## 5 Conclusions  
We have presented a new system, GFZ-WISE, for performing dense 2D seismic ambient-noise array measurements. The system is made up of novel low-cost wireless sensing units (GFZ-WSU) designed to form dense wireless mesh networks (WMN). The GFZ-WSUs employ advances in various technologies to incorporate off-the-shelf sensors, processing and communications components into low-cost seismic sensing units that are linked by advanced, robust and rapid communications routing  
Figure 9: Results of the inversion analysis starting from different input models. (_a_) Under-estimated input model. In the _left-panel_ are shown the experimental phase velocities (_black line_), those for the best-fit model (_green dots_), and the dispersion curves for the input model (_gray line_). In the middle-panel is shown the misfit for each iteration (_green dots_). In the _right-panel_ are shown the S-wave velocities for the input (_thin line_) and final best-fit (_thick line_) models. (_b_) same as (_a_), but for an over-estimated input model. and network organizational protocols appropriate for WMNs. The reduced cost of the instruments (_i.e._, less than one tenth of a standard instrument) and the possibility of creating dense, self-organizing seismic monitoring networks are key attributes that all new approaches to seismic noise surveys to be followed (e.g., within Mega-city and landslides monitoring). Each of the GFZ-WSUs is able to collect, store and undertake preliminary analysis of data when only parameters (e.g., the horizontal-to-vertical curve of a site) are of concern. In addition, the stations create a seismic WMN, through which raw data and computed parameters can be communicated to a user's external laptop running the SeisComP software, which is connected to any node that belongs to the network, allowing a user to perform real-time quality control, and analysis of seismic data. Furthermore, the self-organizing character of the network guarantees the functionality of the network even when some of the sensing units malfunction or are not directly in line-of-sight with the operators. This latter characteristic makes the GFZ-WISE system particularly attractive during survey in urban contexts for microzonation studies, when obstacles as buildings might constraint the geometry of the array for wireless systems centralized or standard cable-dependent systems. Further, during 2D array seismic noise measurements, which generally involve period of recording of few hours, the necessary energy can be provided by small 17 Ah batteries. Finally, thanks to the reduced dimension and weight of the stations, the system is easy to install. Future applications of the GFZ-WISE system will include the monitoring of seismic noise, but when of interest also ground motion or other parameters, for urban sites in earthquake prone areas and landslides. For such long-term monitoring applications, the necessary energy for the GFZ-WSUs should be provided by a buffer battery (e.g., 40 Ah) connected to a solar panel (e.g., 60 W) through a solar controller. In urban context, the reduced cost of the GFZ-WSUs would enable to create dense network for the characterization and real-time monitoring of the variations of the subsoil mechanical properties following the shaking of an earthquakes, as well as to provide new observations concerning the soil-structure interaction effects. However, it worth to specify that for seismological purposes (e.g., studies of the seismic sources, or basins with sedimentary cover thickness of several hundred of meters) where the target signals interest frequencies below 1 Hz, the GFZ-WSUs should be equipped by passive velocimeter sensors with lower corner frequency and higher resolution, but higher cost, than those of the sensor used in this study. Similarly, for strong motion studies, when input signals have very large amplitude, the MEMS accelerometers should be used.