The Friuli Venezia Giulia and Veneto Regional Short Period Network

Centro Ricerche Sismologiche, Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Cussignacco (UD) e Sgonico (TS)

The OGS Seismological Department (CRS) manages the short-period seismometric network of Friuli-Venezia Giulia (RSFVG), property of OGS, and that of Veneto (RSV), property of Regione Veneto. The former has 14 stations, while the latter one has 8. All the stations have similar instrumentation and are mainly installed in mountainous or piedmont low-noise sites: 21 stations are equipped with a 1-Hz three-component Lennartz LE-3D seismometer; one with a Mark L4C 1-Hz vertical seismometer installed in a 100 meter deep borehole; two have a velocimetric/accelerometric couple (BAD and BOO, see OASIS/Sites).

Figure 1. Map of the Friuli Venezia Giulia and Veneto Regional Short Period Networks.

The data are transmitted by UHF radio to the CRS central station in Udine using the Lennartz MARS88/RC digital acquisition system. It is a hardware/software integrated system that supports signal digitalization, data transmission, storage and access to data for automatic and manual processing. Besides the sensors, each remote station includes (see Figs. 2 and 3):

  1. a MARS88 digital data logger with 120 dB dynamic range set to acquire at 62.5 sps or 125 sps with an anti-alias filter at 25 Hz or 50 Hz, respectively. The data are stored temporarily on a 4 MByte RAM memory card. An STA/LTA (Short Time versus Long Time Average) algorithm is used to recognize candidate earthquakes;
  2. a radio modem and a radio for bi-directional communication with the central station;
  3. a DCF signal radio receiver for time synchronization;
  4. solar panels and a battery for power supply.

Figure 2. Instrumentation used for the short-period networks of Friuli Venezia Giulia and Veneto.

Figure 3. Short-period seismic station of Casso (code CSO).

At the central station, located in Udine within the department building (Fig. 4), the acquisition system includes:

  1. a SUN Blade 150 workstation running software for the acquisition, management and on-line access of data for analysis;
  2. a PC acting as interface between the workstation and the radio network. The PC is connected to the workstation through the LAN and to the radio modems via eight serial ports.

Each remote station makes available for transmission only those data recognized as significant by the STA/LTA algorithm, whereas the central station collects these data by cyclical interrogation of the remote stations. For radio communication we use a set of frequencies in the 400 MHz band with 25 KHz channel bandwidth, allowing transmission at 17,400 bps. Due to the limitations imposed by the Italian telecommunication authority, only 7 frequencies are available. Groups of stations share the same frequency based on a token-passing mechanism.

Figure 4. The antenna of the central station at CRS.

The low number of UHF radio frequencies limits the overall number of acquired components and their sampling rate: three-component acquisition is possible for 16 stations, only three of which are sampled at 125 sps. In this configuration, it takes about 4-5 minutes to receive enough data to determine the hypocenter of a local earthquake using P and S arrival times. To obviate this limitation we are implementing spread-spectrum radio links, for which no licence is required by law. The devised communication network includes a backbone that goes across Friuli in a North-South direction, and several links connecting the seismic stations to the backbone. The nodes of the backbone are connected by means of radio devices of type Alvarion BreezeNET B14, transmitting at about 6 Mbps on a 20 MHz-wide channel in the 5.47-5.725 GHz frequency band. During transmission, the devices are able to switch automatically to a different 20 MHz channel if the quality of the signal decays due to weather conditions, noise or radar interference. The connection of the seismic stations to the backbone has been implemented by the less expensive devices WiLAN VIP 110-24, which transmit at about 2Mbps in the 2.4 GHz frequency band. All the devices have bridging capabilities, so that remote stations are seen as IP nodes of the same private network.

To connect the Lennartz MARS88 data loggers, which communicate through a serial interface, we use the serial to Ethernet converter MOXA NPort Express DE-211. Spread-spectrum communication will be also used to connect CRS to the emergency room of the Civil Protection of Friuli-Venezia Giulia (about 15 km apart).