GIANT and LISSA
Field report
May 30, 2009
Authors:
Bergeot Nicolas, Camelbeeck Thierry, Lombardi Denis, Bruyninx Carine and Van Camp Michel
Contents
1. Project personnel and contact details
2. Introduction
3. Key Dates
4. Field sites
5. Field work
Acknowledgements
1. PROJECT PERSONNEL
In 2008-2009, Thierry Camelbeeck, Michel Van Camp, Nicolas Bergeot, Giovanni Rapagnani and Carine Bruyninx from the ROB, prepared the scientific research and solved the technical and instrumental problems for running the geophysical instruments at the Princess Elisabeth base. Pascal De Kimpe and Alain Muls from the Royal Military Academy provided technical support in the preparation of the connection of the ROB GPS system to the optical cable for future internet links. It should also allow connecting the other geophysical instruments.
Nicolas Bergeot conducted the field mission in Antarctica from 10 to 22 February 2009 to install temporarily the GPS equipments from ROB and University of Luxemburg.
Since 1 January 2009, Denis Lombardi is paid by the project. He will be the main scientist involved in the LISSA project and he is already implementing receiver function methodology at the ROB, testing the borehole seismometer and preparing the summer 2009-2010 field mission.
2. INTRODUCTION
GIANT (Geodesy for Ice in Antarctica) is collaborative project between the Royal Observatory of Belgium (ROB) and the Luxembourg University. Its main goal is to quantify the present ice mass variation in the region of the Princess Elisabeth base in Antarctica and to better understand its relation with crust deformation in polar regions. It should improve our knowledge on the Glacial Isostatic Adjustment (GIA) and climate-related mechanisms such as global climate changes. Further investigations will be conducted to study tropospheric and ionospheric atmosphere in the frame of the Solar and Terrestrial Center of Excellence (STCE). The LISSA (Lithospheric and Intraplate Structure and Seismicity in Antarctica) project is a program managed by the ROB. The exceptionally good conditions offered in Antarctica at the Princess Elisabeth station, probably one of the best sites one may find in polar regions, will permit the collection of high-quality seismic recordings which will be used for various kinds of analysis. The recordings of teleseismic earthquakes will allow determining the local lithospheric and mantle structure that will contribute to study present and past geodynamics of the continent. On the other hand, local earth- and ice-quakes will be analysed to understand intra-plate seismic activity and mechanisms of rupture which are so far not fully understood. Finally, data collected by the seismic station will be distributed to international data centres (e.g IRIS) to contribute to the global monitoring of earthquakes.
For that purpose the collection and combination of geodetic (position, gravity) and seismic data over a period of several years is necessary to constrain global and regional geophysical models.
During the first scientific season 2008-2009 at the Princess Elisabeth station, the ROB installed two GPS stations namely ELIS (Belgium station) and ULUX (Luxembourg station). The goal of those GPS stations is to measure surface displacements correlated to crustal deformation in horizontal and vertical components at the sub-mm/yr precision level. To reach such a precision, high precision GPS receivers and antennas, with continuous observations over more than 4 years, as well as the most advanced data processing, are required. The two installed GPS stations allow to compare their measurements and to determine whether the GPS antennas are submitted to the same global movement or to local motion due to monument instabilities which of course must be avoided.
3. KEY DATES
Prior to field campaign 2008-2009, test measurements were conducted at the ROB to evaluate if the gravimeter and seismic instruments could be installed in the vicinity of instruments from the RMI. Indeed, to monitor atmospheric pollution in Antarctica, several pumps with their compressors will be installed by the Royal Meteorological Institute (RMI) and University of Ghent. These tests conducted with 3 pumps, 1 surface and 1 borehole (120 m deep) seismometers revealed a strong influence of the running pumps on the recorded seismic signal by both seismometers. Also the influence on the gravity measurements, though not tested, may be significant since parts of the gravimeter behave as a seismometer. The results lead to the constructing of two separated scientific shelters: one hosts the instruments for atmospheric studies and the other one, the “Emile Danco” hosts the GPS, gravimetric and seismic instruments. During the season 2008-2009, only the GPS instruments were installed due to missing infrastructures at the Princess Elisabeth station. Prior to the installation of the two GPS antennas and receivers, the Department CISS (Communication, Information System and Sensors) of the Royal Military Academy designed a box hosting the receivers and wired to the Princess Elisabeth station through a 300 m long optical fibre allowing continuous connection and monitoring from the Princess Elisabeth station and from Belgium. The box permits in addition to monitor the temperature close to the receivers allowing to remotely turn on an electric heating in the Emile Danco shelter when necessary e.g. when the temperature in the box is below 0°.
