Year-Long Exploration Aims to Add Gravity to the Volcano Monitoring Toolkit
On a typical day, a person walking into the Carnegie Science Earth and Planets seismology and geodesy lab would find the walls neatly packed with the precise instruments that our scientists use to understand the inner workings of our planet. Today, however, the walls are emptier than usual and the floor is spread with scientific equipment, batteries, duffel bags, hiking gear, first-aid kits, energy bars, and helmets. In the middle of this scene is Hélène Le Mével, a staff scientist and geodesist at Carnegie Science, packing her equipment for a year-long expedition to Chile.
In January 2020, Mével will travel to Chile to study Villarrica volcano, one of the most active volcanoes in the world. Once there, she will lead an international team of scientists, including Carnegie's Diana Roman and postdoc Kathleen McKee, to study the elusive inner workings of the volcano. The study, funded by the Brinson Foundation, will include one of the most-extensive monitoring plans to date for the volcano, which is a popular tourist destination. It will also present an opportunity to pioneer the use of gravity as a monitoring tool.
Villarrica, pictured here, is continuously active, which makes it the perfect place to study new volcano monitoring technology such as gravimetry. Credit: Wikimedia Commons/LBM1948
Ask any elementary school student to draw a volcano, and they'll probably come up with a good approximation of Villarrica. With its steep slopes built by millennia of high-viscosity lava flows, Villarrica is your classic "stratovolcano" complete with a lava lake in its crater and icy glacier-capped peak. Scientists seek to study active volcanoes like Villarrica because the chances are high that, with proper instrumentation, they will collect enough data to form a clear understanding of what’s happening underground. As a result, Villarrica provides the perfect proving ground for new monitoring technologies such as gravimetry.
Many people assume the pull of gravity on Earth is constant, but altitude, latitude, topography, and geology all have an effect on the gravity that objects experience. When it comes to measuring the gravity around volcanoes, it's all about the density of the underlying material. Actively moving rock, gas, water, and magma alter the density of the Earth’s crust, which leads to measurable changes in gravity.
“If the gravity has increased,” explains Mevel,” then it means that there is a denser material—magma, for example—that wasn’t there before. On the other hand, if there’s a new void, like a sinkhole, this will decrease the gravity.”
Mével walks toward a large box in the middle of the laboratory. "This box is full of seismometers," explains Mével as she leans down to open the hard plastic case made for rugged travel with sensitive equipment. Inside are 10 round instruments, which Mével examines before removing them from the box. "We are bringing 10 of these, but they are not going to stay in this box once we get there."
Instead, once emptied in Chile, this box and its twin will be buried on the volcano to serve as weatherproof vaults for the project’s two gravimeters.
Hélène Le Mével inspects the ten seismometers that will be traveling tucked safely into large weatherproof cases. The cases serve two purposes, first to transport the seismometers to Chile and then to safely house the gravimeters on the volcano for a year. Credit: Carnegie Science | Katy Cain
With the seismometers removed from the box, Mével moves on to inspect the pièces de résistance. Tucked away in two blue cooler-sized boxes are the gravimeters. Each instrument houses a precision-made spring that is sensitive enough to measure these tiny changes in gravity. They are expensive and delicate, and Mével only has two.
Two gravimeters sit on their short metal tripods, ready to be tested and then packed away. They will first be packed into their blue bags, on the left, which also house a field notebook for each instrument. Credit: Carnegie Science | Katy Cain
To get the gravimeters to Chile without them being dropped or flipped over, which would render them useless, she is flying with one as her carry-on and sending one in a giant, padded, tip-proof crate. To get them on the mountain, her team will place each gravimeter in a backpack intended to carry a baby and hike them carefully over icy glaciers and up the side of a volcano. Then, they will dig a hole and leave them on the site, where they will continuously monitor gravity for an entire year.
The gravimeters cannot be flipped or dropped, so Hélène Le Mével and her team of scientists will hike while carefully carrying them in these baby carriers. Credit: Carnegie Science | Katy Cain
Abandoning the lab's only two gravimeters on the side of an active volcano for a year might seem like a risky bet. Nevertheless, the year of continuous collecting is essential if Mével wants to build the kind of data that gravimetry needs to become a useful monitoring technique.
Mével explains, “It’s a good proof of concept because using gravimetry continuously to monitor volcanoes is quite new. It has only been done twice: in Hawaii and on Etna. We’re still trying to prove that it can work. If I did my experiment somewhere where it’s not active, then I wouldn’t get the amount of data I need to do that.”
One of the challenges in measuring gravity is that the changes are so small that they could be mistaken for instrumental error or normal fluctuations. To address this problem, Mével is using the gravimeters in conjunction with the 10 seismometers to measure the Earth’s movement, one multiGAS machine to measure volcanic gasses, five infrasound monitors to listen for underground explosions, and a GPS unit to measure changes in the position of the volcano. These tried-and-true tools each measure one aspect of the volcano's activity, and when used together they can provide a looking glass that allows scientists like Mével to see what’s happening inside of a volcano.
By adding gravimetry to the mix, Mével hopes to add another tool to the volcano monitoring toolkit. Sudden uplift of a volcano, as measured by GPS or radar satellite, could signal an impending eruption caused by the intrusion of new magma; it could be something less consequential like the release of gas from a cooling magma chamber or the rearrangement of a hydrothermal water system—driven by the hot-rock beneath the volcano. Each of these events will create a different signal on the gravity meters, allowing the true cause of the volcano deformation to be determined. Ultimately, understanding these changes in gravity could allow scientists to predict eruptions with more precision on Villarrica and volcanoes around the world.
Hélène Le Mével prepares to pack the five infrasound monitors, which measure noises coming from the volcano. The infrasound monitors help detect and track the timing of volcanic explosions. Credit: Carnegie Science | Katy Cain
The multiGAS machine identifies volatile gasses, like water and carbon dioxide, degassing from the volcano. Changes in these gasses can be an effective indicator of volcanic activity. Credit: Carnegie Science | Katy Cain
“We are trying to characterize the dynamics of magma in Villarrica’s conduit and in the reservoir. The idea is if we record seismicity, gas, infrasound, position, and gravity at the same time and we have this large time series of all of these parameters, we can constrain what’s happening.” Mével continues, “For example. Let’s say the gravity drops one day. Then I could look over at the data from the multiGAS monitor for that day and see what gasses the volcano is releasing or look at the infrasound data to see if there was an explosion. We won’t have to guess where and when activity is taking place.”
Outside the lab, Hélène Le Mével tests one of two gravimeters in the weatherproof box that will house it for a year on Villarrica. Credit: Carnegie Science | Katy Cain
Back in the lab, Mével takes one of the gravimeters outside for a test. She places the gravimeter inside the sturdy plastic box and closes the lid. Mével hooks the gravimeter up to a battery and watches the screen to make sure that the gravimeter is working inside the weather-proof case that will be its mountain home for the next year. After a moment, the screen lights up with the relative gravity reading–all is working as expected.
Now that everything has been accounted for and tested, all of this equipment has to get to Chile to be installed on the volcano. This feat will require more than six bags of luggage, an entire pallet of batteries, a helicopter, and a team of at least 10 scientists and students. Mével is going ahead of the group to get started. If all goes according to plan, by the end of February 2020, everything will be in place, and the monitoring for this year-long experiment will begin.
Stay tuned for updates as Hélène Le Mével reports back on her year-long study in Villarrica!
Written by Katy Cain