Gravity survey at McMurdo Station, Scott Base, Cape Roberts and Mario Zucchelli Station, Antarctica, 4-30 November 2009 Technical Report by Yves Rogistera, Nicolas Le Moigneb, Larry Hothemc, Dave Collettd, Rachelle Winefieldd and Jacques Hinderera aInstitut de Physique du Globe de Strasbourg, UMR 7516 CNRS-Universit´ede Strasbourg 5 rue Ren´eDescartes, 67084 Strasbourg Cedex, France, [email protected], [email protected] bG´eosciences Montpellier, UMR 5243 CNRS-Universit´ede Montpellier 2, CC 060, 34095 Montpellier, France, [email protected] cUS Geological Survey, DOI, 517 National Center, 12201 Sunrise Valley Dr., Reston, VA 20192 USA, [email protected] dLand Information New Zealand, Toitu te whenua 160 Lambton Quay, PO Box 5501 Wellington 6145 New Zealand, [email protected], [email protected] Abstract From 10 until 23 November 2009, absolute gravity observations have been conducted at the USGS Thiel-1 gravity benchmark at McMurdo Station (US, 10-12 November), LINZ SBG-1 benchmark at Scott Base (NZ, 17-18 November) and Terra Nova Bay AB benchmark at Mario Zucchelli Station (I, 21-23 November) in Antarctica using the gravimeter FG5 #228. The three stations are located in remarkably quiet sites that are close to the sea ice, which is still formed at that time of the year. The absolute gravity measurements at Thiel-1 and SBG-1 stations are the first direct observations, whereas Terra Nova Bay AB Station had already been occupied in 1995 and 1997. From 17 until 28 November 2009, Hut Point Station and Radarsat building at McMurdo Station and ROBE and CRN2 benchmarks at Cape Roberts have been tied to Thiel-1 Station, MMD-N benchmark at Scott Base has been tied to LINZ SBG-1 Station, and the absolute gravity reference station IAGS and relative gravity station IRGS at Mario Zucchelli Station have been tied to Terra Nova Bay AB Station with a Scintrex CG-5 gravimeter. Contents 1 Introduction 6 1.1 Gravity survey in Antarctica . ........ 6 1.2 Financial and logistical supports . ........... 6 1.3 Equipment ....................................... 6 2 McMurdo Station (US) 7 2.1 AGatThiel-1Station .............................. ..... 7 2.2 Gravity links to Hut Point Station and Radarsat building ............... 10 3 Scott Base (NZ) 12 3.1 AGatSBG-1Station ................................ 12 3.2 Gravity link to Seismic Hut Station . .......... 15 4 Cape Roberts 16 5 Mario Zucchelli Station (I) 18 5.1 AG at Terra Nova Bay AB Station . ..... 18 5.2 Gravity links to IAGS and IRGS Stations . ......... 21 6 Conclusions and recommendations for future observations 23 Acknowledgments 25 References 26 A List of Acronyms 27 B Web sites 28 C Event Log 29 D Gravitational attraction by the redundant fuel tank at the Thiel gravity obser- vatory of McMurdo Station 31 D.1 Introduction.................................... ..... 31 D.2 Geometricaldescription . ........ 31 D.3 Gravitational potential . ......... 32 D.4 Ordersofmagnitudeandapproximations . .......... 32 D.4.1 Unitsconversion ............................... 32 D.4.2 Distances ..................................... 33 D.5 Gravitationalfield ............................... ...... 33 D.6 Influenceofthecontainer . ....... 34 2 D.7 Conclusion ...................................... 34 E Set processing log for AG measurements 35 E.1 McMurdoStation.................................. 35 E.2 ScottBase....................................... 38 E.3 MarioZucchelliStation . ....... 41 FG5 #228 and Scintrex CG-5 at Terra Nova Bay AB Station. 3 List of Figures FG5 #228 and Scintrex CG-5 at Terra Nova Bay AB Station. ........... 3 2.1 Yellow Thiel observatory (Building 146) at McMurdo Station. Fuel tank approxi- mately60mbehind. ................................... 7 2.2 Concrete pier and Thiel-1 benchmark inside Thiel observatory (building 146). 8 2.3 FG5 #228 installed over Thiel-1 benchmark. Scintrex CG-5 on the left on the pillar. 9 2.4 AG time series at ground level at Thiel-1 Station. Time is UT. ............ 9 2.5 Tripod of Scintrex-CG5 over Thiel-1 benchmark. ............. 10 2.6 Scintrex-CG5 installed over Hut Point Station. .............. 10 2.7 Radarsat building 71 and mark for gravity tie at Radarsat Station. 11 3.1 Left. Behind the yellow panel, concrete pier holding gravity benchmarks SBG-1 and SBG-3. Right. Gravity benchmarks SBG-1 (centre) and SBG-3 (left). 12 3.2 GravitybenchmarkSBG-1. ...... 13 3.3 AG time series at ground level at SBG-1 Station. Time is UT. ............ 14 4.1 Location of the LINZ ROBE benchmark (right) near the ANZ shelter (left). 16 4.2 Location of the LINZ CRN2 benchmark (bottom left-hand corner of the bottom image) near the tide gauge and GPS antenna (top). ........ 17 5.1 Aerial view of MZS. Indicated is the position of the TNB AB and IAGS stations, first measured in 1990-91 and 1995 respectively. ........... 18 5.2 TNBABStationatMZS. .............................. 19 5.3 AG time series at ground level at TNB AB station. Time is UT. ........... 