4. FIELD SITES VISITED
Utsteinen Arrival and Departure: 10/02/2009 to 22/02/2009
Effective days of work: 11/02/2009 to 19/02/2009
One scientist, Dr. Nicolas Bergeot, from the ROB visited the Princess Elisabeth station for 12 days in order to select the optimal location of the GPS antennas (stable rock, good visibility of the GPS satellites), to monitor the acquisition and to process a first set of GPS data. Two technicians from the station helped Nicolas Bergeot to install the GPS pillars on which the GPS antennas are installed. One complete day was required to install the GPS material (pillar, antenna and box), the cables connecting the receivers and the antenna as well as the fuel generator set to power the GPS receiver in the Emile Danco shelter. The ELIS and ULUX GPS antennas are situated respectively 20 m and 35 m north of the Emile Danco shelter. The distance between the two antennas is 15 m. One of the main difficulties of the installation was to find a stable part of the ridge, composed of many granite blocks, to install the GPS antennas to ensure measurements of the crust deformation and not bloc instability. Data were acquired from 12/02/2009 to 14/02/2009. However, due to problems with the first fuel generator, no continuous data set could be acquired during those two days. Nevertheless, during the 4 following days (15/02/2009 to 18/02/2009) continuous GPS data were acquired, respectively at a 1 s and 30 s sampling rate for the ELIS and ULUX GPS stations. The 4 days of continuous data have been processed to quantify the repeatability of the estimated positioning and estimate the capability of those GPS stations to estimate atmospheric parameters. For that purpose, the GPS observations from 7 other continuously observing GPS stations in Antarctica were also used to process the data in a network approach. However, due to the absence of both power supply in the shelter, of connection between the shelter and the station, and the missing internet connection, the GPS material was removed from the shelter at the end of the mission and brought back to Belgium.
5. FIELD WORK
GPS positioning results
We estimated the position and its precision for the two stations. The mean position estimated during the 4 days are in latitude, longitude and altitude: S71°56’48.1; E23°20’46.8; 1389.8 m for the ELIS station and S71°56’47.7; E23°20’46.2; 1389.8 m for the ULUX station. Those positions varied over the 4 days by 1.9 mm on the East component, 3.2 mm on the North component and by 4.2 mm on the vertical component. Considering the influence of tectonic movements as negligible over this short time period, the variation in the coordinates is therefore due to modelling errors within the GPS data processing (atmospheric loading, solid tides, atmospheric refraction or GPS orbits…) and reflects the best precision available in such a region during this time interval. Further investigations of up-to-date models applied to polar regions will be necessary to improve this precision.
Atmospheric results
Troposphere:
The tropospheric parameter estimated from GPS data is the Zenithal Total Delay (ZTD), which corresponds to the delay of the GPS signal at zenith due to the tropospheric refraction when the GPS signal traverses the troposphere. The ZTD can be converted in terms of water vapour content in the troposphere (1 mm ZTD = 0.15 mm/m2 of water). Due to the close localization of the ELIS and ULUX stations, the ZTD estimated at the two stations is expected to be the same. Consequently, we estimated hourly ZTD from the ELIS and ULUX data during the 4 days of continuous data acquisition. The ZTD time-series from ELIS and ULUX
stations are in very good agreement with a mean difference of 3 mm and a maximum difference of 13 mm. Those estimations encourage us to use GPS to monitoring the Antarctic troposphere for meteorological applications (the required precision for meteorological prediction is 3-10 mm on the ZTD) and for future climatological studies.
Ionosphere:
The ionospheric parameter estimated from GPS data is the Total Electron Content (TEC), which characterizes the ionization state of the upper part of the atmosphere due to solar radiation. In the first order, the TEC is dependant of the Solar exposition of the ionosphere. Additionally, the TEC varies with respect to Solar eruptions and induced geomagnetic storms or to post-seismic (for large earthquakes) atmospheric oscillations. We estimated the ionospheric TEC above the two GPS stations and found a diurnal variation due to the exposition of the ionosphere to the Sun. This result is a first step to better understand the physics of the ionosphere in the polar region.
Conclusion
During the season 2008-2009, the ROB installed two GPS stations close to the Princess Elisabeth station. The first goal of those stations is to estimate variations in the crustal displacement of Antarctica at the station. We acquired 4 days of continuous GPS data, which allowed us to estimate the precision on the position in this region (<5 mm on horizontal and vertical components). However, only a continuous flow of GPS data allows monitoring variations in displacements together with atmospheric behaviour. The acquisition of such measurements was not possible during the season 2008-2009 due to missing infrastructure at the Princess Elisabeth station. Nevertheless, we expect to be able to continuously acquire GPS data together with seismic measurements for the next scientific season 2009-2010. Indeed, in addition to 2 fully permanent GPS stations, the ROB will install 1 borehole (12 m deep) and 1 surface seismometers next (15 m to the south) or within the Emile Danco shelter. The absolute 7 gravimeter will be probably installed for the first time during the season 2010-2011 once the Princess Elisabeth station is fully operational. Continuous and real-time monitoring (using internet connection) will permit the scientists at the ROB to ensure the correct functioning of the instruments (GPS and seismic) and allow the Princess Elisabeth station to serve as a reference station for geodesy and for worldwide earthquake detection representing thus a great contribution of Belgium for global and regional geodesy and seismology.
Acknowledgements
We thank the International Polar Fondation and mainly Johan Berte for his enormous work in the preparation of the shelter “Emile Danco” and his availability to answer our numerous questions and interrogations. We are grateful to the Royal Military Academy for their cooperation in preparing the communication box. We would like also to thank Professor Hugo Decleir for sharing a part of his strong experience of Polar Regions and Maaike Vancauwenberghe for helping us.