The majority of the BSL BARD stations use a low-multipath choke-ring antenna, most of which are mounted to a reinforced concrete pillar approximately 0.5-1.0 meter above local ground level. The reinforcing steel bars of the pillar are drilled and cemented into a rock outcrop to improve long-term monument stability. A low-loss antenna cable is used to minimize signal degradation on the longer cable setups that normally would require signal amplification. Low-voltage cutoff devices are installed to improve receiver performance following power outages. Most use Ashtech Z-12 receivers that are programmed to record data once every 30 seconds and observe up to 12 satellites simultaneously at elevations down to the horizon. The antennas are equipped with SCIGN antenna adapters and hemispherical domes, designed to provide security and protection from weather and other natural phenomena, and to minimize differential radio propagation delays.
Data from most BSL-maintained stations are collected at 15 or 30-second intervals and transmitted continuously over serial connections (Table 3.5.2). Station TIBB uses a direct radio link to Berkeley, and MODB uses VSAT satellite telemetry. Most stations use frame relay technology, either alone or in combination with radio telemetry. Fourteen GPS stations are colocated with broadband seismometers and Quanterra data loggers (Table 3.2). With the support of IRIS, we developed software that converts continuous GPS data to MiniSEED opaque blockettes that are stored and retrieved from the Quanterra data loggers (Perin et al., 1998), providing more robust data recovery from onsite disks following telemetry outages.
Data from BRIB, CMBB, DIAB, HOPB, MHCB, MHDL, MONB, OHLN, OXMT, PTRB, SBRN, SRB1, SVIN, TIBB and UCD1 in the Bay Area, and 13 stations in the Parkfield region (all but PKDB), are now being collected at 1-second intervals. All high-rate observations collected by these stations are currently available from the NCEDC. Collecting at such high frequency (for GPS) allows dynamic displacements due to large earthquakes to be better measured; however, this 30-fold increase in data can pose telemetry bandwidth limitations. We are planning to convert additional stations to 1-second sampling where possible during the next year. The acquisition of the 5 NETRS bundles will help to complete this project (see Subsection 3.5.3). In the Bay Area, we have converted stations that have sufficient bandwidth and are currently assessing bandwidth issues at other stations. Prior to the September 28, 2004 M6 Parkfield earthquake, data from the Parkfield stations were collected on an on-site computer, written to removable disk once per month, and sent to SOPAC for long-term archiving (decimated 30-sec data is acquired daily via the BSL frame relay circuit). In response to the earthquake, we modified the procedures to download 1-second data converted to compact RINEX format at hourly intervals, which does not significantly impact the telemetry bandwidth.
Berkeley Seismological Laboratory
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