The Berkeley Digital Seismic Network (BDSN) is a regional network of very broadband and strong motion seismic stations spanning Northern California and linked to UC Berkeley through continuous telemetry (Figure 3.2 and Table 3.3). The network is designed to monitor regional seismic activity at the magnitude 3+ level as well as to provide high quality data for research in regional and global broadband seismology.
Since 1991, the BDSN has grown from the original 3 broadband stations installed in 1986-87 (BKS, SAO, MHC) to 32 stations, including an autonomous ocean-bottom seismometer in Monterey Bay (MOBB). We take particular pride in high quality installations, which often involve lengthy searches for appropriate sites away from sources of low-frequency noise as well as continuous improvements in installation procedures and careful monitoring of noise conditions and problems. This year, most of the field and operation efforts have been directed toward station upgrades, thanks to the ``American Recovery and Reinvestment Act'' (ARRA). Engineering and research efforts were also devoted to several projects to develop and test new instrumentation (see Operational Section 7). We made progress in testing a new, low-cost sensor for pressure and temperature to be installed at seismic and GPS sites, and have begun testing of the Quanterra environmental add-on, the QEP. Finally, the BSL is part of a team for developing and testing a newly designed VBB sensor to replace the STS-1 seismometer.
The expansion of our network to increase the density of state-of-the-art strong motion/broadband seismic stations and improve the joint earthquake notification system in this seismically hazardous region, one of BSL's long term goals, must be coordinated with other institutions and is contingent on the availability of funding.
Equally important to network growth, data quality and the integrity of the established network must be preserved. The first generation of broadband seismometers installed by the BSL has been operating for almost 25 years. At the same time, the first generation of broadband data loggers are entering their 18th year of service. Fortunately, we received funding and equipment from the ARRA to replace data loggers at the 25 stations with older models between September 2009 and September 2011. These efforts are ongoing. In the meantime, we continue to exercise vigilance and commit time and resources to repairs and upgrades as necessary.
Twenty-eight of the BDSN sites are equipped with three-component broadband seismometers and strong-motion accelerometers, and a 24-bit digital data acquisition system or data logger. Two additional sites (RFSB and SCCB) consist of a strong-motion accelerometer and a 24-bit digital data logger. The ocean-bottom station MOBB is equipped with a three component broadband seismometer and a DPG. Data from all BDSN stations are transmitted to UC Berkeley using continuous telemetry. Continuous telemetry from MOBB was implemented early in 2009. Unfortunately, the underwater cable was trawled and damaged several times, until it finally failed in late February 2010. We are currently preparing a new cable to connect MOBB to the nearby science node. It will be buried in the seafloor, which will hopefully protect it from trawling operations. In order to avoid data loss during utility disruptions, each site has a three-day supply of battery power; many are accessible via a dialup phone line. The combination of high-dynamic range sensors and digital data loggers ensures that the BDSN has the capability to record the full range of earthquake motion required for source and structure studies. Table 3.4 lists the instrumentation at each site.
Most BDSN stations have Streckeisen STS-1 or STS-2 three-component broadband sensors (Wielandt and Streckeisen, 1982; Wielandt and Steim, 1986). A Guralp CMG-3T broadband sensor contributed by LLNL is deployed in a post-hole installation at BRIB. A Guralp CMG-1T is deployed at MOBB. The strong-motion instruments are Kinemetrics FBA-23, FBA-ES-T or Metrozet accelerometers with 2 g dynamic range. Thanks to the ARRA funding, we are replacing the noisier FBA-23 sensors with FBA-ES-Ts. The recording systems at all sites except MOBB are either Q330, Q680, Q730, or Q4120 Quanterra data loggers, with 3, 6, 8, or 9 channel systems. The Quanterra data loggers employ FIR filters to extract data streams at a variety of sampling rates. In general, the BDSN stations record continuous data at .01, 0.1, 1.0, 20 or 40, and 80 or 100 samples per second. However, at some sites, data at the highest sampling rate are sent in triggered mode using the Murdock, Hutt, and Halbert event detection algorithm (Murdock and Hutt, 1983) (Table 3.1). In addition to the 6 channels of seismic data, signals from thermometers and barometers are recorded at many locations (Figure 3.3).
As the broadband network was upgraded during the 1990s, a grant from the CalREN Foundation (California Research and Education Network) in 1994 enabled the BSL to convert data telemetry from analog leased lines to digital frame relay. The frame-relay network uses digital phone circuits which support 56 Kbit/s to 1.5 Mbit/s throughput.
Since frame-relay is a packet-switched network, a site may use a single physical circuit to communicate with multiple remote sites through the use of ``permanent virtual circuits." Frame Relay Access Devices (FRADs), which replace modems in a frame-relay network, can simultaneously support a variety of interfaces such as RS-232 async ports, synchronous V.35 ports, and ethernet connections. In practical terms, frame relay communication provides faster data telemetry between the remote sites and the BSL, remote console control of the data loggers, services such as FTP and telnet to the data loggers, data transmission to multiple sites, and the capability of transmitting data from several instruments at a single site, such as GPS receivers and/or multiple data loggers. Today, 25 of the BDSN sites use frame-relay telemetry for all or part of their communications system.
