Spatial Noise-Field Characteristics of a Temporary Three-Component Small Aperture Array in Central Italy

Abstract

In order to evaluate detection and localization capabilities of a future array in the Upper Tiber Valley (Northern Apennines – Italy), an irregularly configured test array was installed near Città di Castello (CDC) for a period of two weeks, consisting of nine sites with inter-sensor distances between 150 and 2200 m. This test-array installation is particular in its use of three component sensors at all array sites, which allows the application of all array specific analyses techniques for the full seismic wavefield, i.e., also for horizontal-component data. In this study we investigate the inter-sensor coherence of the seismic noise field for the test-array area. In addition to the “classic” noise analysis, where noise cross-correlation values are calculated at single vertical instruments without relative time shifts between the traces, we extend the study by a “dynamic” approach, which accounts for possible slowness characteristics of the noise field. Furthermore, we investigate how the noise characteristics are dependent on the chosen component of the seismic sensors, by analyzing the noise coherence not only between vertical components but also on the radial and the transverse components. The coherence found for noise observed by the different sensors of the test array shows strong azimuthal variations on all components, which are most pronounced for noise within the frequency passband of 1.5 - 4.0 Hz and an apparent velocity of 1.5 km/s (Rg waves). The calculated correlation lengths of noise observed for the CDC array are about half of the values found for the NORES array in Southern Norway. Therefore, a future permanent array installation should be planned for minimum inter-sensor distances between 150 and 200 m.