20 5.4 Italian AG Station (IAGS) at MZS. ....... 21 5.5 IAGSbenchmark................................... 21 5.6 IRGSbenchmark................................... 22 D.1 Position of the gravity mark with respect to the tank. .............. 32 4 List of Tables 2.1 Geographical coordinates of and vertical gravity gradient at Thiel-1 Station. 7 2.2 AG at Thiel-1 Station measured by means of gravity ties. .............. 8 2.3 AG at Thiel-1 Station at ground level and 1 m above ground. ............. 8 2.4 Geographical coordinates of Hut Point Station. .............. 10 2.5 AG at Hut Point and Radarsat stations measured by means of gravity ties. 11 3.1 Geographical coordinates of and vertical gravity gradient at SBG-1 Station . 12 3.2 AG at SBG-1 Station measured by means of gravity ties. ............ 13 3.3 AG at SBG-1 Station at ground level and 1 m above ground. ........... 13 3.4 Geographical coordinates of MMD-N Station . ........... 15 3.5 AG at MMD-N Station (Seismic Hut) measured by means of gravity ties. 15 4.1 Geographical coordinates of ROBE and CRN2 stations . ............. 16 4.2 AG at ROBE and CRN2 stations measured by means of gravity ties.......... 17 5.1 Geographical coordinates of and vertical gravity gradient at TNB AB station. Ele- vation and gravity gradient given by Sasagawa et al. (2004). ............. 20 5.2 AG at TNB AB station at ground level and 1 m above ground. The 1995 value given by Sasagawa et al. (2004) has been corrected back by M¨akinen et al. (2007) for both Earth tides and instrumental factors. So, the same corrections have been applied to Sasagawa et al.’s data and our data, which allows for a comparison of the gravity values............................................. 20 5.3 AG at ground level at IAGS and IRGS stations . ......... 22 5 Chapter 1 Introduction 1.1 Gravity survey in Antarctica Initially planned during the International Polar Year 2007-2008, the gravity survey described in this report was postponed twice until November 2009. It originally consisted in making Absolute Gravity (AG) and Relative Gravity (RG) measurements at McMurdo Station (McM), Scott Base (SB), Mario Zucchelli Station (MZS) and Dumont d’Urville (DdU), as well as RG measurements at Cape Roberts (CR). However, because of logistical incompatibilities, the observation at DdU was cancelled. In the following chapters, we successively report on the observations made at McM, SB, CR and MZS. Where available, we compare the observations to measurements previously made at the same location. 1.2 Financial and logistical supports Initiated as a part of project 337 Variation de gravit´eet mouvement vertical dans les r´egions polaires - Apport aux probl`emes du rebond post-glaciaire et de la d´eglaciation actuelle funded by the French Institut Paul-Emile Victor (IPEV) and led by Jacques Hinderer (IPGS), the 2009 AG survey in Antarctica also benefited from the support of the Italian Programma Nazionale di Ricerche in Antartide (PNRA), Land Information New Zealand (LINZ), Antarctica New Zealand (ANZ), and US Antarctic Program (USAP) managed by the National Science Foundation (NSF). It is a part of the international Polar Earth Observing Network (POLENET) project. 1.3 Equipment The AG observations were conducted with the free-fall FG5 #228 gravimeter designed by Micro- g Solutions Inc., owned by the French Institut National des Sciences de l’Univers (INSU), run by G´eosciences Montpellier and operated by Nicolas Le Moigne who was assisted by Rachelle Winefield, Dave Collett, Larry Hothem and Yves Rogister. Observations consisted of sets of 100 drops, one drop every 10 seconds, which were hourly repeated. The AG raw data are corrected for Earth tide, ocean loading, polar motion and atmospheric pressure. The corrections and instrumental parameters are given, for each station, in Appendix E. The RG observations were made with a Scintrex CG-5 gravimeter owned and run by G´eosciences Montpellier. The operators were RW, NLM, LH, DC and YR. 6 Chapter 2 McMurdo Station (US) 2.1 AG at Thiel-1 Station Brief history and description of McMurdo gravity stations can be found in the technical report by Diehl (2008). Thiel gravity station, whose geographical coordinates are given in Table 2.1, was established in Building 146 (Fig. 2.1) in 2000-2001 to replace the SATGRAV station. Thiel-1 Table 2.1: Geographical coordinates of and vertical gravity gradient at Thiel-1 Station. Station Latitude (◦) Longitude (◦) Elevation (m) dg/dz (µGal/cm) Thiel-1 77.8490 S 166.6794 E 46.21 -3.012 ± 0.030 Figure 2.1: Yellow Thiel observatory (Building 146) at McMurdo Station. Fuel tank approximately 60 m behind. benchmark is on a concrete pier inside the building (Fig. 2.2) and Thiel-2 benchmark is on a concrete pier outside the building. We only occupied Thiel-1 station. No direct AG measurements had been made at Thiel station before November 2009, but the AG had been determined by means of gravity ties (Table 2.2).