As described in Operational Section 7, data from the BDSN are acquired centrally at the BSL. These data are used for rapid earthquake reporting as well as for routine earthquake analysis (Operational Section 2 and 8). As part of routine quality control (Operational Section 7), power spectral density (PSD) analyses are performed continuously and are available on the internet (http://www.ncedc.org/ncedc/PDF/html/). The occurrence of a significant teleseism also provides the opportunity to review station health and calibration. Figure 3.4 displays BDSN waveforms for the great 8.8 earthquake that occurred in Chile on February 27, 2010.
BDSN data are archived at the Northern California Earthquake Data Center. This is described in detail in Operational Section 6.
In 1995, in collaboration with Dr. Frank Morrison, the BSL installed two well-characterized electric and magnetic field measuring systems at two sites along the San Andreas Fault which are part of the Berkeley Digital Seismic Network. Since then, magnetotelluric (MT) data have been continuously recorded at 40 Hz and 1 Hz and archived at the NCEDC (Table 3.2). At least one set of orthogonal electric dipoles measures the vector horizontal electric field, E, and three orthogonal magnetic sensors measure the vector magnetic field, B. These reference sites, now referred to as electromagnetic (EM) observatories, are collocated with seismometer sites so that the field data share the same time base, data acquisition, telemetry, and archiving system as the seismometer outputs.
The MT observatories are located at Parkfield (PKD1, PKD), 300 km south of the San Francisco Bay Area, and Hollister (SAO), halfway between San Francisco and Parkfield (Figure 3.2). In 1995, initial sites were established at PKD1 and SAO, separated by a distance of 150 km, and equipped with three induction coils and two 100 m electric dipoles. PKD1 was established as a temporary seismic site, and when a permanent site (PKD) was found, a third MT observatory was installed in 1999 with three induction coils, two 100 m electric dipoles, and two 200 m electric dipoles. PKD and PKD1 ran in parallel for one month in 1999, and then the MT observatory at PKD1 was closed. Starting in 2004, new electromagnetic instrumentation was installed at various Bay Area sites in cooperation with Simon Klemperer at Stanford University. Sensors are installed at JRSC (2004), MHDL (2006) and BRIB (2006/2007).
Data at the MT sites are fed to Quanterra data loggers, shared with the collocated BDSN stations, synchronized in time by GPS, and sent to the BSL via dedicated communication links.
In 2009, the BSL led a joint effort toward improving operation and maintenance of these sites with Jonathan Glen and Darcy McPhee from the USGS, and Simon Klemperer at Stanford University.
Engineers from the BSL met scientists from the USGS and Stanford at the station SAO in October of 2008 to assess the condition of the EM/MT system. At that time, the EM coils were found to be not working. They were removed and returned to the manufacturer (EMI Schlumberger). In June 2010, the EM coils had not been reinstalled at SAO. EM/MT equipment at PKD was evaluated in August of 2008. There, the data logger was removed from the PKD EM/MT system and has not yet been returned.
Since it began in 1995, the EM/MT effort has suffered from minimal funding, in part due to the misconception that the EM/MT data could be recorded on unused channels in the seismic data logger. These data loggers had no channels available, however. Thus, for each site, an additional data logger was purchased. In 2008, the BSL began in-house development of a low cost digitizing solution. While not as feature-rich as commercially available data loggers, the prototype 24 bit digitizer was developed and is being tested in the field.
Given the remoteness of the off-campus stations, BDSN data acquisition equipment and systems are designed, configured, and installed so that they are both cost effective and reliable. As a result, there is little need for regular station visits. Nonetheless, many of the broadband seismometers installed by BSL are from the first generation and are about 25 years old. Concurrently, the first generation of broadband data loggers is now 18 years old. Computer systems are retired long before this age, yet the electronics that form these data acquisition systems are expected to perform without interruption.
In the summer of 2009, the USGS received ARRA funds, among other things, to upgrade and improve seismic stations operated as part of the Advanced National Seismic System (ANSS). The BSL is benefitting from those funds. We are receiving the newest model of Quanterra data logger, the Q330, as government-furnished equipment (GFE) to replace the old Quanterras at 25 of the BDSN seismic stations. In addition, all remaining Kinemetrics FBA-23 accelerometers will be replaced with Kinemetrics' newer model, the FBA-ES-T. Stations for which the upgrade is complete are marked in Table 3.4. As of June 2010, we have replaced equipment at about half of the BDSN sites that will be upgraded. Finally, we have also received support through the ARRA project to investigate and implement alternative, and less expensive, telemetry options.
As always, some of the BSL's technical efforts were directed toward maintaining and repairing existing instrumentation, stations, and infrastructure. While expanding the network continues to be a long term goal of BSL, it is equally important to assure the integrity of the established network and preserve data quality.
