|Observatories and Research Facilities for EUropean Seismology|
|Volume 3, no 1||June 2001||Orfeus Newsletter|
A New Broad-Band Seismic Network with Satellite Transmission in Catalonia (Spain)X. Goula, J.A. Jara, T. Susagna and A. Roca
Contributors: O. Olivera, S. Figueras, J. Fleta, J.C. Olmedillas and M. Tàpia
Figure 1. Implementation phase of the seismic network.
In a third phase about 12 more stations will be installed. Moreover it is planned to share data from stations belonging to other institutions as the Ebro Observatory, the Institut d'Estudis Catalans and the Instituto Geogràfico Nacional. A big effort is devoted to select adequate sites and to design and construction of the different elements of a remote station with the aim to have reliable, robust, low noise and durable stations. In Figure 2 a view of Organyà seismic station (CORG), in the Pyrenees, is shown with the seismometer vault, the instrumental house, the solar cells and the VSAT antenna. All stations are provided with high performance electrical and environmental protections.
Figure 2. View of Organyà seismic station.
Records of high quality are obtained from the operating stations. An example of low magnitude (ML=1.9) earthquake recorded at short distance (40 km) in Llívia station (CLLI) is shown in Figure 3, where broadband records obtained on a CMG-40T sensor are presented together with high-pass filtered records at 1 Hz. Fourier spectrum of E-W component is also shown together with the noise spectrum. A good signal/noise ratio is observed for frequencies higher than 1 Hz. Another example is shown in Figure 4 for a Japanese earthquake of M=6.5 recorded at Llívia station (=90°) on a broadband CMG-40T sensor. A record of 1 hour of duration is shown and the Fourier spectrum of surface waves recorded on the E-W component is presented together with the noise spectrum. A good signal/noise ratio is observed between 10s and 100s.
Figure 3. Left: Broadband records obtained on a CMG-40T sensor and high-pass filtered records at 1 Hz. Right: Fourier spectrum of E-W component (red) and noise spectrum (black).
Figure 4. Left: Broadband records obtained on a CMG-40T sensor Right: Fourier spectrum of E-W component (red) and noise spectrum (black).
Nanometrics, Canada). The Hub and remotes communicate via the Hispasat 1, a geostationary satellite using 100 kHz of bandwidth providing 112/64 kbps of data throughput. The system uses the same carrier for the inbound and the outbound, minimizing the required bandwidth (see Figure 5). At each station in the network (Hub and remotes) a slot within a time domains multiple access protocol (TDMA) is assigned, and it transmits only when authorized. At each remote site an HRD24 Nanometrics digitizer receives seismic signal from a broadband sensor installed in a vault, and samples it at 100 sps and streams the data to the remote VSAT terminal. The data are transmitted to the central Hub using UDP/IP protocol and a header (NMXP) containing a unique sequence number. When data are received at the Hub the sequence numbers are checked for continuity, and if data are missing, a retransmission request is automatically sent from the Hub to the remote. Data are stored in ring buffers at remotes (2.5 hours of backup) and are retransmitted to the Hub if requested. At the data acquisition center in Barcelona, received data are sent from the Hub over a LAN to one computer, in which they are stored into ringbuffers.
Figure 5. Libra VSAT network.
Acquisition systemThe digitizer uses a fixed resolution sigma-delta A/D converter on each channel providing 120 dB of resolution after digital filtering and a sample rate of 256 kHz. Antialiasing filtering protection is applied to the signal using a DSP processor, which decimate the data to obtain the expected sample rate, in this case 100 sps. The digitizer can be modified to obtain the desired sensivity in nm/s/count, so we can combine different seismometers from the same network without significant differences in the output sensitivity. Seismic data are timed at each remote site using a temperature compensated crystal oscillator phase locked to a GPS time code receiver. Data are assembled into packets with CRC for error correction. Each packet includes a comprehensive header which holds parameters such as the sequence number, time in long seconds and the oldest packet available for retransmissions if required. To ensure efficient use of communications system the data are compressed prior to transmission. The compression scheme used yields approximately 1.2 bytes per sample and is fully recoverable with no errors. These data are recorded into local ringbuffers before be forwarded to the VSAT modem. So, system has a remote data backup of the last 2.5 hours of acquired data. One more feature of the digitizer is its capability for monitoring some parameters of the remote stations and generating multiple state of health messages. All this information are sent to the Hub. Thus, it is possible to monitor some parameters of the remote station, such as battery voltage, digitizer temperature, GPS status, bytes sent, log messages, etc. from the central site and have them stored into separate ringbuffers.
Communications systemEach remote station has a full satellite communications system, which basically consists of a Ku-band 1.8 m diameter antenna, a Ku-band VSAT modem, a GPS time code receiver, a Ku-band LNB and a Ku-band SSPB. The VSAT modem receives the seismic data packets from the digitizer and formats them as UDP/IP data prior to modulating the data for transmission over the satellite link. The transmission data rate of the VSAT modem can be set for either 64 or 112 Kbps. The Libra network distributes the bandwidth between a number of Lynx remote stations using a time domain multiple access protocol (TDMA). In a TDMA system a number of stations are configured to share the same frequency, each station transmitting during a precisely defined time window or slot. During one transmission slot, the selected remote site transmits at the full rate of 64 or 112 kbps. The durations of the slot is set to provide the required continuous data rate. The satellite communications system provides a half duplex communications link between each remote site and the central data acquisition facility. The TDMA configuration includes a number of inbound slots during each remote site transmitting seismic data to the central Hub and one outbound slot during which the central Hub communicates with all the remotes sites. The communications data rate for the Hub and the remotes is configurable by the user. In general, the traffic from the Hub to the remotes is very little, and it is used for data retransmission request, TDMA configuration command and remote control. Similar to the acquisition system, communications system has the capability to produce a fully state of health summary and to be remotely controlled. Thus, the user from the central site can monitor the state of each remote communications modem and perform a remote control of the communications parameters. Inbound seismic data from the remote field stations are received at the central site via a 3.8 m antenna. The indoor assembly consists of a Carina combiner/splitter module and a number of Carina transceivers. Each Carina transceiver tunes to a single space segment frequency (100 kHz) and receives all data from stations transmitting on that frequency. Typically, 16, 3-componnent remote field stations are configured to operate a single 100 kHz space segment channel. The Carina transceiver demodulates de seismic data and forwards it as TCP/IP packets to the central acquisition computer(s) via a 10Base-T LAN connection. We can consider the entire network as a WAN IP network, which is customized depending of the requirements. So, the network can include some subnetworks to obtain the desired topology. This facility allows different networks to share data in real time.
Data reception, storage and processing centerAt the central site, an acquisition computer stores the seismic data into ringbuffers with a capacity of 16 days of data using NAQS Server software. NAQS Server software is a primary software element for data adquisition and seismic data handling. This software performs the following functions:
Figure 6. Gaps of transmission in seconds per hour, observed on two stations, for the year 2000.
The analysis of the retransmissions carried out by the system during 2000 for Llívia station is shown on Figure 7. It can be seen that the daily percentage of retransmissions never exceeded 15% of data. Retransmissions have been efficient and so loss of data is very scarce. The few percentage of retransmissions indicate that almost real time transmission is efficiently achieved.
Figure 7. Retransmission of data per day, in percentage observed on Llívia station for the year 2000.