Subsections


Data Acquisition and Quality Control



Introduction

Stations from the networks operated by the BSL transmit data continuously to the BSL facilities on the UC Berkeley campus for analysis and archival. In this section, we describe activities and facilities which pertain to the individual networks described in Operational Sections 1, 3, and 4, including procedures for data acquisition and quality control, and sensor testing capabilities and procedures. Some of these activities are continuous from year to year and have been described in prior BSL annual reports. In this section, we describe changes or activities which are specific to 2009-2010.

Figure 3.28: Data flow from the BDSN, NHFN, mPBO, HRSN, and BARD networks into the BSL central processing facility.
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Figure 3.29: Flow of data from comserv areas through network services processing. One stream of the network services provides picks (and currently still provides codas) determined using the programs shown in the right flow path. Every 5 seconds, ground motion parameters are also determined, including PGA, PGV, PGD, and ML100 (left flow column). Parameters from the network services are available to the CISN software for event detection and characterization. Data are also logged to disk (via datalog), distributed to other computers (mserv), and spooled into a trace ring for export.
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Figure: Annual summary of noise on all components of the broadband sensors of the BDSN for the band from 32 s to 128 s.
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Data Acquisition Facilities

The computers and the associated telemetry equipment are now located in the campus computer facility in Warren Hall at 2195 Hearst Avenue. This building was constructed to current ``emergency grade'' seismic codes and is expected to be operational even after a $M$ 7 earthquake on the nearby Hayward Fault. The hardened campus computer facility within was designed with special attention for post-earthquake operations. The computer center contains state-of-the art seismic bracing, UPS power and air conditioning with generator backup, and extensive security and equipment monitoring.

Data Acquisition

Central-site data acquisition for data from the BDSN/HRSN/NHFN/mPBO networks is performed by two computer systems in the Warren Hall data center (Figure 3.28). These acquisition systems also collect data from the Parkfield-Hollister electromagnetic array and the BARD network. A third system is used primarily for data exchange with the USNSN (U.S. National Seismograph Network) and transmits data to the USNSN from HOPS, CMB, SAO, WDC, HUMO, MOD, MCCM, and YBH. Data for all channels of the HRSN are now telemetered continuously from Parkfield to the BSL over the USGS T1 from Parkfield to Menlo Park, and over the NCEMC T1 from Menlo Park to Warren Hall.

The BSL uses the programs comserv and qmaserv developed by Quanterra for central data acquisition. These programs receive data from remote Quanterra data loggers and redistribute it to one or more client programs. The clients include datalog, which writes the data to disk files for archival purposes, wdafill, which writes the data to the shared memory region for processing with the network services routines, and other programs such as the seismic alarm process, the DAC480 system, and the feed for the Memento Mori Web page.

The two computers performing data acquisition are also ``network services'' computers that reduce waveforms for processing with the CISN software (Figure 3.29). To facilitate processing, each system maintains a shared memory region containing the most recent 30 minutes of data for each channel.

BDSN data loggers which use frame relay telemetry are configured to enable data transmission simultaneously to two different computers over two different frame relay T1 circuits to UCB. Normally, only one of these circuits is enabled. The comserv/qmaserv client program cs2m receives data and multicasts it over a private ethernet. The program mcast, a modified version of Quanterra's comserv program, receives the multicast data from cs2m, and provides a comserv-like interface to local comserv clients. Thus, each network services computer has a comserv/qmaserv server for every station, and each of the two systems has a complete copy of all waveform data.

We have extended the multicasting approach to handle data received from other networks such as the NCSN and UNR (University of Nevada, Reno). These data are received by Earthworm data exchange programs and are then converted to MiniSEED and multicast in the same manner as the BSL data. We use mserv on both network services computers to receive the multicast data and handle it in the same way as the BSL MiniSEED data.

In 2006, the BSL established a real-time data feed of all BSL waveforms between the BSL acquisition systems and the NCEDC computers using the open source Freeorb software. This allows the NCEDC to provide near-real-time access to all BSL waveform data through the NCEDC DART (Data Availabile in Real Time) system.

