ALBERT

Description: As part of the CryoSat Cal/Val activities and the pre-site survey for an ice core drilling contributing to the International Partnerships in Ice Core Sciences (IPICS), ground based kinematic GPS measurements were conducted in early 2007 in the vicinity of the German overwintering station Neumayer (8.25° W and 70.65° S). The investigated area comprises the regions of the ice tongues Halvfarryggen and Søråsen, which rise from the Ekströmisen to a maximum of about 760 m surface elevation, and have an areal extent of about 100 km x 50 km each. Available digital elevation models (DEMs) from radar altimetry and the Antarctic Digital Database show elevation differences of up to hundreds of meters in this region, which necessitated an accurate survey of the conditions on-site. An improved DEM of the Ekströmisen surroundings is derived by a combination of highly accurate ground based GPS measurements, satellite derived laser altimetry data (ICESat), airborne radar altimetry (ARA), and radio echo sounding (RES). The DEM presented here achieves a vertical accuracy of about 1.3 m and can be used for improved ice dynamic modeling and mass balance studies.

Description: Die Oberflächentopographie der Eisschilde ist von großer Bedeutung für exakte Berechnungen von Massenbilanzen und Modellierungen der Eisdynamik. Aus diesen Studien kann der Beitrag der Eisschilde im Hinblick auf die globalen Veränderungen des Meeresspiegels ermittelt werden. Es ist jedoch notwendig, hierfür ein möglichst genaues Höhenmodell einzusetzen. Die Küstenregionen des antarktischen Eisschildes spielen dabei eine wichtige Rolle, da sie die Übergangszone zwischen gegründetem und schwimmendem Eis bilden. Bereits vorhandene Höhenmodelle zeigen in diesen Regionen abweichende Höhen und besitzen oft nicht die nötige Genauigkeit um genaue Berechnungen zu ermöglichen.Im Rahmen der CryoSat Cal/Val-Aktivitäten (CryoVEx) und der International Partnerships in Ice Core Sciences (IPICS) Vorerkundung, wurden Anfang 2007 im Umkreis der deutschen Neumayer-Station bodengebundene kinematische GPS Messungen vorgenommen. Das Untersuchungsgebiet erstreckt sich von 5° bis 11° W und 70° bis 72° S. Im Fokus stehen die beiden das Ekstrømisen umgebenden gegründeten Eiszungen Halvfarryggen und Søråsen. Die kinematischen GPS Messungen sind um lokale GPS Referenzstationen zentriert, was zu einer Minimierung der systematischen Fehler bei der Post-Prozessierung führt. Jedoch sind diese GPS Messungen nur sehr kleinräumig, weshalb sie mit weiteren Datensätzen ergänzt werden. Hierfür wurden Laseraltimeterdaten über den Eisschilden des ICESat, GLAS12 Release 28, verwendet. Um eine noch höhere Datendichte zu erreichen, wurden flugzeuggestützte Radaraltimeterdaten (RA) bzw. Eisradardaten genutzt. Der neue Topographiedatensatz beruht auf der Kombination dieser Daten. Da das kinematische GPS weder von Wolken, noch durch die Hangneigung beeinflußt wird, wurden diese Daten als Referenz genommen. Alle weiteren Datensätze wurden hinsichtlich ihrer Höhenunterschiede zum GPS untersucht und ggf. korrigiert. Der neue Datensatz basiert auf korrigierten Datensätzen, die mittels dem Ordinary Kriging Algorithmus auf ein 1 km x 1 km Raster interpoliert wurde. Durch die Kombination der bodengebundenen GPS Messungen mit Flugzeug- und Satellitenaltimetrie wurde ein sehr genaues Höhenmodell der Region um das Ekstrømisen erstellt. Ältere Topographiedatensätze, welche ohne Bodenreferenzmessungen erstellt wurden, weisen deutliche Höhendifferenzen von bis zu 400 m gegenüber dem hier vorgestellten neuen Modell auf.

