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PBO Strainmeter Installation Update: A Whole New Frontier!

Overview

After a year of planning and organizing, today marks the onset of drilling for the first of 175 strainmeters. PBO is installing a network of state-of-the-art strainmeter instruments. The strainmeters use a mechanical extensometers and very sensitive displacement transducers to measure extension in three directions across a borehole. Sensitive enough to easily detect solid earth tides, observed changes in strain will enable scientists to monitor tectonic deformation between the Pacific and North America plates. To move the strainmeters away from surface sources of noise, each strainmeter is installed in a 600’ deep borehole. Borehole drilling is done in two stages. Stage 1 involves mobilizing a drill rig to the site to drill a hole about 500’ deep. The most desirable installation requires a section of unfractured rock. However, even at this depth, competent rock is not assured. Stage 2 is aimed at characterizing the borehole more fully using a wireline coring technique.

Wireline core drilling will be done at each borehole by the DOSECC (Drilling, Observation and Sampling of the Earth’s Continental Crust) who will mobilize a second rig to the site to deepen the hole another 30-100 feet deep, extracting 8-10 foot section of rock for identification of a suitable target area for installation. If the rock is found to be competent, the drilling part of the installation is finished, and the hole will be capped until the strainmeter instrument is ready to be installed. As of now, the delivery of the first instrument is expected in February 2005 and will be installed as soon as weather permits.

Within the next year, PBO plans to install eight borehole strainmeters along the Olympic Peninsula, four installations on Vancouver Island in Canada, and four installations along the San Andreas Fault zone in Parkfield, CA. An additional 159 instruments will be installed by the end of PBO construction in October of 2008.

November 1, 2004 - Day One

The PBO Strainmeter engineers, Bob Mueller, Mike Hasting, and Wade Johnson, woke early to prepare for the arrival of the drill rig to begin drilling the hole for the first PBO strainmeter.

The sites for the first three strainmeter installations are located on private property in Sequim, WA. Unlike the common rainfall of 100+” per year for the surrounding towns, Sequim is located just north of the Olympic Mountains, and therefore sheltered from major rain fall (on average, Sequim gets only 15” of rainfall per year).

This first installation is one of the more difficult of the Olympic Peninsula strainmeter sites to access, so a bit of prep work was required before the drilling could begin. To avoid the risk of the heavy drill rig and equipment sinking or getting stuck in the moist soft earth surrounding the area, 150 tons of rock was brought in from a near by quarry (a total of 10 truck loads!). Mike Hasting and Wade Johnson spread the angular gravel over the area to create a driveway and work area, enabling the rig direct access to the target drilling spot. Additionally, an easement needed to be cleared to create the most level and direct driving route for the rig to drive from the paved public road onto the private property. The team cleared the easement of fallen trees and re-growth using a tractor borrowed from the landowner. In total, preparation for the site took about three full days.

Waking up to drizzling rain and foggy skies on November 1st, the PBO crew made their way to the installation site to meet the drillers. Driving up from Tacoma, WA, the big rig arrived around 11 AM. By noon, a 10” tri-cone drill bit was in place, burrowing its way into the ground of the first site (referred to as Clarke West). At the same time that the drill bit is working its way into the ground, a 10” hollow steel casing is inserted into the hole. This casing will minimize any damage that could be caused to the hole, such as fallen rock or debris that could potentially block or clog the hole. During the next hour, the drillers reached a depth of 20 feet, using a very powerful generator to blow the cuttings (at 350 psi) out of the hole. After the first 20 feet, crews switched bits, attaching an 8” tri-cone. This size bit will be used for the rest of stage one drilling.

Wade Johnston bravely took on the job of collecting cutting samples every 20 feet. Trying to avoid getting covered in what looks like mud to the non-geophysicist, Johnson holds a kitchen strainer in the path of the spewing cuttings to gather a sample of the precious cuttings in order to learn the composition of the matter at different depths.

Sunny one minute and raining the next, fickle weather kept the crews on their toes as the rig drilled on. Sand made up the majority of the cuttings until about 93’, when materials composed of clay and shale was extracted. Only 7’ further down, crews discovered what appeared to be wet, fractured basalt. By 4 PM, when the hole was drilled to 116’ below the surface, dryer ground cuttings flew through the air, indicating that dry basalt was reached. Just as the dry material was discovered, the 8” bit became clogged from the mixture of all the different material consistencies. Without water to flush the coated bit, it would’ve taken over an hour to clean off the coated tri-cone. At this point, with it already starting to become dark, tired & cold crew members headed back to their hotel to discuss the day’s progress and plan for day number two. After today there is no turning back for PBO’s strainmeter program.


Last modified: Sunday, 11-May-2014 18:02:08 UTC

 

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