Recent technological advances in the area of digital recording and telemetry now make it possible to design and implement remote instrumentation that can record a variety of geophysical parameters at required sampling rates, and can transmit those in close to real time to a central processing facility. We here describe how, at the Berkeley Seismological Laboratory (BSL, formerly Berkeley Seismographic Station), we are taking advantage of this state-of-the-art technology to acquire and telemeter broadband and strong motion seismic data on the one hand, geodetic data from continuously operating GPS receivers and/or electromagnetic data on the other, as well as other auxiliary channels such as barometric pressure and temperature. The BSL is involved in three expanding regional geophysical networks. The Berkeley Digital Seismic Network (BDSN) and the Hayward Fault Seismic Network (HFN) currently count, respectively, 15 and 5 operational stations. BARD (Bay Area Regional Deformation Network), a geodetic network of permanent GPS receivers deployed in cooperation with other institutions*, counts ~30 stations, 11 of which have been installed and are operated by the BSL. Of the latter, 9 are collocated with BDSN stations.
The BDSN stations are equipped with three component broad-band seismometers (STS-1 or STS-2 except for the Farallon Island site, which has a three-component Guralp CMG-40T), three component FBA-23 strong motion accelerometers (2g), and 24-bit Quanterra data loggers (of the Q935 or Q4120 type, with GPS clocks). Broadband data sampled continuously at 20Hz, 1Hz and 0.1 Hz, as well as triggered 80Hz broadband and strong motion data, are transmitted continuously to UC Berkeley, where data processing, along with information from the short-period Northern California Seismic Network (NCSN) operated by the U.S. Geological Survey is performed in quasi-real time and key earthquake parameters, such as moment magnitude and seismic moment tensor are broadcast in automatic mode (REDI program, Gee et al., 1996; Pasyanos et al., 1996).
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Figure 1. BDSN (broadband seismic), HFN (borehole seismic) and BARD (geodetic GPS) sites. |
The borehole HFN stations are designed to study microearthquakes along the Hayward Fault (down to magnitudes smaller than 0) and potentially gain insight into nucleation processes of large earthquakes, and are equipped with 6-component downhole packages (3 component Wilcoxin 731A accelerometers and 3 component Oyo HS-1 velocity transducers installed at depths of 40m to 170m) with Quanterra Q4120 recorders, affording sampling rates up to 1000 Hz. Auxiliary channels logged on the Quanterra´s and sampled at 1Hz transmit data from temperature and pressure sensors located in the vicinity of the seismic sensors, and provide valuable information to effect noise reduction corrections, either through installation improvements or using correlation procedures directly on the time series (figure 2). In addition, two of our sites (SAO and PKD, figure 1) represent prototype ULF electromagnetic (EM) observatories, and are equipped with 3 orthogonal magnetic field sensors (10-4 to 20Hz) and two electric dipoles (DC to 20Hz), sampled at 40 Hz and recorded in the Quanterras. These observatories are part of a monitoring program to document potential precursory signals to earthquakes.
 | Figure 2. Plot comparing the UHZ raw data (0.01 Hz sampling rate) at BDSN station SAO (in counts), for a period of 40 days, with the theoretical gravitational tides, the relative atmospheric pressure, the STS-1 seismometer temperature, and the Q935 datalogger temperature. The diurnal temperature variation of the digitizer and the annual temperature variation of the seismometer have the largest effect on the raw data for the vertical component while the pressure has a smaller effect (for horizontal components the latter effect is larger). Procedures for optimum broadband station installations as developed (and still in progress for improvements) can be found on our WWW page at http://www.seismo.berkeley.edu |
Until a year and a half ago, our telemetry system relied on standard analog telephone lines and, for some sites, piggy-backed on the microwave network maintained in northern California by the US Geological Survey. The Quanterra data loggers made it possible, through packetized transmission, to telemeter continuously all seismic and auxiliary data from the BDSN stations, but the data rates required for transmission of the high frequency HFN data were not suitable for this type of standard telemetry. Also, BARD data were downloaded once every 24 hours over separate dial-up phone lines. Over the past two years, with the aid of a grant from PacBell´s California Research and Educational Network (CalREN) to Caltech and U.C. Berkeley, the BSL has installed a state-of-the-art frame relay digital network to replace the point-to-point analog leased lines used to acquire real-time data from its regional stations.