Description: SummaryRecently, a variety of gravity observations in Antarctica has become available through extensive e orts of airbornesurveys. Aircrafts serving as multi-instrumentation platforms provide measurements on gravity, bedrocktopography, ice surface topography and ice thickness. Collected datasets are valuable in terms of resolution andhomogeneity, which make them suitable for studying regional geoid determination in selected Antarctic regions.Within this context the German joint project VISA provided an excellent database for improving the regionalgeoid by combining gravity and topographic data from aerogeophysical observations with long-wavelength informationfrom global gravity eld models. Using the remove-compute-restore technique in conjunction withleast-squares collocation a regional geoid for Dronning Maud Land, East Antarctica, will be presented. A signalthreshold of up to 6 m added to the global model that was used as a basis can be expected. The accuracy ofthe regional geoid will be estimated to be at the level of 15 cm.Citation: J. Muller, S. Riedel, M. Scheinert, M. Horwath, R. Dietrich, D. Steinhage, H. Anschutz, W. Jokat(2007), RegionalGeoid and Gravity Field from a Combination of Airborne and Satellite Data in Dronning Maud Land, East Antarctica { OnlineProceedings of the 10th ISAES, edited by A.K. Cooper and C.R. Raymond et al., USGS Open-File Report 2007-xxx, ExtendedAbstract yyy, 1-4.IntroductionThe new datasets provided by the satellite missions CHAMP, GRACE and GOCE (to be launched by theend of 2007) enable a homogeneous determination of the gravity eld. Furthermore, in the polar regions icesurface heights could be determined in a similar quality by ICESat. These new satellite data shall be validatedand densi ed by the German joint project VISA (Validation, Densi cation and Interpretation of Satellite Datafor the Determination of Magnetic Field, Gravity Field, Ice Mass Balance and Structure of the Earth Crust inAntarctica, uitilizing Airborne and Terrestrial Measurements) of TU Dresden and AWI Bremerhaven.For this purpose western and central Dronning Maud Land (DML), East Antarctica, were chosen as areaof investigation. Airborne as well as terrestrial observation campaigns were carried out to provide appropriatedatasets on height and height changes, gravity and gravity changes, magnetics, glaciology and seismology. Incombination with the satellite data these measurements will be applied to yield more detailed models of thegravity eld and the regional geoid, of the crustal structure and litosphere dynamics and of the dynamics andmass balance of the Antarctic ice sheet in the working area.Observation campaignsBetween 2001 and 2005 four airborne observation campaigns and two terrestrial observation campaigns werecarried out in western and central DML in order to conduct geodetic and geophysical measurements (Fig. 1,left). The scienti c program of the aerogeophysical campaigns for the observation of the gravity eld, magnetic eld, ice surface height and ice thickness (Radio Echo Sounding (RES)) contains more than 350 ight-hourswith a line-spacing between 10 and 20 kilometers. The terrestrial eld work took place at two di erent areas,during the season 2003/04 at Schirmacher Oasis - Potsdam Glacier - Wohlthat Mountains and one year later(season 2004/05) at Heimefrontfjella - Kirwanveggen. GPS and seismometer stations on bedrock were installed,kinematic GPS pro les, relative gravimetry on ice and ground penetrating radar (GPR) measurements werecarried out as well as samplings of rn cores and snow pits (Anschutz et al., 2007; Anschutz et al., 2006;Scheinert et al., 2005; Nixdorf et al., 2004).Regional Geoid ImprovementCombining satellite observations from CHAMP and GRACE with terrestrial data, high-resolution models ofthe Earth gravity eld have been obtained. Latest examples of these combination models are EIGEN-CG03C, EIGEN-GL04C (Forste et al., 2005; Forste et al., 2006) and GGM02C (Tapley et al., 2005). In Antarctica, thedetermination of the global gravity eld is problematic becausen due to the remoteness (often inaccessibility)and harsh conditions the terrestrial gravity data coverage features very large gaps. Only for a few smallerregions ground-based or airborne measured gravity was included into the combination. In order to improve theterrestrial gravity coverage and to determine the Antarctic geoid, the IAG Commission Project 2.4 "AntarcticGeoid" (chaired by M. Scheinert) was set into action, which is closely linked to SCAR Expert Group on GeodeticInfrastructure in Antarctica (GIANT) project 3 "Physical Geodesy". An overview on the situation is given in(Scheinert, 2005), and the strategy of regional geoid improvement is discussed in (Scheinert et al., 2007b) for thePrince Charles Mountains region, East Antarctica (PCMEGA), as well as for Palmer Land, Antarctic Peninsula(Scheinert et al., 2007a).Within this context, the VISA observation campaigns de-Figure 2: Free-air Anomalies (preliminary resultswith a spatial resolution of 14 kilometers)scribed above provide an excellent database for the validationof the gravity eld and, more importantly, for the determinationand improvement of the regional geoid. Fig. 2 showspreliminary results for the free-air anomalies derived from airbornemeasurements over the western and central DML witha resolution of 14 kilometer (Riedel and Jokat, 2007). Comparedwith the subglacial topography (Fig. 3, left panel) thestrong