentral area of the 2016-18 seismic sequence and also the geothermal areas in the western sector. The array called CISA (Central Italy Seismic Array) has a maximum aperture of 1000 m and is composed of 9 three-component seismic sensors installed at inter-station distances between 100 and 500m. When designing a seismic array, the number of sensors, their respective inter-station distances and the array aperture must be tuned to the wavenumber characteristics of the target signals and the local noise field, in order to optimize the signal-to-noise ratio. For this purpose, the statistical structure of signal and noise have been analyzed by means of auto-correlation and cross-correlation functions for all combinations of sensor pairs in order to determine suitable interstation distances, such that the array response-once tuned to the signal-suppresses energy reaching the array with wavenumbers, which are characteristic for the local noise wave-field. The Rayleigh-wave dispersion curve retrieved combining results of auto-correlation and frequency-wavenumber analysis on ambient noise data, has also been inverted by using a neighborhood algorithm in order to retrieve the shear-wave velocity profile for the site down to a depth proportional to the spatial configuration of the array.