Gravity measurements in Antarctic summer

Three people walking in Antarctica.
Photo:
Jyri Näränen

Geodetic and geodynamic research takes the research scientist around the world, sometimes as far as Antarctica. Last November, I did absolute gravity measurements in summery Antarctica.

This time, during my fourth measurement campaign on the continent, measurements were taken at New Zealand's Scott Base station on Ross Island, at the United States' McMurdo Station and the nearby Crater Hill. No measurements had been taken at Crater Hill before. During the expedition, I enjoyed the hospitality of my New Zealander hosts at Scott Base.

Geodesy is an international field of research

Intercontinental flights aren't much fun, which is why it pays off to take as much advantage as possible of a trip to the Southern Hemisphere. The first stop of the trip was in Australia, where I participated in the biennial International Workshop on Laser Ranging, which was this time held in Canberra. There, my colleague Arttu Raja-Halli and I gave a presentation on the progress of the system integration of the new satellite laser system at the Metsähovi Geodetic Research Station.

From Australia, my trip continued to Christchurch, New Zealand, where I met representatives of our partner, Land Information New Zealand. We discussed the aims of the expedition and received equipment for our trip to the Antarctic from Antarctica New Zealand, which was responsible for the logistics of the expedition.

We travelled from Christchurch to Ross Island on the military transport plane C17 Globemaster III operated by the United States Antarctic Program. During the summer season, there are flights almost daily. When you travel to and work in the Antarctic, you are at the mercy of the raw and unpredictable weather of the continent. When booking flights to and from the Southern Hemisphere and Antarctica, you need to reserve a few spare days in your schedule, because flights can be moved by a few days in either direction. However, this time the weather was perfect, and the trips and measurement campaigns could be completed on schedule.

Why do we measure gravity in Antarctica?

This time, the measurement campaign was a part of the GRAVLASER research project funded by the Academy of Finland, which was undertaken in cooperation with Land Information New Zealand, LINZ. International cooperation is important, because in geodetic research we measure the entire planet, not just a part of it within the borders of a country.

In absolute gravity measurements, acceleration of free fall (g) as m/s^2 is measured at an accuracy of eight decimals. Through the measurements in November 2018, the geodetic infrastructure of Ross Island and the surrounding Antarctica is built and maintained.

The measurements can be used in land uplift research and, e.g., as starting points for airborne gravity surveys. Precise gravity measurements are needed for defining the geoid, for example. A precise geoid model is, in turn, the basis of precise positioning. In addition, understanding and modelling the mechanisms of land uplift is important when interpreting the movements of ice masses in the Antarctic.

For Finland, the current decrease in the ice masses of the Antarctic is interesting, because most of the ice that is melting in the Antarctic will raise sea levels in the Northern Hemisphere. Ross Island is volcanic, which brings additional challenges to researching changes in gravity. Here in the Nordic countries, we are researching the changes in gravity due to land uplift following the last ice age, but on Ross Island, there are movements in the Earth's crust and upper mantle caused by volcanic activity as well.

Gravity monitored from diving hut

It was summer in the Southern Hemisphere, which meant that temperatures varied between 0 and -20 °C, which is, however, considerably milder than local winter conditions. At times, winds were brisk, so the weather was slightly colder than in Finland during the weeks preceding Midsummer.

To ensure that the poor researcher wouldn't freeze solid next to his instruments, the absolute gravimeter was placed in a mobile container normally used for sea bottom research. The measuring instrument needs temperatures above 0 °C to function as well. The hole in the floor of the container was just about right for the equipment, as it was possible to place it through the hole on a pillar set on icy ground and isolate it from the environment at the same time. Normally, the hole in the floor of the container would be used as a way for divers to get under the ice, but this time, the researcher was able to sit in peace inside the warm container.

Jyri Näränen in a diving hut measuring gravity.
Photo: Jyri Näränen

The measuring instruments travel frequently

The absolute gravimeter used in these measurements is an important part of the measuring instrumentation at the National Land Survey's Metsähovi Geodetic Research Station. It is used to measure gravity in various places in Finland, and in cooperation projects around the world. The instrument is also the national measurement standard of acceleration of free fall. There are approx. 50 such instruments around the world, and there are no instruments more precise than this, which measure directly in SI units.

We have previously visited more than 15 countries with our instrument and its predecessors to share our knowledge. We have also visited the same locations repeatedly to do measurements, because gravitational time series are an important tool in geodynamics research.

The Department of Geodesy and Geodynamics has researched land uplift in Antarctica since the early 1990s. The recent measurements are a part of the GRAVLASER research project funded by the Academy of Finland and led by Prof. Juha Hyyppä, head of the Department of Remote Sensing and Photogrammetry at the Finnish Geospatial Research Institute FGI. This project also involves cooperation between different departments at FGI, not only between Finland and New Zealand.

Kuvassa blogin kirjoittaja Jyri Näränen
Jyri Näränen
research manager
Twitter: @Dr_Jyri

The author works at the FGI's Department of Geodesy and Geodynamics as the manager of the Metsähovi Geodetic Research Station. His research focuses on the gravitational changes caused by the movements of the Earth's crust.

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