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 3.1 and Table 3.1). This network is designed to monitor regional seismic activity at the magnitude 3+ level as well as to provide high quality data for research projects in regional and global broadband seismology.
The network upgrade and expansion initiated in 1991 has continued, and it has grown from the original 3 broadband stations installed in 1986-87 (BKS, SAO, MHC) to 28 stations in 2003, including the ocean-bottom seismometer in Monterey Bay. Two new stations were added in the past year (PACP and MNRC).
We take particular pride in high quality installations, which involves often 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 at existing stations.
Future expansion of our network is contingent on the availability of funding and coordination with other institutions for the development of a denser state-of-the-art strong motion/broadband seismic network and joint earthquake notification system in this seismically hazardous region.
Twenty-five of the BDSN sites are equipped with 3 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. Data from all BDSN stations are transmitted to UC Berkeley using continuous telemetry. In order to insure against data loss during utility disruptions, each site has a 3-day supply of battery power and is 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 for source and structure studies. Table 3.2 lists the instrumentation at each site.
Most BDSN stations have Streckeisen three-component broadband sensors (Wielandt and Streckeisen, 1982; Wielandt and Steim, 1986). Guralp CMG-3T downhole broadband sensors contributed by LLNL are deployed in post-hole installations at BRIB and FARB. The strong-motion instruments are Kinemetrics FBA-23 or FBA-ES-T with 2 g dynamic range. The recording systems at all sites are either Q730, Q680, Q980 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 and these have been implemented as acausal filters in the BDSN. In general, the BDSN stations record continuous data at .01, 0.1, 1.0, and 20.0 samples per second and triggered data at either 80 or 100 samples per second using the Murdock, Hutt, and Halbert event detection algorithm (Murdock and Hutt, 1983) (Table 3.3). In addition to the 6-channels of seismic data, signals from thermometers and barometers are recorded at nearly every site (Figure 3.2).
In parallel with the upgrade of the broadband network, a grant from the CalREN (California Research and Education Network) Foundation enabled the BSL to convert data telemetry from analog leased lines to digital frame-relay connections. The frame-relay network uses digital phone circuits that can 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 multiple interfaces such as RS-232 async ports, synchronous V.35 ports, and ethernet connections. In practical terms, the upgrade to frame relay communication provides faster data telemetry between the remote sites and the BSL, remote console control of the data loggers, additional services such as FTP and telnet to the data loggers, data transmission to multiple sites, and the ability to communicate and transmit data from multiple instruments such as GPS receivers and/or multiple data loggers at a single site. Today, 20 of the BDSN sites use frame-relay telemetry for all or part of their communications system.
As described in Chapter 9, data from the BDSN are acquired centrally at the BSL. These data are used in the Rapid Earthquake Data Integration System as well as in routine earthquake analysis (Chapter 10). As part of routine quality control (Chapter 9), power spectral density analyses are performed weekly and Figure 3.3 shows a summary of the results for 2002-2003. The occurrence of a significant teleseism also provides the opportunity to review station health and calibration and Figure 3.8 displays the response of the BDSN to a 7.3 deep focus earthquake in the Fiji Islands region.
BDSN data are archived at the Northern California Earthquake Data Center and this is described in detail in Chapter 11.
Given the remoteness of the off-campus stations, BDSN data acquisition equipment and systems have been designed, configured, and installed for both cost effectiveness and reliability. As a result, the need for regular station visits has been reduced. Most station visits are necessitated by some catastrophic failure. The 2002-2003 fiscal year was no exception.
The seismic vault at Yreka, CA (YBH) is sited in an abandoned hard rock mining drift in the Klamath National Forest in northern California. YBH was previously chosen as an alternative monitoring station by both IMS and DTRA. In collaboration with the IMS, BSL installed a VSAT data link, long-period microbarograph, separate battery back-up, a stand-alone data validation computer and door switch in 2001-2002.
This year, an STS-2 seismometer was deployed, bringing YBH into compliance as an auxiliary station of the IMS. This seismometer (0.0083-50 Hz passband) joins the three-component set of Streckeisen STS-1 broadband seismometers (0.0027-5 Hz passband), a three-component Kinemetrics FBA-23 strong motion accelerometer (0-32 Hz passband; ±2g full scale); an Ashtech Z-XII3 geodetic GPS receiver; a YSI 44031 thermistor (to sense seismic pier temperature); a Motorola MPX-2010 pressure transducer and a Druck PTX-1240 microbarograph (to sense atmospheric pressure); and a Quanterra GPS clock for an accurate time base. Additionally, the sensor temperature, data logger temperature, broadband sensor mass position, clock quality, and telemetry through-put are utilized for status of health monitoring.