We monitor seismic stations and telemetry using the program seisnetwatch. This program extracts current information such as time quality, mass positions, and battery voltage and allows it to be displayed. If the parameter departs from the nominal range, the station is marked with yellow or red to indicate a possible problem.

Seismic Noise Analysis

BSL seismic data are routinely monitored for state-of-health. An automated analysis is computed regularly to characterize the seismic noise level recorded by each broadband seismometer.

PSD Noise Analysis

The estimation of the Power Spectral Density (PSD) of the ground motion recorded at a seismic station, as documented in the 2000-2001 BSL annual report (http://seismo.berkeley.edu/annual_report/), provides an objective measure of background seismic noise characteristics over a wide range of frequencies. It also provides an objective measure of seasonal and secular variation in noise characteristics and supports early diagnoses of instrumental problems. In the early 1990s, a PSD estimation algorithm was developed at the BSL for characterizing the background seismic noise and as a tool for quality control. The algorithm generates a bar graph output in which all the BDSN broadband stations can be compared by component. We also use the weekly PSD results to monitor trends in the noise level at each station. Cumulative PSD plots are generated for each station and show the noise level in 5 frequency bands for the broadband channels. The plots make it easier to spot certain problems, such as failure of a sensor. In addition to the station-based plots, a summary plot is produced for each channel. The figures are presented as part of a noise analysis of the BDSN on the web at http://www.seismo.berkeley.edu/seismo/bdsn/psd/.

PDF PSD Noise Analysis

In addition to the PSD analysis developed by Bob Uhrhammer, the BSL has implemented the Ambient Noise Probability Density Function (PDF) analysis system developed by McNamara and Buland (2004). This system performs its noise analysis over all the data of a given time period (week or year), including earthquakes, calibration pulses, and cultural noise. This is in contrast to Bob Uhrhammer's PSD analysis, which looks at only the quietest portion of data within a day or week. Pete Lombard of the BSL extended the McNamara code to cover a larger frequency range and support the many different types of sensors employed by the BSL. Besides the originally supported broadband sensors, our PDF analysis now includes surface and borehole accelerometers, strain meters, and electric and magnetic field sensors. These enhancements to the PDF code, plus a number of bug fixes, were provided back to the McNamara team for incorporation in their work. The results of the PDF analysis are presented on the web at http://www.ncedc.org/ncedc/PDF/. One difficulty with using these plots for review of station quality is that it is necessary to look at data from each component separately. To provide an overview, we have developed summary figures for all components in two spectral bands, 32 - 128 s and 0.125 - 0.25 s (Figure 3.30). The figures are also available on the web at http://seismo.berkeley.edu/~taira/NCEDC/NCEDC_PSD.html.

Sensor Testing Facility

The BSL has an Instrumentation Test Facility in the Byerly Seismographic Vault where the characteristics of up to eight sensors can be systematically determined and compared. The test equipment consists of an eight-channel Quanterra Q4120 high-resolution data logger and a custom interconnect panel. The panel provides isolated power and preamplification, when required, to facilitate the connection and routing of signals from the sensors to the data logger with shielded signal lines. The vault also has a GPS rebroadcaster, so that all data loggers in the Byerly vault operate on the same time base. Upon acquisition of data at up to 200 sps from the instruments under test, PSD analysis, coherence analysis, and other analysis algorithms are used to characterize and compare the sensor performance. Tilt tests and seismic signals with a sufficient signal level above the background seismic noise are also used to verify the absolute calibration of the sensors. A simple vertical shake table is used to assess the linearity of a seismic sensor. The sensor testing facility of the BSL is described in detail in the 2001-2002 Annual Report (http://www.seismo.berkeley.edu/).

Several projects made use of the sensor testing facility in 2009-2010. This included testing of the STS-1 type sensors being developed jointly by Metrozet and the BSL.

