Description: Ice core records and ice-penetrating radar data contain complementary information on glacial subsurface structure and composition, providing various opportunities for interpreting past and present environmental conditions. To exploit the full range of possible applications, accurate dating of internal radar reflection horizons and knowledge about their constituting features is required. On the basis of three ice core records from Dronning Maud Land, Antarctica, and surface-based radar profiles connecting the drilling locations, we investigate the accuracies involved in transferring age-depth relationships obtained from the ice cores to continuous radar reflections. Two methods are used to date five internal reflection horizons: (1) conventional dating is carried out by converting the travel time of the tracked reflection to a single depth, which is then associated with an age at each core location, and (2) forward modeling of electromagnetic wave propagation is based on dielectric profiling of ice cores and performed to identify the depth ranges from which tracked reflections originate, yielding an age range at each drill site. Statistical analysis of all age estimates results in age uncertainties of 5 10 years for conventional dating and an error range of 1 16 years for forward modeling. For our radar operations at 200 and 250 MHz in the upper 100 m of the ice sheet, comprising some 1000 1500 years of deposition history, final age uncertainties are 8 years in favorable cases and 21 years at the limit of feasibility. About one third of the uncertainty is associated with the initial ice core dating; the remaining part is associated with radar data quality and analysis.

Description: Measurements of ice thickness on the Antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database. From these data, a seamless suite of digital topographic models have been compiled for Antarctica and its surrounding ocean. This includes grids of ice thickness over the grounded ice sheet and ice shelves, water column thickness beneath the floating ice shelves, bed elevation beneath the grounded ice sheet, and bathymetry to 60°S, including the sub-ice-shelf cavities. These grids are consistent with a recent high-resolution surface elevation model of Antarctica. While the digital elevation models have a nominal spatial resolution of 5 km, such high resolution is justified by the original data density only over a few parts of the ice sheet. The suite does, however, provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps. The toal volume of the Antarctic ice sheet calculated from the BEDMAP grid is 25.4 million km3, and the total sea level equivalent, derived from the amount of ice contained within the grounded ice sheet, is 57 m, comprising 52 m from the East Antarctic ice sheet and 5 m from the West Antarctic ice sheet, slightly less than earlier estimates. The gridded datasets can be obtained from the authors.

Description: In 1996 and 1997 an airborne RES survey was flown in the context of the North Greenland IceCore Project (NGRIP). At 75.10 N and 42.30 W a deep ice core is being drilled forpalaeoclimate studies with the objective to recover an undisturbed Eemian sequence. The present survey was specially designed to determine with high lateral resolution the bedtopography and the layering of internal reflectors in the vicinity around the drill site. Atotal of 19,000 km of profiles were flown yielding a rectangular flight pattern with sidelengths of 222 km respectively 210 km on 16 flights. The spacing between two flight tracksis about 10 km for most of the area and about 2.5 km in the center of the grid. Detailed mapsof ice thickness and subglacial topography have been produced. Moreover a data set was acquired that enables an independent stratigraphic control on the ice core record of NGRIP.From the map of subglacial topography and from selected profiles the bed of NGRIP can be denoted as perfectly flat particularly when compared to the region of GRIP/GISP2. There is noobvious reason, why flow disturbances eventually might have altered the stratigraphic orderin the lower part of the ice sheet at least up to 30 km upstream NGRIP. On the basis of thepresent study NGRIP seems to be an ideally chosen drill site in order to recover an undisturbed Eemian sequence.

Description: During five austral summers, from 1994/95 until 1998/99, the Alfred Wegener Institute (AWI)carried out a large airborne radio echo sounding (RES) survey in Dronning Maud Land (DML),Antarctica. These ice thickness measurements are part of the AWI contribution to the pre-site survey for a deep ice core drill site in DML within the European Project for Ice Coringin Antarctica (EPICA). The survey encompasses more than 90,000 km RES profiles over DML and the adjacent coastal area, covering more than 1 million km^2. The lower boundary ofthe ice sheet could be determined area-wide. Internal horizons occuring in the upper two thirds of the ice column can also be traced for several hundred kilometers. This work presents the latest maps of the subglacial topography of the investigated area as well as of an internal horizon.

