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Borehole instruments: The hidden component of geophysical monitoring in Yellowstone

August 15, 2021

When it comes to data, Yellowstone is a geophysicist’s dream. There is continuous activity from earthquakes, geysers, and of course, the volcano itself. A keen eye may be able to spot one of the park’s numerous GPS or seismometer stations hard at work, but some of the park’s data collectors are buried deep within the Earth, hidden from sight in boreholes.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Courtenay Duzet, graduate student at the University of Montana and intern at UNAVCO.

When it comes to monitoring data, Yellowstone is one of the best-covered volcanic systems in the world! Numerous instruments are distributed throughout the park to collect data for assessing the state of the volcanic system and researching related to earthquakes, geyser eruptions, and slow but steady rates of ground deformation. Not all geological activities are the same, however, and neither are the instruments! Volcano monitoring is like a toolbox—we have lots of different tools for different jobs. 

The Yellowstone borehole geophysical network
The Yellowstone borehole geophysical network, installed by UNAVCO in 2007–2008. The placement of the boreholes is focused primarily around the caldera, and the boreholes contain a mix of instruments, including strainmeters, seismometers, tiltmeters, and pore pressure sensors.

While most monitoring equipment, including seismic stations, GPS stations, and temperature probes, are located at the surface, boreholes can be used to install scientific instruments deep underground and record signals not perceptible on the Earth’s surface. In 2007, UNAVCO began the process of implementing a small network of six boreholes across the park to better obtain geophysical data from earthquakes and sources of deformation as part of the Network of the Americas (NOTA). Within NOTA boreholes, instruments are installed up to 250 meters below ground. NOTA boreholes house up to four instruments: strainmeters, seismometers, tiltmeters, and pore pressure sensors, all of which are able to collect distinct sets of measurements that help us better understand the small changes that happen at Yellowstone

In contrast to the NOTA GPS stations, which are installed above ground, the installation of borehole instruments requires heavy equipment to drill deep into the Earth. Sometimes, this can prove more difficult than it sounds.

The first step in drilling a scientific borehole is to find a suitable location with access and space to set up a drill rig and good sky view for solar panels and data transmission. The NOTA tensor strainmeter instrument has some strict installation requirements, so drilling progress must be constantly monitored for bedrock type, quality, fractures, as well as borehole stability, verticality, and water production. Once completed, the borehole must be logged with specialized equipment to determine if there is a suitable strainmeter installation location within the target zone of 100-250 meters below the surface. Groundwater flow or fractures can prevent good coupling between the instrument and surrounding rock, which in turn will reduce the quality of the subsurface measurements.  

A unique constraint at Yellowstone is the presence of hot geothermal fluids. When drilling the boreholes, UNAVCO engineers encountered fluids that ranged from 67 to 91 degrees Celsius (152 to 195 degrees Fahrenheit), which surpasses the instruments’ safe operating limit of 65 degrees Celsius. These high temperatures, coupled with high concentrations of chloride, sodium, silica, and hydrogen sulfide, create a harsh environment for electrical equipment.

Once a suitable borehole has been drilled and a suitable location in the borehole identified, engineers can begin the process of installing the instruments.

Strainmeters are installed at the bottom of the borehole and permanently secured with a specialized grout. These instruments are capable of measuring changes in the shape of the surrounding bedrock as small as four picometers—that’s ten million times smaller than the width of a human hair! As the ground around the borehole deforms and changes shape, the borehole itself is also subjected to deformation. This change will then squeeze, stretch, or shear the strainmeter inside the borehole. These deformations can be brought about by processes such as pressure changes in magma, changes in water levels of the park’s lakes, and earthquake activity. Such measurements have helped scientists locate and characterize the magma beneath Yellowstone with greater accuracy.

Installing geophysical boreholes in Yellowstone
UNAVCO engineers drilling a borehole for instrument installation (left). Aerial view of borehole casing used to protect instruments from the elements found below the surface (right).

A few meters above the strainmeter, the seismometer is installed to record earthquake activity. Since Yellowstone is one of the most seismically active areas in the US, seismometers are an essential part of data collection in the park. A majority of the park’s other seismometers are buried within two meters or less below the surface, but at these shallow depths, it is difficult to record microseisms, which are faint tremors like those caused by water waves in Yellowstone Lake. That’s where borehole seismometers save the day, as they are buried at a depth that enables them to detect and locate even the smallest seismic waves. While borehole seismometers are not completely immune to noise at the surface—they still sense things like high winds and storms—they are far quieter than surface instruments and more capable of seeing smaller changes.

Also installed in the NOTA boreholes, and closer to the surface, tiltmeters and pore pressure sensors monitor deformation and hydrological processes. Just like the strainmeters, the tiltmeters are incredibly sensitive, measuring changes in “tilt” of less than a millionth of a degree. Tiltmeters are best for monitoring short-term variations and very effectively pick up tides and even variations due to distant earthquakes, but so far they have never recorded anything local to Yellowstone, since deformation there is slow and steady. Meanwhile, pore pressure sensors help geologists by tracking minute changes in groundwater pressure at timescales of seconds to years. This can be especially useful for enhancing our understanding of the numerous geysers scattered throughout the park.

The journey of drilling a borehole and installing geophysical instruments may not be the simplest, but it’s easy to see how rewarding the effort can be. Since its inception in 2007, the data collected by the network of boreholes in Yellowstone has shed light on the many mysterious geological processes that take place far below the surface. NOTA boreholes are a unique tool in the kit that we use to better understand how Yellowstone works!

Interested in seeing the data collected by boreholes? Point your web browser at https://www.unavco.org/borehole-instruments/borehole-data-access/ to start your exploration!

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