Frame relay technology offers a number of significant benefits over analog leased lines. With point-to-point analog leased lines, each seismic station required a dedicated circuit between the seismic station and UC Berkeley, and used a standard modem with a maximum of 9600 baud to 18,000 bit/second to provide a single RS-232 async circuit. The frame relay network uses digital phone circuits that can support 56Kbit/second to 1.5Mbit/second throughput. Frame relay is a packet -switched network, which allows a site to use a single frame relay phone circuit to communicate with multiple remote sites through the use of permanent virtual circuits (PVCs). Frame Relay Access Devices (FRADs), which replace modems in a frame relay network, can simultaneously support multiple interfaces such as RS-232 async ports, synchronous V.35 ports, and ethernet connections. Our CalREN grant provided for a 56Kbit/second line at each of the remote seismic stations, and a single 1.5Mbit/second T1 line at UC Berkeley to receive the data from all stations. We were also granted an additional 1.5Mbit/second T1 circuit at UC Berkeley and a corresponding T1 circuit at USGS Menlo Park to be used for the real-time exchange of seismic data between the Seismological Laboratory and Menlo Park.
Multiple goals have been reached through the use of the frame relay network:
(1) Increased bandwidth between our remote observatories and UC Berkeley to support higher rate seismic data and new data channels from existing data loggers. This, in particular, has made it possible to telemeter high-frequency data in near real time from the borehole HFN stations. In particular, we can now make use of the central UC Berkeley site triggering capabilities to remotely enable triggered recording at all these stations, a powerful procedure to eliminate false triggers and reduce the amount of data recorded at high sampling rates.
(2) Ability to telemeter data from multiple instruments such as GPS receivers, electromagnetic sensors, or multiple seismic data loggers from a single site to UC Berkeley. In particular, with the installation of the frame-relay hardware, we are now continuously acquiring GPS data, sampled every 30 sec, from 10 of our 11 BARD sites. This opens up the possibility, in the very near future, to process geodetic data in near-real time (within minutes after the occurrence of an earthquake), along with the seismic data, if sufficiently accurate satellite orbits are available within the same time frame. Fluctuations in the regional and local deformation field can thus be followed at different time scales and, in particular, the geodetic data can provide complementary constraints to seismic data for source parameter estimation of large earthquakes. We are working on the development of algorithms towards this goal (Murray et al., 1996). A particularly important application is for tsunami warning, in our case in the northernmost part of California, in that geodetic data can provide instantaneous estimates of local displacements (Ellsworth, personal communication).
(3) Practical improvements in the operation of the telemetry: this includes (a) better control of telemetry costs, since frame relay costs are determined by a combination of flat rate and data throughput need instead of mileage based rates, (b) improved control of our remote seismic sites by providing Òremote consoleÓ control at UC Berkeley of our seismic data loggers (c) possibility of remote configuration, monitoring and diagnostics for communications hardware and telemetry circuits, (d) error free communications handling for async circuits.
One of our most recent installations that has particularly benefitted from the frame-relay system is the Farallon Island site (Figure 3), a BDSN and BARD site located 30 km off-shore beyond the San Francisco Golden Gate bridge, which includes a radio connection to the relay station at Mount Tamalpais. This remote site is a wild-life refuge and is unaccessible for visits for over half of the year. The frame-relay connection allows, for example, remote two-way monitoring of station state-of-health as well as upgrade of station software. Our Parkfield (PKD) and Sago (SAO) sites, on the other hand, represent prototype multiparameter observatories, with continuous acquisition and telemetry of broadband (and strong motion) seismic, geodetic, electromagnetic and auxiliary (temperature and pressure) data.
 | Figure 3. Photograph of the BDSN Farallon Islands (FARB) equipment rack and battery installation. The Quanterra Q4120 data logger (orange) is strapped to the top of the rack and the batteries are to the left of the rack in a restraining enclosure on the floor. The equipment in the rack from top to bottom is: Cylink spread spectrum radio; seismometer control panel; Freewave spread spectrum radio and frame relay access device (FRAD), Ashtech GPS receiver, power supply distribution equipment and, finally, duplicate power supplies. |
The availability of powerful data-loggers and reliable, large bandwidth telemetry opens up further opportunities for system enhancement and potential cost savings. For example, up till now, we have been transmitting continuous GPS and seismic data channels to UC Berkeley through separate virtual circuits on our frame relay lines. Our most recent development, with support from the IRIS/GSN, has been to integrate the GPS datastream into the Quanterra data logger, where the GPS data can be written to disk and/or tape, and incorporated with the seismic and related data from the Quanterra into a single real-time telemetry channel. This provides on-site buffering and storage of GPS data and should create a more reliable data path for the continuous telemetry of the GPS data. This software, initially designed for Ashtech receivers, is now under final testing and will soon be ported to other brands of GPS receivers with the aid of the IRIS/GSN and UNAVCO. It will be made available to the IRIS community.