Description: Successful simulation of ground penetrating radar (GPR) traces in polar ice is achieved bynumerical finite-difference time-domain (FDTD) forward modeling.Properties of the modeled medium are taken from high resolution dielectric profiling (DEP)of the upper 100~m of an ice core from Dronning Maud Land, Antarctica.The GPR reference trace is calculated from stacking of a normal moveout correctedcommon-midpoint survey, carried out near the borehole location.The excellent agreement of synthetic and GPR-based results demonstrates the capability ofFDTD models to reproduce radargrams from ice core properties forinterpretation of radio echo sounding data, and emphasizes the exploitation of radar datafor improved interpretations of glaciological climate proxys.In addition to the presentation of modeling results, we perform sensitivity experiments toinvestigate the nature and origin of radar reflection in ice,discuss reasons for the failure of modeling studies in the past, and indicate new approaches.

Description: The investigation of glaciers and ice sheets by means of ice penetrating radar has become one of the most commonly used geophysical techniques in glaciology.Although many different applications utilise internal reflection horizons, assuming that an individual reflector is isochronous, open questions concerningthe different reflection mechanisms remain.We demonstrate successful simulation of ice penetrating radar traces in polar ice by numerical finite-difference time-domain forward modeling.Based on a combined analyses of ice core records, radar surveys, and numerical sensitivity studies, we are able to provide better insights into the origin ofelectromagnetic reflections.By means of high resolution dielectric profiling (DEP) the physical properties of the upper 100 m of an ice core from Dronning Maud Land, Antarctica, aredetermined.As the considered medium is an ice-air composition, the real and imaginary partsof the constituents are subject to complex-valued mixing to form the measuredcomplex dielectric constant $\epsilon$.In contrast to earlier assumptions, correct frequency-scaling of $\epsilon$ requires complex decomposition of the dielectric constant at the 250~kHz DEP frequency to yield the frequency-independent properties, i.e.~density and conductivity,and has to be composed again at the 200 MHz radar frequency.Forcing of the numerical model with differently scaled and altered DEP data setsreveals the role of the initial physical parameters for the formation ofreflection horizons and enlights their isochronic characteristics, an importantapproach for interpreting internal reflection horizons.

Description: The accuracy of the traveltime-velocity and traveltime-depth profile derived from ground-penetrating radar (GPR) common-midpoint (CMP) surveys at different frequencies is investigated for the first time ever by direct comparison with the profile calculated from high resolution dielectric-profiling (DEP) ice core data.In addition, we compare two traveltime profiles calculated from ice core density data by means of different dielectrical mixture models with the DEP based profile.CMP surveys were carried out at frequencies of 25, 50, 100 and 200 MHz near the new European deep drilling site DML05 in Dronning Maud Land, Antarctica, during the 1998/99 field season.An improved scanning capacitor for high resolution DEP and a Gamma-densiometer for density measurements were used to determine thecomplex dielectric constant and the density at 5 mm increments along the ice core B32, retrieved in 1997/98 at DML05.The comparisons with DEP and density based velocity series show that the CMP velocity series are slightly higher, but asymptotically approach the core based velocities with depth.Root-mean-square differences of the DEP velocity series range between 8% for the 25 MHz CMP and 2% in the case of the 200 MHz survey.Density based velocities differ from the DEP velocities by less than 1%.The traveltime-depth series calculated from the interval velocities show a better agreement between all series than the velocity series.Differences are between 5.7 and 1.4% for the 25 and 200 MHz CMP measurements, and less than 0.6% for the density data.Based on these comparisons we evaluate the accuracy with which the depth of electromagnetic reflectors observed in common-offset profiles can be determined and discuss reasons for the observed differences between CMP- and core based profiles.Moreover, we compare the errors determined from the field measurements with those estimated from GPR system characteristics to provide a measure that can be used to estimate the accuracy of GPR analyses for the planning of GPR campaigns.Our results show that CMP surveys are a useful technique to determine the depth of radar reflectors in combination with common-offset measurements, especially on a region-wide basis.