Volcano scientists to deploy ultra-sensitive gravity sensor

Do you find this article interesting? It could be the first in a series of features on unique or unusual facilities, instruments, or installations in the Aloha State. If so, next I’ll tell you about a nearly forgotten atom smasher currently in mothballs in Manoa. Know of an obscure bit of tech or gadget here? Please share!

As a geek and sci-fi fan, I’d like to think I have a better than passing understanding of energy, mass, and squares.

On the mind-boggling scale, you have spiral galaxies and black holes and forces that are so massive that they bend space and time, twisting light and swallowing whole stars. Zoom in a bit and you’re in solar system territory, a giant star keeping a mix of planets in its thrall, orbiting in concentric circles almost endlessly.

But on planet Earth, I can’t really process things much better than Sir Isaac Newton and his apple, or even a toddler drop-testing her breakfast from a high chair. The fact that the moon is close enough and big enough to mess with our ocean tides is enough knowledge to make my head spin.

It turns out that the seemingly universal and constant pull of gravity does change, ever so slightly, depending on where you are, and what’s going on beneath your feet. And Hawaii is now home to a state-of-the-art, 200-pound piece of equipment that can measure these tiny shifts in Earth’s gravitational pull.

When magma moves

“Our mission at the Hawaiian Volcano Observatory is to monitor, investigate, and assess hazards from active volcanoes and earthquakes in Hawaii, issue warnings, and advance scientific understanding in order to reduce impacts of volcanic eruptions,” explains Ashton F. Flinders, a U.S. Geological Survey (USGS) Research Geophysicist at the Hawaiian Volcano Observatory.

This monitoring and research takes many forms, from non-stop visual observation, to measuring air quality, to tracking hundreds of daily earthquakes scattered across the seismically active Hawaiian Islands.

One of the main measures of volcanic activity is the movement of magma beneath the summits and flanks of a volcano. Scientists rely on these measurements to assess the likelihood of a pending eruption or other threat. And I knew these movements could be tracked seismically, atmospherically, and visually — the rise and fall of the lava lake in or under Halemaumau Crater atop Kilauea, for example.

It turns out that these magma movements, the shifting of molten rock mass underground, also change the force of gravity above… on a very, very faint scale.

“As we continually improve our monitoring and eruption response capabilities, we acquired a new, state-of-the-art instrument called an Absolute Quantum Gravimeter (AQG),” Flinders’ colleagues recently announced. “HVO scientists are excited about the AQG’s ability to measure very small mass changes beneath the ground surface, which will help detect underground volcanic processes.”

Magma, like anything else, has mass, and thus its own gravity field. More molten lava, more gravity. Since underground magma can’t be directly observed, shifts in gravity are a great way to assess how much magma is present at a given time and place.

The evolution of gravimeters

What constitutes the state-of-the-art in Earthbound gravity measurements has changed over the decades.

In the 1800s and 1900s, scientists used relative gravimeters. These were as simple as a weight hanging from a spring, with the amount of stretch used to determine the amount of gravity. Of course, this basic mechanical configuration was subject to “drift,” as the spring’s resistance changed over time. Inconsistencies among gravimeters limited the accuracy of their measurements over time.

Even so, relative gravimeters have been employed on Hawaii Island since the 1970s, measuring gravity at more than 65 fixed locations (benchmarks) scattered across the island every two years. Thus, HVO has accumulated a fair amount of data over a long time scale to track the movement of magma in Kilauea and Mauna Loa volcanoes.

At Pu‘uo‘o at the summit of Kilauea, a handful of continuous relative gravimeters had been operating since 2010. These instruments were key to monitoring the 2018 Kilauea erupting, allowing scientists to assess the volume and movement of magma along the East Rift Zone. But that especially destructive eruption ended with the gravimeters lost or non-functional, leaving HVO scientists without any gravity measurements for several years.

That brings us to 2019, and absolute gravimeters.

Following the 2018 eruption, the Hawaii Volcano Observatory received funding from Washington, D.C. to rebuild its gravity monitoring network. This upgrade included two new relative gravimeters, an absolute gravimeter on the floor of Halema’uma’u Crater, and the Absolute Quantum Gravimeter.

Absolute gravimeters measure the acceleration of gravity directly, using lasers to observe an object as it falls within a vaccum chamber. A free-fall absolute gravimeter points a laser at a reflecting prism, whereas an Absolute Quantum Gravimeter drops clouds of ultra-cool rubidium atoms. While both do not suffer from “drift,” a free-fall absolute gravimeter is a permanent installation with delicate mechanical parts that don’t get along well sitting on an active volcano.

Even the much more sensitive Absolute Quantum Gravimeter is just barely portable, comprising two crates about the size of a large drink cooler and a standing cylinder that looks like a small home water heater. The whole instrument weights more than 200 pounds.