The STS-2 became operational as a part of the BDSN and telemetry of the signals to CTBTO and to BSL commenced on 27 November 2002. The three-component STS-2 and STS-1 data are continuously telemetered at 80, 20, 1, 0.1 and 0.01 samples per second.
As one part of BSL routine testing, the vertical-component background noise levels observed by the STS-1 and STS-2 were compared (see Figure 3.5). From the manufacturers specifications, the self noise of the high-gain STS-2 is lower that of the STS-1 at frequencies above 1 Hz. Here the high-gain STS-2 noise floor rises above the background earth noise observed by the STS-1 at 0.5 Hz and there are also narrow band spectral peaks at 1 Hz and its harmonics present with amplitudes of up to 20 dB. We suspect that the abnormally high noise levels observed on the STS-2 signals are related to the installation of the IMS satellite transmitter and computer equipment in the YBH vault. We also need to track down the source of the 0.7-8 Hz frequency noise peaks on the STS-1 (see Figure 3.5) which we suspect are also related to the installation of the IMS equipment.
In the coming year, we need to trouble shoot and cure the high-frequency noise observed on the STS-2 channels and on the STS-1 channels. The LD2 channel was configured to record the output from the Druck microbarograph in early January. We have experimented with installing opto-isolators in all digital signal lines at BSL in order to minimize the number of potential ground loop paths and the results are very encouraging. We will install opto-isolators in all digital signal lines at YBH, the next time the station is visited, to break the ground loop paths which are most likely contributing to the high noise level observed on the STS-2.
Since replacement hinges are not available from Streckeisen - and since as many as 20 BDSN sensors could develop this problem, BSL staff began efforts to manufacture replacement hinges. During a visit to BSL, Erhard Wielandt recommended replacing all 4 hinges simultaneously, using material similar to the original. BSL staff has spent time attempting to develop a reproducible recipe for the hinges, including laser cutting the edges for smoothness.
The first set of replacement hinges was tested and found to be too thin. In order to center the mass of the horizontal seismometers, the feet are adjusted such that the glass bell jar no longer clears the instrument itself. A second attempt at fabricating the hinges is ongoing.
In the fall of 2000, we began a search for a site to extend BDSN north of the California/Oregon border, as part of a collaboration with the USGS National Seismic Network and the Global Seismic Network of IRIS, to be located north of the midpoint between the existing sites at MOD and YBH.
During the fall of 2002, VSAT connection to the National Seismic Network VSAT was established. Because the equipment is underground and the site is located within a mature forest, it was necessary to locate the VSAT dish approximately 300 meters away from the data logger in a location with a view of the southern sky (where the NSN satellite is located). To achieve digital telemetry over this distance, power lines and fiber optic cable were trenched and buried. The fiber optic link connects the data logger with the VSAT hardware. Additionally, the station is accessible via a dial-up phone line.
HUMO is a collaborative effort; the USGS/NSN provided the STS-2 seismometer, the BSL supplied the Episensors and the Quanterra data logger, and IRIS provided additional installation funding. The US Bureau of Land Management assisted in locating the historical mine adit, and provides the site permit.
During 2002-2003, BSL acquired a permit from the State of California, Department of Forestry and Fire Protection to install a broadband observatory at Pacheco Peak in south Santa Clara County. The mountain top site has two existing concrete radio vaults and 30 meter antenna towers. Police, fire, and state, and county communication equipment are housed there. Santa Clara County has furnished BSL a microwave channel from the site to transmit data back to their main facility in San Jose where the BSL station SCCB was previously installed in 2000. There data from both stations (PACP and SCCB) are aggregated with two BARD stations (LUTZ and SODA) onto a single digital data circuit.
Data from the Pacheco Peak station was first recorded in late June of 2003. At this time, additional efforts to minimize site, and thermally induced noise is being undertaken, such as the insulation over the seismometer.
The mutual cooperation of the State of California, Department of Forestry and Fire Protection (CDF), Santa Clara County Communications, together with BSL made this site possible.