Enhanced Pressure Vessels for STS-1 Seismometers

As part of the NSF (National Science Foundation) funded STS-1 development grant, BSL has been working on the design, fabrication and testing of an enhanced pressure vessel for the original STS-1 seismometers. Originally, these seismometers were deployed on a glass base plate and covered with a glass dome that could be evacuated. Later the glass base plate was replaced by a warpless base plate developed by the Albequerque Seismological Laboratory (ASL). For either base plate, the dome was sealed against a rubber gasket only by the weight of the glass dome and atmospheric pressure, once air had been evacuated through a rubber stopper and glass stopcock. Vacuum leaks, and the resulting intrusion of atmospheric humidity/moisture are believed to be the principal source of the long term degradation of the sensors.

The enhancements designed by the BSL are based on the pressure vessel design for the new MetroZet very broadband seismometers. The glass dome is replaced by an aluminum cover vessel, complete with a mounting flange and holes for bolting the cover in place. The cover is attached to a newly designed mounting ring, that attaches to the warpless baseplate. O-rings replace the flat gaskets used on both the original glass base plates and the later ASL warpless base plates. In addition, the BSL design eliminates the glass stopcock for the vacuum port. Two threaded holes were added to the mounting ring: one for a ball valve through which the vessel is evacuated, the second so that a vacuum gauge can be attached. If a solid state sensor were used in place of the vacuum gauge, the vacuum could be monitored continuously by sampling its output at a low rate. Figure 3.31 shows the test setup in the Byerly Vault.

One advantage of the newly designed vessel is that all seismometer components at a single site can be plumbed together to achieve a single, consistent level of pressure. Individual pressure differences seen by the seismometers would be eliminated and the vacuum within monitored by a single solid state pressure device.

BSL engineers have installed the enhanced pressure vessel and mounting ring at Byerly vault and are performing evaluation and comparison testing.

Figure 3.31: STS-1 seismometers in the Byerly Vault installed in a new pressure vessel (left) and on a warpless baseplate with a glass bell (right).
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Acknowledgements

Doug Neuhauser, Bob Uhrhammer, Taka Taira, Peggy Hellweg, Pete Lombard, Rick McKenzie, and Jennifer Taggart are involved in the data acquisition and quality control of BDSN/HRSN/NHFN/MBPO data. Development of the sensor test facility and analysis system was a collaborative effort of Bob Uhrhammer, Tom McEvilly, John Friday, and Bill Karavas. IRIS (Incorporated Research Institutions for Seismology) and DTRA (Defense Threat Reduction Agency) provided, in part, funding for and/or incentive to set up and operate the facility, and we thank them for their support. Bob Uhrhammer, Taka Taira, Peggy Hellweg, Pete Lombard, Doug Neuhauser, and Barbara Romanowicz contributed to the preparation of this section.

References

Ekström, G. and M. Nettles, http://www.ldeo.columbia.edu/~ekstrom/Projects/WQC.html, 2005.

Gardner, W. A., A unifying view of coherence in signal processing, Signal Processing, 29, p. 113-140, 1992.

Ingate, S. et al, Workshop Report from Broadband Seismometer Workshop, Lake Tahoe, CA, http://www.iris.edu/stations/seisWorkshop04/report.htm, 2004.

McNamara, D. and R. Buland, Ambient Noise Levels in the Continental United States Bull. Seism. Soc. Am., 94, 4, 2004.

Scherbaum, Frank. Of Poles and Zeros: Fundamentals in Digital Seismology, Volume 15 of Modern Approaches in Geophysics, G. Nolet, Managing Editor, Kluwer Academic Press, Dordrecht, xi + 257 pp., 1996.

Tapley, W. C. and J. E. Tull, SAC - Seismic Analysis Code: Users Manual, Lawrence Livermore National Laboratory, Revision 4, 388 pp., March 20, 1992.

Wielandt, E. and G. Streckeisen, The leaf spring seismometer: design and performance, Bull. Seis. Soc. Am., 72, 2349-2367, 1982.

Wielandt, E. and Steim, J. M., A digital very broad band seismograph, Annales Geophysicae, 4 B(3), 227-232, 1986.

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