HVO is using the same model of Absolute Quantum Gravimeter that was installed on the north flank of Mount Etna volcano in Italy. Despite frequent eruptions and high levels of vibration, it has successfully recorded many months of high-quality data.

Gravimeter and benchmark sightings

Flinders, who was involved in the Absolute Quantum Gravimeter acquisition, is now conducting experimental testing and calibration before it is deployed and producing data.

“We won't be taking it into the field until later this summer, and unfortunately, the public will not be able to see it under normal circumstances,” he says.

Eventually, its long-term home will be co-located with other instruments at the observatory’s temperature- and humidity-controlled Uēkahuna vault at Kīlauea’s summit, which is currently in the midst of a a major refurbishment. The other instruments housed there include a GPS receiver, tilt meter, and seismometer, as well as the HVO’s historic watertube tilt meter, built in 1956 out of spent artillery shells.

The Absolute Quantum Gravimeter won’t be confined to the vault forever, however.

“Several times a year we plan to take the instrument out of the vault and perform field measurements with it,” Flinders notes. “During that time, visitors will likely be able to catch us using it.”

Until then, Flinders was kind enough to take behind-the-scenes photos for Hawaii Bulletin.

As for those field measurements, Flinders notes that the first order of business will be to re-establish a gravity calibration line on Hawaii Island.

“Starting in the late 1960s USGS scientists began to establish a series of ‘benchmarks’ at precisely known and repeatable locations, where they could measure gravity on a reoccurring basis,” he explains. “You've probably walked right past one of these small benchmarks and not even noticed it — they are hard to spot unless you're looking for them.”

These “Gravity Stations” or “Gravity Control” benchmarks are similar to other government survey benchmarks — popular landmarks for hikers and geocachers. The oldest ones, installed in the 1960s and 1970s along Mauna Kea Access Road, are aluminum hexagons that are four inches in diameter and installed flush with rock or concrete surfaces like building foundations, drainage culverts, and lava outcroppings.

“We do need to make sure that we return the exact same spot every time, which is why we use the benchmarks,” Flinders explains.

Why do scientists need a gravity calibration line? Although the measurements from gravimeters atop active volcanoes are important, scientists also need to compare those measurements against measurements far away from volcanic activity.

“Since these benchmarks were far away from the active volcanic processes of Mauna Loa and Kilauea, any changes seen at these benchmarks during repeated measurements were due to changes in the way the instrument, the gravimeter, was functioning,” Flinders says.

He compared the shift to the periodic need to align the suspension of a car, which might otherwise “drift” to the left or right over time.

“For example, scientists might notice that the gravity measured at a site near Wilder Road in Hilo was gradually and predictably increasing through the years, even though we know that the area is gravitationally stable and not affected by volcanic forces,” he elaborated. “This meant the gravimeters were slowly, but steadily, drifting, and changes measured at these locations allowed USGS scientists to calibrate the instruments.”

The original gravity calibration line was maintained through the late 1990s, but over time, several benchmarks have been lost due to construction, and others are now too close to road traffic or other activity to safely set up instruments for the eight-hour calibration session. The next closest set of benchmarks are over 2,000 miles away.

“For the past fifteen years, we have been flying our campaign gravimeters to the next closest gravity calibration line on Mount Hamilton, in Santa Clara County, California,” Flanders says. “While the calibration line in California has served HVO scientists well, it is not very practical for our monitoring needs, does not cover a wide enough range in gravity to be very useful for calibrating monitoring measurements made on Mauna Loa, and past air travel and shipping of our monitoring gravimeters has led to costly damage.”

To return to homegrown calibration, HVO hopes to install up to a handful of new benchmarks across Hawaii Island in the next few years.

“We can make measurements pretty much anywhere as long as they are close to a road,” he says. “The Absolute Quantum Gravimeter is 200 pounds, and so we're looking at existing benchmarks that are easily accessible.”

The new instrument will allow scientists to “measure the precise value of gravity at those older benchmarks and potential new ones, to re-establish this calibration line and ensure our annual campaign gravity surveys of Kilauea and Mauna Loa are providing the best data possible for volcano monitoring,” Flinders says.

He notes that he and his colleagues will be talking to land managers in the next year to get permission to set up and make measurements.

“We recognize and appreciate that places we work are sacred to many in the Hawaiian community, and our goal is to work with those communities to accomplish our mission while remaining respectful,” he says.

Want to see live data from HVO’s many instruments? Click on a specific Hawaii volcano (like Kilauea) and on the map, click on one of the instrument icons (triangles are seismometers, stars are GPS, etc.), and you can pull up graphics of recent data.

Illustration by Andriy Onufriyenko/Getty Images.