Hybrid PBO Sites Shore-NW and Floequarry Will Offer a Comprehensive Look at the Plate Boundary of the Pacific Northwest at all Frequencies

Overview

September 30, 2005

The Plate Boundary Observatory has met another significant milestone in the MREFC phase. On September 12, 2005, PBO engineers Mike Hasting and Peter Gray completed the first co-located borehole strainmeter (BSM) and GPS site. The combination of these two instruments at one site will provide comprehensive coverage of deformation from 100ths of seconds to 10s of years by collocating seismometer, borehole strainmeter, pore pressure monitor, and precision GPS sensors at a single site.

A BSM site located on the Olympic Peninsula named “Shore-NW” (B005) was chosen to host the first collocated site because of its rock type, shallow depth to bedrock, and good sky view. The Shore-NW site is also part of a three-station cluster with stations Shore-NE and Shore-S located about 300 and 400 meters apart, respectively. Using a prototype adapter designed by Mike Hasting, a GPS monument was attached to the top of the Shore-NW BSM station’s wellhead. The design of the adapter was critical to allow the strainmeter, seismometer, and pore pressure sensors to be installed and their associated data cables routed out of the wellhead casing to their respective data loggers. The adapter also allows access to a 1-inch PVC tube installed for thermal gradient measurements and copper tubing used to pressurize the pore pressure packer, and provides a stable monument for the GPS antenna. For these BSM/GPS stations, the top several feet of the 6-7/8” casing is isolated from the ground by leaving in the 10” conductor casing during the drilling process and only cementing to a few feet of the surface with neat cement inside the annulus. As such, these sites must be pre-selected prior to commencement of drilling operations for the strainmeter to ensure the wellhead can be completed accordingly. The Shore-NW GPS installation went well, taking only four hours from start to finish to get the equipment installed and working.

On September 13, 2005, another GPS monument was installed at the BSM station “Floequarry” (P403), located about 50 miles west of Port Angels, Washington. Later this spring, a third collocated site will be constructed at a BSM site in Parkfield, California, located just 50 feet from a “Deep Braced” GPS station. For the first time ever, this will allow side-by-side data comparison between the two GPS monuments, revealing the stability of the combined sites. In total, about 20 hybrid sites will be built by the end of the project.

Not only is merging the strainmeter and GPS operations together an important avenue for obtaining significant scientific data, it also provides an opportunity for cross-training between the two different installation crews, ultimately building a stronger network of specialized PBO personnel. It will also provide additional GPS stations at a reduced cost as most of the GPS adapters cost thousands of dollars less to manufacture than the total cost for installing a deep braced monument. Additionally, these hybrid stations combine infrastructure and communications equipment, which also increases the “earned value” of the site.