The McLaughlin Mine site is on property owned and formerly operated by Homestake Mining Company as a surface gold mine. The geology of the area is extremely varied and complex. With the conclusion of mining operations, the property will be managed as a UC-Davis reserve for research. The seismographic vault is the first new research project on the reserve. The site is located approximately 20 kilometers east of the town of Lower Lake, California in an area of Franciscan sandstone.
A steel and concrete vault from a shipping container similar to those found at stations JCC, PKD, and HOPS was constructed. Power and telephone lines were trenched approximately 300 meters to the site. Because of the remoteness of the site, digital telephone data circuits are not available. To address our desire for continuous telemetry, BSL engineers have proposed and applied for permits to install a wireless radio bridge to a site 50 km away where digital phone service is offered. If and when permits are acquired, data would reach the Berkeley hub via a combination of land telco lines and spread spectrum radios. Continuous telemetry should be achieved in 2003-2004. In the meantime, data are being retrieved by dial-up access.
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 BSL, its goal has been to install and operate a permanent seafloor broadband station as a first step towards extending the on-shore broadband seismic network in northern California, to the seaside 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). In this context, evaluating the possibility of a posteriori noise deconvolution using auxiliary data (e.g. current meter, differential pressure gauge) as well as comparison with land based recordings.
This project follows the 1997 MOISE experiment, in which a three component broadband system was deployed for a period of 3 months, 40 km off shore in Monterey Bay, with the help of MBARI's Point Lobos ship and ROV Ventana (Figure 3.6). MOISE was a cooperative program sponsored by MBARI, UC Berkeley and the INSU, Paris, France (Stakes et al., 1998; Romanowicz et al., 1998; Stutzmann et al., 2001). During the MOISE experiment, valuable experience was gained on the technological aspects of such deployments, which contributed to the success of the present MOBB installation.
The successful MOBB deployment took place April 9-11, 2002 and the station is currently recording data autonomously (e.g. Romanowicz et al., 2003). In the future, it may be linked to the planned (and recently funded) MARS (Monterey Accelerated Research System; http://www.mbari.org/mars/) cable, or to the MBARI MOOS buoy, and provide real-time, continuous seismic data to be merged with the rest of the northern California real-time seismic system, although there are plans to eventually replace it by a quieter bore-hole installation.
The ocean-bottom MOBB station currently comprises a three-component seismometer package, a current-meter, and a recording and battery package. A differential pressure gauge (DPG) with autonomous recording (e.g. Cox et al., 1984) was deployed in the vicinity of the seismometer package in December 2002.
The seismic package contains a low-power (2.2W), three-component CMG-1T broadband seismometer system, built by Guralp, Inc., with a three-component 24-bit digitizer, a leveling system, and a precision clock. The seismometer package is mounted on a cylindrical titanium pressure vessel 54 cm in height and 41 cm in diameter, custom built by the MBARI team and outfitted for underwater connection.
Because of the extreme sensitivity of the seismometer, air movement within the pressure vessel must be minimized. In order to achieve this, after extensive testing at BSL (Chapter 9), the top of the pressure vessel was thermally isolated with two inches of insulating foam and reflective Mylar. The sides were then insulated with multiple layers of reflective Mylar space blanket, and the vessel was filled with argon gas.
The current-meter is a Falmouth Scientific 2D-ACM acoustic current meter. It is held by a small standalone fixture and measures the magnitude and direction of the currents about 1 meter above the seafloor.
The recording system is a GEOSense LP1 data logger with custom software designed to acquire and log digital data from the Guralp system and digital data from the current meter over RS-232 serial interfaces. The seismic data are sampled at 20 Hz and current-meter data at 1 Hz, and stored on a 3 GB, 2.5 in disk drive. All the electronics, including the seismometer and the current meter, are powered by a single 10kWh lithium battery.
All installations were done using the MBARI ship Point Lobos and the ROV Ventana. Prior to the instrumentation deployment, the MBARI team manufactured and deployed a 1181 kg galvanized steel trawl-resistant bottom mount to house the recording and power systems (Figure 3.7), and installed a 53 cm diameter by 61 cm deep cylindrical PVC caisson to house the seismometer pressure vessel. The bottom mount for the recording system was placed about 11m away from the caisson to allow the future exchange of the recording and battery package without disturbing the seismometer. Prior to deployment, the seismometer package was tested extensively at BSL, then brought to MBARI where its internal clock drift was calibrated in the cold room against GPS time. The details of the deployment which took place on 04/09/02-04/11/02 were described in the 2001-2002 BSL Annual Report.
