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Authors: Braun, T.* 
Schweitzer, J.* 
Title: Spatial Noise-Field Characteristics of a three component small aperture test arrayin Central Italy
Issue Date: 2006
Keywords: Noise-Field
Subject Classification04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis 
Abstract: In order to evaluate detection and localization capabilities of a future array in the Northern Apennines an irregularly configured test array was installed in the Upper Tiber Valley (Italy) near Città di Castello (CDC), for a period of two weeks, consisting of nine sites with inter-sensor distances between 150 and 2200 m. The peculiarity of this test-array installation was the use of three component sensors at all array sites, which allowed the application of all array specific analyses techniques for the full seismic wavefield, i.e., also for horizontal-component data. The optimal array geometry and the number of sensors depend on the characteristics of the seismic signals, as well as on the local noise conditions. 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 characteristic is dependent on the chosen component of the seismic sensors, i.e., we analyze 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 show on all components strong azimuthal variations, 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 noise coherence on the horizontal components show slightly smaller azimuthal variations than on the vertical components. Comparison of the radial and transversal components suggests that the observed coherent noise from the E-NE-direction is mainly composed by 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.
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