correlation between these two datasets is clearly visible.The right panel of Fig. 3 shows the ice surface heightin the area of investigation. The datasets of Fig. 3 a ord toderive the ice-thickness, which will be needed in addition tothe subglacial topography for the computation of an improvedgeoid. The high resolution of these datasets make them muchmore suitable than BEDMAP data (Lythe et al., 2000), whichwere a valuable source of information prior to the VISA radarobservations in DML.Especially in Antarctica problems occur when satellite observationsfrom CHAMP and GRACE up to a certain spherical harmonic degree (typically 120) should be combined with terrestrial data. Geophysically extrapolated gravity anomalies do not necessaily reect the actualgravity eld in Antarctica, though they are inevitable to provide a globally complete data coverage neededfor the solution of the closed surface integrals. For this reason, shorter wavelength information (higher thanspherical harmonic degree 120) is unreliable for most Antarctic areas (Fig. 1, right). This evinces when comparingthe gravity anomalies from EIGEN-GL04C for a harmonic window (degrees 121 to 360) (Fig. 1, right)with the free-air anomalies derived from VISA airborne measurements (Fig. 2). While a higher correlation canbe seen near the coastline, it diminishes in the southern part of DML.For the calculation of the regional geoid the remove-compute-restore technique (RCRT) was applied, whichis discussed in detail e.g. in (Forsberg and Tscherning, 1997) and (Sjoberg, 2005) and which was also usedin the PCMEGA case (Scheinert et al., 2007b). In the remove step, a long-wavelength part (predicted by aglobal gravity eld model) and a short-wavelength part (predicted by topography) are removed from the originalgravity data. In the compute step, the obtained band-pass ltered gravity anomalies are transformed into geoidheights, using least-squares collocation in this study. Least-squares collocation o ers the advantage of providingerror estimates for the resulting geoid. After having carried out the compute step, the long-wavelength part andthe short-wavelength part are restored in the geoid. For the computations, we could make use of the programpackage GRAVSOFT (Forsberg et al., 2003; Tscherning, 1974), which o ers a variety of tools for the geodeticgravity eld modelling.ConclusionCombining gravity and topographic data from VISA aerogeophysical campaigns with a global gravity eldmodel a regional geoid for Dronning Maud Land, East Antarctica, will be presented. Studies in other regionsof Antarctica (Scheinert et al., 2007a; Scheinert et al., 2007b) have shown that a signal threshold of up to 6 mto the global gravity eld model that was used as a basis can be expected when comparing the improved geoidwith the global model up to spherical harmonic degree 120. The accuracy of the regional geoid is estimated tobe at the level of 15 cm. Considering the current data situation in Antarctica, the accuracy level of 1 dm is arealistic and appropriate goal for this area of the world. The data coverage in Antarctica will most likely besubject to major improvements when further airborne surveys are carried out. The International Polar Year2007/ 2008 provides a reasonable framework for international and interdisciplinary cooperation in that eld.SCAR-GIANT project 3 "Physical Geodesy" and IAG Commission Project 2.4 "Antarctic Geoid" work towardsthe goal of closing the gaps in the gravity data coverage and at improving the geoid in Antarctica.

Description: IntroductionThe evolution of Antarctica and the Antarctic Ocean is vital to understanding the growth and breakup of super continent Gondwana. The reconstruction models of Gondwana have been established by many authors using geophysical data set as well as geological data (e.g. Norton and Sclater, 1978). The area around Syowa Station, the Japanese Antarctic wintering Station in Lützow- Holm Bay, is considered to be a junction of Africa, India, Madagascar, and Antarctic continents from the reconstruction models of Gondwana. Therefore this area is a key to investigate the formation and fragmentation of Gondwana. However, the tectonic evolution is still speculative because geological evidence is limited to a few isolated outcrops and the coverage with geophysical surveys in this area is poor.Joint Japanese-German airborne geophysical surveys around Syowa Station had been conducted in January 2006 during the 47th Japanese Antarctic Research Expedition to reveal tectonic evolution of the area around Syowa Station. The observation lines are shown in Figure 1.Data The airborne geophysical surveys had been made along almost N-S observation lines with spacing of about 20 km. Ice radar measurements had been carried out onshore area and ice thickness data are obtained. Bed rock topography are estimated using RAMP surface elevation data set. Magnetic and gravity measurements had been conducted both onshore and offshore areas. Magnetic anomalies are determined after correcting diurnal geomagnetic variations at Syowa Station. Precise positions of the aircraft are determined using DGPS techniques and free-air gravity anomalies are also obtained. Those data are girded and plotted using GMT software (Wessel and Smith, 1998).ResultsThe results of bed rock topography, gravity and magnetic anomalies are shown in Figures 2, 3 and 4, respectively. Characteristic features possibly related to the tectonic evolution