Monitoring of Mt. Vesuvius with seismic antennas: first results of VAS array

A seismic array named VAS (Vesuvius Array South) has been installed at the beginning of 2012 on the south flank of Mt. Vesuvius to improve the seismic monitoring of the volcano. It is composed of 10 short period seismometers connected by cable to the acquisition system. One three-component and nine vertical components are acquired by two six-channel data loggers. Data acquisition is continuous, on local memory, 100 samples per second, with 24 bit dynamic range. Data are periodically collected, converted and analyzed by applying four different array methods in several frequency bands in the range 1 Hz - 5 Hz. Array methods furnish propagation direction and apparent velocity of coherent signals, giving an important contribution to the identification of seismic events that could be related to the volcanic activity. Preliminary results of array analysis show that the array VAS has an optimal resolution in the 2 Hz - 5 Hz frequency range. Higher frequency signals may have a poor coherence due to the short wavelength, while at frequency lower than 2 Hz, the coherence of seismic noise may be comparable to the signal of interest, thus preventing the identification of natural events potentially related to changes in the dynamic of the volcano.