Since the installation in April 2002, 5 data recovery dives have taken place (Jun 22, 2002; Sep 20, 2002; Jan 7, 2003; Mar 24, 2003; and Jun 9, 2003). Each time, the data recording and battery packages are exchanged for new ones, and the data transferred to BSL for analysis. While the seismometer package functioned well since installation, we have experienced several serious problems with malfunction of the data loggers, so that since that time, new seismic data are available only over short intervals. Both hardware and software problems appear to be involved, but BSL and MBARI staff are optimistic that the problems have been identified and corrected in the June 2003 deployment. We are hoping that the recovery dive scheduled for Sept 16th will result in augmenting our existing 2002 collection with 3 months of valuable uncorrupted MOBB seismic, DPG and current meter data.
Available MOBB data are being systematically analyzed assess the data quality and possible improvements, through post-processing and/or installation adjustments. We plan to evaluate the long term time evolution of background noise, as the system continues to settle and stabilize, and the shorter term noise fluctuations in relation to tides and currents as recorder by the current-meter as well as the DPG. Since the auxiliary data are sampled at sufficiently high rates (1 sps) compared to what was available for the MOISE experiment, we are investigating ways to reduce the background noise correlated with the pressure and current data at periods longer than 10 sec (see the Research contribution in Chapter III)
Under Barbara Romanowicz's general supervision, Lind Gee and Doug Neuhauser oversee the BDSN data acquisition operations and Bill Karavas is head of the engineering team. John Friday, Dave Rapkin, Cathy Thomas, and Bob Uhrhammer contribute to the operation of the BDSN. Bill Karavas, Bob Uhrhammer, and Lind Gee contributed to the preparation of this chapter.
Support for the installation of HUMO was provided by the USGS/NSN and IRIS. The California Governor's Office of Emergency Services provided funding toward the development of sites MNRC and PACP as part of the CISN.
MOBB is a collaboration between the BSL and MBARI, involving Barbara Romanowicz, Bob Uhrhammer, and Doug Neuhauser from the BSL and Debra Stakes and Paul McGill from MBARI. The MBARI team also includes Steve Etchemendy (Director of Marine Operations), Jon Erickson, John Ferreira, Tony Ramirez and Craig Dawe. The MOBB effort at the BSL is supported by funds from NSF/OCE and UC Berkeley.
Cox, C., T. Deaton and S. Webb, A deep-sea differential pressure gauge, J. Atm. Ocean. Tech., 1, 237-245, 1984.
Murdock, J., and C. Hutt, A new event detector designed for the Seismic Research Observatories, USGS Open-File-Report 83-0785, 39 pp., 1983.
Romanowicz, B., D. Stakes, J. P. Montagner, P. Tarits, R. Uhrhammer. M. Begnaud, E. Stutzmann, M. Pasyanos, J.F. Karczewski, S. Etchemendy, MOISE: A pilot experiment towards long term sea-floor geophysical observatories, Earth Planets Space, 50, 927-937, 1999.
Romanowicz, B., D. Stakes, R. Uhrhammer, P. McGill, D. Neuhauser, T. Ramirez and D. Dolenc, The MOBB experiment: a prototype permanent off-shore ocean bottom broadband station, EOS Trans A.G.U., Aug 28 issue, 2003.
Stakes, D., B. Romanowicz, J.P. Montagner, P. Tarits, J.F. Karczewski, S. Etchemendy, D. Neuhauser, P. McGill, J-C. Koenig, J.Savary, M. Begnaud and M. Pasyanos, MOISE: Monterey Bay Ocean Bottom International Seismic Experiment, EOS Trans., A.G.U., 79, 301-309, 1998.
Stutzmann, E., J.P. Montagner et al., MOISE: a prototype multiparameter ocean-bottom station, Bull. Seism. Soc. Am., 81, 885-902, 2001.
Wielandt, E., and J. Steim, A digital very broad band seismograph, Ann. Geophys., 4, 227-232, 1986.
Wielandt, E., and G. Streckeisen, The leaf spring seismometer: design and performance, Bull. Seis. Soc. Am., 72, 2349-2367, 1982.
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