from the results are summarized as followings.. Large negative gravity anomalies are observed along the Shirase Glacier (A in Figure 3) and those almost correspond to deep bed rock topography. Two sets of positive and negative gravity anomalies are shown along ocean-continental transition (B in Figure 3). However, magnetic anomalies along ocean-continental transition indicate only one set. NW-SE trending positive magnetic anomalies are observed between 40°E and 43°E near Antarctic continental margin (A in Figure 4). Those almost correspond to the transitional zone from Amphibolite to Granulite faces in the Lützow-Holm Complex. NE-SW trending magnetic anomalies in offshore area possibly indicate magnetic anomaly lineations (B in Figure 4). Positive magnetic anomalies surrounded by negative ones are observed around Cape Hinode (C in Figure 4). DiscussionLarge negative gravity anomalies and deep bed topography along the Shirase Glacier (A in Figure 4) possibly indicate major geological boundaries. It has been inferred that the peak metamorphic grade of the Lützow-Holm Complex progressively increases in a southwestern direction from amphibolite-facies to granulite-facies conditions and higher metamorphic grade are observed near the Shirase Glacier (Hiroi et al., 1983). Therefore large negative gravity anomalies and deep bed topography along the Shirase Glacier most likely delineate southwestern boundary of the Lützow-Holm Complex.Characteristic magnetic anomaly features around Cape Hinode (C in Figure 5) may indicate an allochthonous unit in the Lützow-Holm Complex. The main orogenic activities of the Lützow-Holm Complex took place during the Latest Proterozoic ro Early Paleozoic times. However, older rocks around 1000 Ma were documented at Cape Hinode within the Lützow-Holm Complex (Shiraishi et al., 1994). Magnetic anomaly data will provide new constraints for constructing tectonic evolution model in this area.NE-SW trending magnetic anomalies (B in Figure 5) possibly represent M sequence magnetic anomaly lineations. ENE-WSW and E-W magnetic anomaly lineation trends, possibly belonging to the Mesozoic magnetic anomaly lineation sequence, accompanied by the NW-SE and NNW-SSE trending fracture zones are deduced from vector magnetic anomalies just seaward of the continental slope of Antarctica to the east of Gunnerus Ridge (Nogi et al. 1996). NE-SW trending magnetic anomalies show similar strikes of magnetic anomalies from vector magnetic anomalies. Two sets of positive and negative gravity along ocean-continental transition (B in Figure 4) possibly reflect initial breakup conditions of Gondwana. Magnetic anomalies along ocean-continental transition do not show two sets of positive and negative magnetic anomalies. However, possible magnetic anomaly lineation trends in this area are almost parallel to the trends of gravity anomalies along ocean-continental transition and those imply that the direction of initial extension are normal to present coast line of Antarctica in this area. Two sets of positive and negative gravity along ocean-continental transition may suggest initial extension of Gondwana breakup.ConclusionsThe outline of ice thickness, bed rock topography, gravity and magnetic anomaly results in the area around Syowa Station are shown. Characteristic features from the results are indicated and discussed. These results provide new constraints on the tectonic evolution in the area a junction of Africa, India, Madagascar, and Antarctic continents of Gondwana. Further data analysis are carrying out and detailed discussion will be made based on those results.

Description: The area around Syowa Station, the Japanese Antarctic wintering Station in Lützow-Holm Bay, is a key area to investigate the formation of Gondwana, because this area is considered to be a junction of Africa, India, Madagascar, and Antarctic continents from the reconstruction model of Gondwana. However, the tectonic evolution is still speculative because geological evidence is limited to a few isolated outcrops and the coverage with geophysical surveys in this area is poor. Joint Japanese-German airborne geophysical surveys around Syowa Station had been conducted in January 2006 during the 47th Japanese Antarctic Research Expedition to reveal the tectonic evolution related to Gondwana formation and breakup in this area. Ice radar, magnetic, and gravity data are obtained using the AWI owned, Dornier aircraft (Polar-2). The airborne geophysical surveys had been made along almost N-S observation lines with spacing of about 20 km.Several characteristic features possibly related to the tectonic evolution of Gondwana are inferred from magnetic and gravity anomaly maps as well as bedrock topography. Large negative gravity anomalies are observed along the Shirase Glacier and those almost correspond to deep bed rock topography. This structure most likely delineate southwestern boundary of the Lützow-Holm Complex. Northeastern boundary of the Lützow-Holm Complex is also deduced from magnetic and gravity anomalies. Moreover, Lützow-Holm Complex seems to be divided into three segments by the boundaries with almost ENE-WSW strike. These structures may reflect tectonic movements of the post collision. Positive magnetic anomalies surrounded by negative ones are also observed around Cape Hinode within the Lützow-Holm Complex. These data will provide new constraints for constructing tectonic evolution model in this area.