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.

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