The general configuration of borehole instrument installation at each Mini-PBO station is shown in Figure 8.3. A 6.625" steel casing was cemented into a 10.75" hole to 500-650' depth to prevent the upper, most unconsolidated materials from collapsing into the hole. Below this depth a 6" uncased hole was drilled to the target region for the strainmeter and seismometer packages. Coring, in order to identify the region with the most competent rock for the strainmeter, was attempted with only moderate success at a few of the holes and was not attempted at St. Vincents. We found that video logs provided a reasonable substitute. The target region of each hole was filled with a non-shrink grout into which the strainmeter was lowered, allowing the grout to completely fill the inner cavity of the strainmeter within the annulus formed by the sensing volumes to ensure good coupling to the surrounding rock.
The 3-component seismometer package was then lowered to just above the strainmeter, on a 2" PVC pipe, and neat cement was used to fill the hole and PVC pipe to entirely enclose the package. The pipe above this depth was left open for later installation of the pore pressure sensor. To allow water to circulate into the pipe from the surrounding rock for the pore pressure measurements, the steel casing was perforated, a sand/gravel pack was emplaced, and a PVC screen was used at this depth. At each hole, the casing was then cemented inside to about 200', and outside to about 20' depth. A 12" PVC conductor casing was cemented on the outside from the surface to 20' to stabilize the hole for drilling and to provide an environmental health seal for shallow groundwater flow. The annulus between the 12" conductor casing and the 6.625" steel casing was cemented to about 10' depth and above was left decoupled from the upper surface to help minimize monument instability for the GPS antenna mounted on top of the steel casing.
The BSL developed a GPS mount for the top of the borehole casings to create a stable, compact monument. This design will be used at the more than 140 PBO strainmeter stations to be installed over the next 5 years. The antennas, using standard SCIGN adapters and domes for protection, are attached to the top of the 6-inch metal casing, which will be mechanically isolated from the upper few meters of the ground. The casing below this level is cemented fully to the surrounding rock. The GPS mount design consists of two 11-inch diameter stainless steel flanges. The lower slip- and- weld type flange is welded onto the top of the borehole casing providing a level surface for the second flange. The upper blind-type flange, to which the 1 1/ 4" stainless steel pipe used to connect to the SCIGN DC3 adaptor is attached, is bolted to the lower flange and includes two offset stainless steel dowels to insure precise self-centering alignment. During the period July 2003-June 2004, the BSL completed the installation of GPS systems using the borehole mount at St. Vincents School for Boys (SVIN) near San Rafael and at the Ox Mt site (OXMT) near Half Moon Bay.
Two-component tiltmeters were installed at all the stations by the USGS in 2003. Data from these sensors are recorded at 10-minute intervals and telemetered using the GOES system. Pore pressure sensors are also installed at all the stations and data are recorded at 1 Hz on the Quanterra dataloggers, except at Marin Headlands, where 10-minute interval data are also recorded on the Zeno datalogger.
The 1-Hz GPS, and 100-Hz strainmeter and seismometer data is acquired on Quanterra data loggers and continuously telemetered by frame relay to the BSL. Low frequency (600 second) data (including strainmeters, for redundancy) is telemetered using the GOES system to the USGS. All data is available to the community through the Northern California Earthquake Data Center (NCEDC) in SEED format, using procedures developed by the BSL and USGS to archive similar data from 139 sites of the USGS ultra-low-frequency (UL) geophysical network, including data from strainmeters, tiltmeters, creep meters, magnetometers, and water well levels.
The BSL is supervising GPS, power, frame relay telemetry, and Quanterra 4120 datalogger installation and maintenance at all the broadband deformation stations. Power, telemetry, and dataloggers are currently installed at all stations except MHDL, where we are waiting for PG&E to install the power drop. Telemetry at SVIN was established in June 2003 using Wi-LAN radios, a new type of radio that the BSL is currently beginning to adopt. These radios act as Ethernet bridges, providing superior access to console control on the Quanterras. The radios can also provide a spanning tree network structure for a regional wireless network, which allows greater flexibility for future network installations. We will use Wi-LAN radios at MHDL as well to provide a link between the borehole site and the frame-relay network interface circuit, which is located about 0.5 miles from the site.
Berkeley Seismological Laboratory
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