Joint IRIS/UNAVCO/SCIGN GPS Installation
For GPS/Seismic Integration
At the IRIS PASSCAL Facility
Jim Fowler, Marcos Alvarez, Bruce Beaudon (IRIS/PASSCAL)
Mike Jackson, Karl Feaux, Victoria Andretta, Chuck Meertens (UNAVCO)
Shane Ingate (IRIS)
November 13, 2001
Goal of the Installation
The purpose of this installation is to provide a testing ground for the integration of GPS and seismic data for future large-scale crustal deformation initiatives. The site will be used to test a variety of GPS monument types, receivers, communications methods, seismic sensors, and data integration software. The installation is broken down into three phases. Phase 1 involves the installation of a deep anchored GPS monument (PAS1), Ashtech mZ receiver, with GPS data being sent to a download computer via a Freewave radio link. Phase 1 was completed on November 13, 2001. Phase 2 involve installing a Nanometrics VSAT terminal at the site with separate GPS and seismic data streamed to a VSAT hub. We anticipate this phase will be completed by March of 2002. The third phase involves the integration of different GPS and seismic sensors, different communications methods, and different monumentation strategies. This work will be ongoing throughout 2002. This document details the first phase of the installation.
Introduction
The PAS1 site is located behind the IRIS/PASSCAL facility outside of Socorro New Mexico in an area set aside as a test bed for PASSCAL seismic installations. There is no exposed bedrock, only massive, course alluvial fan deposits with clasts of rhyolite, volcanic tuff, and breccias cemented with CaC03. Due to the lack of bedrock, we install modified NGS CORS pillar monument (Figure 1). The poured concrete monument consists of a 2-ft diameter below ground base to a depth of 12-feet and a 12-inch diameter above ground pillar to a height of 5 feet. A 2-inch diameter stainless steel antenna mast is placed such that its threads are 8 inches above the top of the concrete monument. The top of the concrete monument is beveled to shed water. A SCIGN antenna adapter holds the antenna to the mast and a tall radome covers the antenna. A deeply anchored concrete monument of this type should provide sufficient mechanical stability for this study as well as for precise geodetic surveying. We should be able to test the stability of the monument by monitoring the displacement of PAS1 relative to SC01 (10 km away on M-Mountain) and PIE1 (120 km away at Pie town NM).
Figure 1. Modified NGS CORS pillar monument installed at the Socorro PASSCAL Facility.
Other modifications to the monument include using standard construction grade rebar (Figure 2) for below and above ground structural cages, and the addition of ground running from the steel antenna mast to a copper ground rod (Figure 3).
Figure 2. Scheme for supporting the stainless steel antenna mast from the rebar cage. Six-inch PVC and wire ties were used to separate the rebar cage. All-thread was used to provide an attachment point for the ground rod.
Figure 3. Engineering drawing of the antenna mast and rebar cage.
Site Design
The site consists of an Ashtech mZ GPS receiver broadcasting data via a Free Wave radio modem (Figure 4). The site is powered entirely off of DC solar. The GPS equipment and batteries sit inside enclosures mounted on 2-inch metal conduit. The solar panels sit on top of the enclosures. The data are transmitted to a Free Wave master radio modem connected to a computer running Linux 7.1 computer located inside the PASSCAL Facility.
Figure 4. PAS1 site installation diagram.
Monument Construction
The poured concrete monument consists of a 2-ft diameter below ground base to a depth of 12-feet and a 12-inch diameter above ground pillar to a height of 5 feet. A 2-inch diameter stainless steel antenna mast is placed such that its threads are 8 inches above the top of the concrete monument (Figures 5 &6). A SCIGN antenna adapter holds the antenna to the mast and a tall radome covers the antenna (Figure 7). The completed monument is shown in Figure 8 surrounded by IRIS and UNAVCO personnel who participated in the installation.
Figure 5. Pouring the concrete into the 12-foot below ground hole. The above ground form has been removed to facilitate pouring the concrete.
Figure 6. Finishing the monument base and beveling the top.
Figure 7. GPS monument (foreground) with solar enclosure (right, background) and receiver/modem enclosure (left, background).
Figure 8. Completed GPS monument with M-Mountain in the background. From left to right Karl Feaux (UNAVCO), Victoria Andreatta (UNAVCO), Bruce Beaudoin (IRIS/PASSCAL), Marcos Alvarez (IRIS/PASSCAL), Shane Ingate (IRIS), Noel Barstow (IRIS/PASSCAL), Steve Azevedo (IRIS/PASSCAL).