## 4 Field Tests of the GFZ-WISE
### Data Recording, Analysis and Results  
On the 6 th of October 2009, the first field test with the GFZ-WISE system was performed at the Alfred Einstein Science Park in Potsdam, Germany (Figure 5). Figure 5: Field test with the GFZ-WISE system at the Alfred Einstein Science Park in Potsdam, Germany. In the _inner inset_ it is shown an example of GFZ-WSU installation. The geology of Brandenburg, the region where the site is located, is representative for large areas of northern Germany, being characterized by Quaternary sediments formed during the last glacial period (Weichsel), overlying Tertiary clays. In particular, at the investigated site the Quaternary sediments are mainly fluvial and glacial sands [40]. The array consisted of 15 GFZ-WSUs (Figure 5 and Figure 6) equipped with standard 3D SM-6/B 4.5 Hz external geophones that were installed so as to obtain a good coupling between the instrument and soil (Figure 5, inner inset). We selected a simple cross-shaped 2D geometry (Figure 6a), and ambient seismic noise was recorded at a sampling rate of 100 Hz for about 1 hour, which guarantees the statistical stabilization of the signal [41]. The energy required by the GFZ-WSUs was provided by 17 Ah batteries. The maximum inter-station distance, which controls the array resolution, and the minimum one which constrains which wavelengths will be affected by spatial aliasing, were 43 and 1.5 meters, respectively. Despite the selected geometry being quite simple, the array response function (_i.e._, the transfer function of the array, which depends only on the distribution of stations in the array, and controls the spatial accuracy and resolution of signals that can be recorded by that array) computed for the frequency 10 Hz [42]  
Figure 6: (a) Array geometry. (b) array response function. (c) experimental (_black lines_) and self-noise (_gray lines_) WSU spectra. does not show for the range of wave numbers of interest other peaks that would cause aliasing than the central one (Figure 6b). The seismic noise data were recorded contemporary in real-time by two different operators, which established a WLAN-connection with two different GFZ-WSUs of the network, and were running the SeisComP software on their laptops. The observed delays during the multi-hops transmission of data from all the stations were constantly less than two seconds. Before the analysis, all recordings were corrected for the instrumental response, considering the calibration parameters of each sensor. Then, the spectra of the recorded signals were compared with the self-noise spectra in order to identify the usable frequency range in the analysis. It is worth to point out that when the amplitudes of seismic noise at certain frequency become smaller than the self-noise (_i.e._, also known as internal noise) of the instrument, the recorded signals do not provide anymore information about the propagation of waves into the ground. Hence, they must be discharged from the analysis. Of course, since the self-noise is a steady characteristic of the instrument, while the amplitude of the ambient noise changes with time, the comparison of the experimental spectra with the self-noise one should be always performed during each survey. Figure 6c shows that the experimental spectra tend to approach the self-noise for frequencies lower than 1 Hz. Therefore, we limited the RWDC and H/V analysis to frequencies higher than this 1 Hz threshold. The RWDC was computed using data from all stations of the array, following the procedure described previously. We used non-overlapping time windows 30 seconds long and tapered with a 5 per cent cosine function before the computation of the spectra, extracted from the 1 hour seismic noise recording signal. Figure 7 provides a view of the real-time analysis performed for retrieving the RWDC.