1986 - 2001

Installation of the HRSN deep (200-300m) borehole sensors initiated in 1986, and recording of triggered 500 sps earthquake data began in 1987. The HRSN sensors are 3-component geophones in a mutually orthogonal gimbaled package. This ensures that the sensor corresponding to channel DP1 is aligned vertically and that the others are aligned horizontally. Originally a 10 station network, completed in 1988, the HRSN was expanded to 13 borehole stations in 2001, and the original recording systems (see previous BSL Annual Reports) were upgraded to 24 bit acquisition (Quanterra 730s) and 56K frame relay telemetry to UCB. Properties of the sensors are summarized in Table 3.9.

Table 3.8: Stations of the Parkfield HRSN. Each HRSN station is listed with its station code, network id, location, date of initial operation, and site description. The latitude and longitude (in degrees) are given in the WGS84 reference frame. The surface elevation (in meters) is relative to mean sea level, and the depth to the sensor (in meters) below the surface is also given. Coordinates and station names for the 3 new SAFOD sites are given at the bottom.
Site Net Latitude Longitude Surf. (m) Depth (m) Date Location
EADB BP 35.89525 -120.42286 466 245 01/1988 - Eade Ranch
FROB BP 35.91078 -120.48722 509 284 01/1988 - Froelich Ranch
GHIB BP 35.83236 -120.34774 400 63 01/1988 - Gold Hill
JCNB BP 35.93911 -120.43083 527 224 01/1988 - Joaquin Canyon North
JCSB BP 35.92120 -120.43408 455 155 01/1988 - Joaquin Canyon South
MMNB BP 35.95654 -120.49586 698 221 01/1988 - Middle Mountain
RMNB BP 36.00086 -120.47772 1165 73 01/1988 - Gastro Peak
SMNB BP 35.97292 -120.58009 699 282 01/1988 - Stockdale Mountain
VARB BP 35.92614 -120.44707 478 572 01/1988 - 08/19/2003 Varian Well
VARB BP 35.92614 -120.44707 478 298 08/25/2003 - Varian Well
VCAB BP 35.92177 -120.53424 758 200 01/1988 - Vineyard Canyon
CCRB BP 35.95718 -120.55158 595 251 05/2001 - Cholame Creek
LCCB BP 35.98005 -120.51424 640 252 08/2001 - Little Cholame Creek
SCYB BP 36.00938 -120.53660 945 252 08/2001 - Stone Canyon

Table 3.9: Instrumentation of the Parkfield HRSN. Most HRSN sites have L22 sensors and were originally digitized with a RefTek 24 system. After the failure of the WESCOMP recording system, PASSCAL RefTek recorders were installed. In July of 1999, 6 of the PASSCAL systems were returned to IRIS and 4 were left at critical sites. The upgraded network uses a Quanterra 730 4-channel system. For the three new stations (bottom) horizontal orientations are approximate (N45W and N45E) and will be determined more accurately as available field time permits.
Site Sensor Z H1 H2 RefTek 24 RefTek 72-06 Quanterra 730          
EADB Mark Products L22 -90 170 260 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
FROB Mark Products L22 -90 338 248 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
GHIB Mark Products L22 90 failed unk 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
JCNB Mark Products L22 -90 0 270 01/1988 - 12/1998 12/1998 - 06/2001 03/2001 -          
JCSB Geospace HS1 90 300 210 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
MMNB Mark Products L22 -90 175 265 01/1988 - 12/1998 12/1998 - 06/2001 03/2001 -          
RMNB Mark Products L22 -90 310 40 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
SMNB Mark Products L22 -90 120 210 01/1988 - 12/1998 12/1998 - 06/2001 03/2001 -          
VARB Litton 1023 90 15 285 01/1988 - 12/1998 12/1998 - 07/1999 03/2001 -          
VCAB Mark Products L22 -90 200 290 01/1988 - 12/1998 12/1998 - 06/2001 03/2001 -          
CCRB Mark Products L22 -90 N45W N45E - - 05/2001 -          
LCCB Mark Products L22 -90 N45W N45E - - 08/2001 -          
SCYB Mark Products L22 -90 N45W N45E - - 08/2001 -          

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).

Table 3.10: Data streams currently being acquired at each HRSN site. Sensor type, channel name, sampling rate, sampling mode, and type of FIR filter are given. C indicates continuous; T triggered; Ac acausal; Ca causal. ``?" indicates orthogonal, vertical, and 2 horizontal components.
Sensor Channel Rate (sps) Mode FIR
Geophone DP? 250.0 T and C Ca
Geophone BP? 20.0 C Ac

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 $M_{w}$ 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.

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