The 3 2001 borehole stations were added, with NSF support, at the NW end of the network as part of the SAFOD project to improve resolution of the structure, kinematics and monitoring capabilities in the SAFOD drill-path and target zones. Figure 3.12 illustrates the location of the drill site, the new borehole sites, and locations of earthquakes recorded by the initial and upgraded/expanded HRSN.
The three newest SAFOD stations have a similar configuration to the original upgraded 10 station network and include an additional channel for electrical signals. Station descriptions and instrument properties are summarized in Tables 3.8 and 3.9. All HRSN Q730 dataloggers employ FIR filters to extract data at 250 and 20 Hz (Table 3.10).
The remoteness of the drill site and new stations required an installation of an intermediate data collection point at Gastro Peak, with a microwave link to the CDF facility. The HRSN stations use SLIP to transmit TCP and UDP data packets over bidirectional spread-spectrum radio links between the on-site data acquisition systems and the central recording system at the CDF. Six of the sites transmit directly to a router at the central recording site. The other seven sites transmit to a router at Gastro Peak, where the data are aggregated and transmitted to the central site over a 4 MBit/second digital 5.4 GHz microwave link. All HRSN data are recorded to disk at the CDF site.
The upgraded and expanded system is compatible with the data flow and archiving common to all the elements of the BDSN/NHFN and the NCEDC (Northern California Earthquake Data Center), and is providing remote access and control of the system. It has also provided event triggers with better timing accuracy and is also now recording continuous 20 and 250 sps data for all 38 channels of the HRSN, which flow seamlessly into both the USGS automated earthquake detection system and into the Berkeley's NCEDC for archiving and online access to the community. The new system also helps minimize the problems of timing resolution, dynamic range, and missed detections, in addition to providing the added advantage of conventional data flow (the old system recorded SEGY format).
Another feature of the new system that has been particularly useful both for routine maintenance and for pathology identification has been the Internet connectivity of the central site processing computer and the station dataloggers with the computer network at BSL. Through this connection, select data channels and on-site warning messages from the central site processor are sent directly to BSL for evaluation by project personnel. If, upon these evaluations, more detailed information on the HRSN's performance is required, additional information can also be remotely accessed from the central site processing computer at Parkfield. Analysis of this remotely acquired information has been extremely useful for trouble shooting by allowing field personnel to schedule and plan the details of maintenance visits to Parkfield. The connectivity also allows certain data acquisition parameters to be modified remotely when needed, and commands can be sent to the central site computer and dataloggers to modify or restart processes when necessary.
The network connectivity and seamless data flow to the NCEDC also provides near-real-time monitoring capabilities that are useful for rapid evaluation of significant events as well as the network's overall performance level. For example, shown in Figure 3.13 are P-wave seismograms of the 8.1 Solomon Islands earthquake of April 1, 2007 20:39:56 (UTC) (9892 km S54E of Parkfield, CA; depth 10 km) recorded on the DP1 (vertical) channels of the 13 HRSN borehole stations. This event killed 54 people. The seismic data from the quake was telemetered to Berkeley and available for analysis within a few seconds of being recorded by the HRSN. The consistency of the first motions and subsequent arrivals across all the stations of the local HRSN for this global event also show that the entire ensemble of stations for the network was performing well at this time. By routinely performing PSD analysises of the HRSN data, rapid assessment of the HRSN seismometer responses across their wide band-width is also easily done and corrective measures applied in a relatively short time-frame.
Berkeley Seismological Laboratory
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