Sensor Array Discovers an Underground Reservoir at Old Faithful

(Image Credit JillWellington/pixabay)

The Old Faithful geyser has a secret. And University of Utah scientists uncovered it. Beneath the area around the geyser lies a network of fractures and pores that forms part of its hydrothermal system. This contains hot water that helps fuel the eruptions. Using seismic sensors, the team mapped the shallow subsurface around the geyser. They discovered a large hydrothermal reservoir within the fractured rock.  

To characterize the hydrothermal system under Old Faithful, the researchers deployed an array of 133 autonomous three-component 5 Hz nodal geophones. These instruments, spaced 50 metres apart, were spread out across a 1 km-wide area in Yellowstone’s Upper Geyser Basin. Each geophone measured ground motion in three directions. Then, the scientists measured how seismic waves traveled through the subsurface.

Scientists relied on the vibrations generated by Yellowstone’s hydrothermal activity instead of creating seismic signals. These signals were produced by hydrothermal fluid and steam migration, while higher-frequency signals come from boiling water and bubbles collapsing. Researchers recorded those signals for 13 days and used the seismic waves as a source to image the subsurface.

The 133 seismic array around Old Faithful. It also shows hydrothermal seismic signals detected at different stations. (Image: Advanced Earth and Space Sciences)

They used ambient-noise seismic methods, such as cross-correlation of signals recorded at different stations, for analysis. Comparing seismic waves detected by varying sensors made it possible to measure their travel speeds. The researchers evaluated Rayleigh waves at different frequencies, which are sensitive to varied depths. Seismic velocity variations helped differentiate between rock types and locate hydrothermally altered zones.  

Imagery revealed a strong variation in velocities between the northeastern and southwestern portions of the study zone. Higher seismic velocities in the northeast were consistent with solid rhyolitic lava flows. Meanwhile, slower southwest-based velocities corresponded to glacial deposits in the Firehole River valley. At certain frequencies, the velocity difference between the two regions reached 40%. Results showed the geological boundary at the surface continued below the ground.

An underground 200-metre-wide hydrothermal reservoir was found by seismic imagery. It extends from 10 to 60 meters and is centered around 100 metres southwest of the geyser. Based on a 30% estimated porosity, the researchers calculated that it holds 300,000 m³ of water. It has fractures and pores inside the surrounding rock.

Additionally, the data revealed that Old Faithful’s eruption tremor differed across the array. 16 geophones placed within 120 metres of the geyser detected signals at 10-20 Hz. The signals happened on a 93-minute eruption cycle. Stations west of Old Faithful picked up weaker tremors than those to the east, suggesting the western subsurface strongly attenuates seismic waves. The team believes this attenuation is caused by the highly fractured and porous rock in the hydrothermal zone.

The seismic measurements point to complex fractures supplying Old Faithful. Analyzing the natural waves helped the scientists assess the subsurface structure. They also found evidence that the hydrothermal reservoir provides a steady influx of fluid to the shallower plumbing system.

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