The Monterey Ocean Bottom Broadband observatory (MOBB) is a collaborative project between the Monterey Bay Aquarium Research Institute (MBARI) and the BSL. Supported by funds from the Packard Foundation to MBARI, NSF/OCE funds, and UC Berkeley funds to the BSL, its goal has been to install and operate a long-term seafloor broadband station as a first step toward extending the onshore broadband seismic network in Northern California to the seaward side of the North-America/Pacific plate boundary, providing better azimuthal coverage for regional earthquake and structure studies. It also serves the important goal of evaluating background noise in near-shore buried ocean floor seismic systems, such as may be installed as part of temporary deployments of ``leap-frogging" arrays (e.g. Ocean Mantle Dynamics Workshop, September 2002). The project has been described in detail in BSL annual reports since 2002 and in several publications (e.g. Romanowicz et al., 2003, 2006).
The MARS (Monterey Accelerated Research System) observatory (Figure 3.5, http://www.mbari.org/mars/) comprises a 52 km electro-optical cable that extends from a shore facility in Moss Landing out to a seafloor node in Monterey Bay (Figure 3.5). The cable was deployed in the spring of 2007, and node installation was completed in November 2008. It now can provide power and data to as many as eight science experiments through underwater electrical connectors. MOBB, located 3km from the node, is one of the first instruments to be connected to the cable. The connection was established on February 28, 2009, through an extension cable installed by the ROV Ventana, with the help of a cable-laying toolsled. The data interface at the MARS node is 10/100 Mbit/s Ethernet, which can directly support cables of no more than 100 m in length. To send data over the required 3 km distance, the signals pass through a Science Instrument Interface Module (SIIM) at each end of the extension cable (Figure 3.6). The SIIMs convert the MARS Ethernet signals to Digital Subscriber Line (DSL) signals, which are converted back to Ethernet signals close to the MOBB system. Power from the MARS node is sent over the extension cable at 375 VDC, and then converted to 28 VDC in the distal SIIM for use by the MOBB system. The connection to the MARS node eliminates the need for periodic exchange of the battery and data package using ROV and ship. At the same time, it allows us to acquire seismic data from the seafloor in real time (Romanowicz et al., 2009).
The electronics module in the MOBB system has been refurbished to support the connection to the MARS observatory. The low-power autonomous data logger has been replaced with a PC/104 computer stack running embedded Linux. This new computer runs an Object Ring Buffer (ORB), whose function is to collect data from the various MOBB sensors and forward it to another ORB running on a computer at the MARS shore station. There, the data are archived and then forwarded to a third ORB running at the UC Berkeley Seismological Laboratory. The Linux system acquires data from the various systems on the sea floor: from the Guralp digitizer included in the seismometer package (via RS232) and from a Q330 Quanterra 24 bit A/D converter which digitizes data from the DPG (via Ethernet). It also polls and receives data (via RS232) from the current meter. The data are available through the NCEDC. Procedures to include the MOBB data in the Northern California real time earthquake processing are under development.
Recently, we have been exploring ways to routinely remove low frequency noise generated by infragravity ocean waves, which are also observed on the DPG. Figure 3.7 shows the transfer function from the DPG to MOBB-Z in the period band (35 - 200 sec). The stability of the transfer function over the course of a year indicates that automatic implementation of the noise reduction procedure will allow us to more effectively use MOBB data jointly with data from other BDSN stations to constrain regional moment tensors in real time. Figure 3.8 shows the vertical component trace before and after removing the DPG correlated noise, in the case of a regional event of 4.3. Unfortunately, the cable that links the MOBB instrumentation to the MARS science node was trawled several times since February 2009 leading to a failure on February 27, 2010. We have obtained funds from NSF/OCE to purchase and install a new cable. This time, the cable will be buried in the seafloor to avoid further trawling incidents. This has required the construction of a cable burying tool for the MBARI ROV Ventana, which is in the process of being tested, for a planned installation of the new cable in early 2011.
Under Barbara Romanowicz's general supervision, Peggy Hellweg and Doug Neuhauser oversee the BDSN data acquisition operations, and Bill Karavas heads the engineering team. John Friday, Jarrett Gardner, Rick Lellinger, Taka'aki Taira, and Bob Uhrhammer contribute to the operation of the BDSN. The network upgrades and improvements are funded through the ARRA (American Reinvestment and Recovery Act), under the USGS Award Number G09AC00487.
MOBB is a collaboration between the BSL and MBARI, involving Barbara Romanowicz, Taka'aki Taira, and Doug Neuhauser from the BSL, and Paul McGill from MBARI. The MBARI team also has included Steve Etchemendy (Director of Marine Operations), Jon Erickson, John Ferreira, Tony Ramirez, and Craig Dawe. The MOBB effort at the BSL is supported by UC Berkeley funds. MBARI supports the dives and data recovery. The MOBB seismometer package was funded by NSF/OCE grant #9911392. The development of the interface for connection to the MARS cable is funded by NSF/OCE grant #0648302.
Bill Karavas, Taka'aki Taira, and Peggy Hellweg contributed to the preparation of this section.
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