Hawaii volcano continues to erupt, quantum gravimeter is in use

The Kilauea volcano in Hawaii is continuing to erupt, and scientists are using a variety of instruments to make uninterrupted observations of the island's volcanoes, including a new "absolute quantum gravimeter."

Kilauea volcano eruption

To continually improve its ability to monitor and respond to volcanic eruptions, the U.S. Geological Survey's (USGS) Hawaii Volcano Observatory (HVO) uses a new, state-of-the-art instrument, the Absolute Quantum Gravimeter (AQG). HVO scientists are excited about AQG's ability to measure very small changes in mass below the surface, which will help probe subsurface volcanic processes.

All objects have mass and therefore have a gravitational field. Earth's gravity is slightly stronger in regions of greater mass and weaker in regions of lower mass. A gravimeter can measure gravity. In volcano research, gravimeters help scientists detect subtle changes in gravity caused by the movement of magma. When measuring gravity in volcanically active regions, stronger gravity indicates more magma beneath the surface.

There are two main types of gravimeters: relative gravimeters and absolute gravimeters.

Relative gravimeters are the most common and consist of a weight attached to a sensitive vertical spring. Gravity stretches the spring, and the amount of stretch is proportional to the measurement of local gravity. A relative gravimeter measures the difference in gravity between different locations.

However, these instruments are subject to "drift," which adds noise to the measurements over a period of weeks to months and gradually degrades their accuracy. Drift needs to be characterized and eliminated through frequently repeated measurements.

An absolute gravimeter directly measures the acceleration of gravity. The most common free-fall absolute gravimeter uses a laser to measure the free-fall acceleration of a small reflecting prism (test mass) in a vacuum.

Unlike relative gravimeters, absolute gravimeters do not drift or lose accuracy over time. However, they are bulky, have delicate mechanical parts, require ample power, and are not designed for use in harsh field conditions such as volcanoes. Those instruments that can be carried in the field cannot make long-term continuous measurements, nor are they sensitive enough to detect the tiny changes needed for volcano monitoring.

Similar to a free-fall absolute gravimeter, HVO's new AQG measures the acceleration of a small test mass in a vacuum. However, AQG overcomes the limitations of classical free-fall absolute gravimeters by replacing small prisms with laser-cooled clouds of rubidium atoms. This allows precise and long-term continuous measurements. AQGs are also small enough to be deployed in active volcanic environments and operate continuously without "drifting."

"The Hawaii Volcano Observatory's absolute quantum gravimeter is being set up, tested and calibrated prior to Kilauea's installation," the USGS said. The AQG, pictured below, is from French startup Muquans, which launched It was acquired by iXblue, a French manufacturer of optical communication equipment, last year.

The absolute quantum gravimeter is being set up, tested and calibrated

The Muquans instrument is relatively compact, consisting of a cylinder with a height of 70 centimeters and a slightly larger electronics and laser module.

Bruno Desruelle, president of Muquans, revealed: "In the next four years, the weight of our instrument will be halved, from the current 100kg to 50kg, which will further expand its application."

The same AQG model has been installed on the north side of the frequently erupting volcano Etna in Italy. Despite the high level of vibration and noise in this environment, the instrument has successfully recorded high-quality data for several months. It continued to help Italy improve volcanic hazard reporting and crisis management planning.

Muquans installed  AQG on Mount Etna

Currently, HVO's AQG is undergoing experimental testing and calibration. Ultimately, its long-term base will be co-located with other instruments on Kilauea. In addition to occasional field deployments, where it will take continuous measurements. 

While AQG is new to HVO, gravity monitoring has a long history in Hawaii. Since the 1970s, relative gravimeters have been successfully used to measure more than 65 datum points on the island of Hawaii. These benchmarks are measured every few years to detect magma buildup or shallowing over long time scales at Mauna Loa and Kilauea.

Also, starting around 2010, there are some continuous relative gravimeters at Kilauea. They played a vital role in monitoring the 2018 Kilauea eruption, allowing scientists to estimate the speed and volume of magma accumulation and withdrawal in the eastern rift belt and mountaintops.

Unfortunately, these instruments were either lost during the summit collapse caused by the eruption or have ceased to work, leaving HVO without a continuously operating gravimeter for several years.

Following the events of Kilauea in 2018, HVO received funding from the Disaster Relief Supplementary Appropriation Act of 2019. To rebuild Kilauea's continuous gravity network, a gravimeter was reinstalled at the base of Kilauea's Halemaʻumaʻu crater in June 2021. In April 2022, two more serial relative gravimeters are planned to be installed at other sites in Kilauea.

The combination of the new absolute quantum gravimeter, the new continuous gravimeter and ongoing activity measurements make the prospect of using gravimetry to monitor, investigate and assess the dangers of Hawaiian volcanoes exciting and promising.

Link:

[1]https://www.bigislandvideonews.com/2022/03/19/volcano-watch-absolute-quantum-gravimeter-to-measure-kilauea-mass-changes/

[2]https://www.nps.gov/media/webcam/view.htm?id=BA08FE10-B9F1-6EF1-3EE8A0E9DC683143