Uwe Nixdorf

Electromagnetic wave speed in polar ice: validation of the common-midpoint technique with high-resolution dielectric-profiling and γ-density measurements

Abstract The accuracy of the travel-time–velocity and travel-time–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 travel-time 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 γ-densiometer for density measurements were used to determine the complex 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 51 %. The travel-time–depth series calculated from the interval velocities show a better agreement between all series than the velocity series. Differences are 5.7–1.4% for the 25 and 200 MHz CMP measurements, and <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.

New maps of the ice thickness and subglacial topography in Dronning Maud Land, Antarctica, determined by means of airborne radio-echo sounding

Since the austral summer 1994/95 the Alfred Wegener Institute (AWI) hascarried out airborne radio echo sounding measurements (RES) in Antarctica withits newly designed RES system. Since 1995/96 an ongoing presite survey foran ice coring drill site in Dronning Maud Land (DML) has been carried out.This survey is part of the EPICA programme (European Project for Ice Coring inAntarctica). It covers an area of 948000 km2 with more than 49500 km ofairborne RES obtained from more than 200 hours of flight operation flown from1994-1997. Within this paper first results of the airborne RES survey will begraphically summarized as newly derived maps of the ice thickness and thesubglacial topography as well as a 3D view of surface and subglacial bed andoutcrop topography, revealing a total icevolume of 1.48 million km3.

Alpine ice cores and ground penetrating radar: combined investigations for glaciological and climatic interpretations of a cold Alpine ice body

Accurate interpretation of ice cores as climate archives requires detailed knowledge of their past and present geophysical environment. Different techniques facilitate the determination and reconstruction of glaciological settings surrounding the drilling location. During the ALPCLIM1 project, two ice cores containing long-term climate information were retrieved from Colle Gnifetti, Swiss-Italian Alps. Here, we investigate the potential of ground penetrating radar (GPR) surveys, in conjuction with ice core data, to obtain information about the internal structure of the cold Alpine ice body to improve climatic interpretations. Three drill sites are connected by GPR profiles, running parallel and perpendicular to the flow line, thus yielding a three-dimensional picture of the subsurface and enabling the tracking of internal reflection horizons between the locations. As the observed reflections are of isochronic origin, they permit the transfer of age—depth relations between the ice cores. The accuracy of the GPR results is estimated by comparison of transferred timescales with original core datings, independent information from an older ice core, and, based on glaciological surface data, findings from flow modeling. Our study demonstrates that GPR is a mandatory tool for Alpine ice core studies, as it permits mapping of major transitions in physical-chemical properties, transfer of age—depth relations between sites, correlate signals in core records for interpretation, and establish a detailed picture of the flow regime surrounding the climate archive. Environmental and Climate Records from High Elevation Alpine Glaciers.

Spatial distribution of surface mass balance on Amundsenisen plateau, Antarctica, derived from ice-penetrating radar studies

Abstract The distribution of surface mass balance on Amundsenisen, Dronning Maud Land, Antarctica, is investigated along a continous profile line. Ice-penetrating radar is used to map variations in ice-layer thickness within the upper 100 m of the ice sheet. The route passes several firn- and ice-core drilling sites over a distance of 320 km. Dielectric-profiling data of ice cores are used to calculate the depths of selected reflection horizons and the cumulative mass of the ice column. The local surface mass balance is determined as a temporal average, covering a time-span of almost two centuries. The findings indicate a complex accumulation pattern superimposed on a generally low surface mass balance, which is related to small-scale surface undulations. The results of the radar soundings are in general in good agreement with surface mass-balance data derived from firn-core studies. Discrepancies between these two datasets can be explained by spatial mismatch or by minor quality of either ice-core profiles or radar data. For regional comparison of radar-based accumulation data we use an accumulation distribution interpolated from point measurements. The surface mass balance varies up to 50% over short distances, with correlation lengths of <10 km. We conclude that the current utilization schemes of point sampling are only capable of reproducing local values and regional trends but provide no information on the small-scale variability of surface mass balance.

Identifying isochrones in GPR profiles from DEP-based forward modeling

Abstract Isochronic continuous horizons between 20 and 90 m depth in a ground-penetrating radar (GPR) profile, recorded in Dronning Maud Land, Antarctica, are identified by comparison of synthetic and measured single radar traces. The measured radar-gram is derived from a stacked GPR profile; the synthetic radargram is computed by convolution of the complex reflection coefficient profile, based on dielectric profiling (DEP) data of a 150 m ice core, with a depth-invariant wavelet. It reproduces prominent reflections of the measured radargram to a considerable degree. Analyzing matching peaks in both radargrams enables us to identify isochronic reflections and transfer individual volcanic-event datings to the GPR profile. Reflections are primarily caused by changes in permittivity; changes in conductivity are of minor importance. However, several peaks in permittivity andconductivity show a good correlation and indicate that some reflections are related to acidic layers. The results demonstrate the possibility of reproducing radargrams from ice-core property profiles, a necessary step for the interpretation of remotely sensed radar data and the general significance of connecting ice-core and radar data for correct interpretations. Problems related to forward modeling, data gaps, origin of permittivity peaks, and GPR profiles used for comparison, are discussed.

Subglacial topography and internal structure of central and western Dronning Maud Land, Antarctica, determined from airborne radio echo sounding

Abstract During five austral summers, from 1994/1995 until 1998/1999, 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 Coring in Antarctica (EPICA). The survey encompasses more than 90,000 km of RES profiles over DML and the adjacent coastal area, covering more than 1 million km 2 . The lower boundary of the ice sheet could be determined area-wide. Internal horizons occurring 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.

Characteristics of accumulation around the EPICA deep-drilling site in Dronning Maud Land, Antarctica

Abstract Based on ground-penetrating radar profiles, we analyze area-wide spatial and temporal characteristics of accumulation rate in the vicinity of the EPICA (European Project for Ice Coring in Antarctica) deep-drilling site in Dronning Maud Land, Antarctica (EDML). An area of 1600km2 is covered by 500km of radar profiles, organized in a star-like pattern with eight 20–25km legs and a 10km grid with 1–3km spacing, each pattern being centred on the EDML drilling location. Distributions of density, cumulative mass, age and the electromagnetic wave speed with depth are available from physical ice-core records. Nine internal reflection horizons are continuously tracked within the upper 110m of ice over the whole area, yielding a spatial picture of accumulation rate history over >1000 years. The mean accumulation rate over the last 153 years varies between 50 and 75 kgm–2 a–1 over 50 km perpendicular to the ice divide; the spatial average is ~61 kgm–2 a–1. This general pattern is overlain by small-scale variations of accumulation rate on the order of 10% of the mean. Maximum local gradients in accumulation rate are ~2–3 kgm–2 a–1 km–1, about five times the regional accumulation rate gradient. Comparison of topography and accumulation rate along a 20 km profile in the direction of the mean winds indicates that variations in accumulation rate over short distances are linked to surface undulations. Our results show that advected spatial variations of accumulation rate are on the same order and even exceed temporal changes over the investigated periods. Ice flow and upstream effects therefore might influence accumulation rates reconstructed from the EDML ice core.

Investigations of the mass balance of the southeastern Ronne Ice Shelf, Antarctica

Data from the Filchner V campaign were used to investigate the mass-balance conditions in the southeastern Ronne Ice Shelf (RIS), Antarctica. Radio-echo sounding and seismic measurements over this area show a maximum ice thickness of >2000 m close to the grounding line of Foundation Ice Stream. The measurements also revealed that the position of this grounding line is 40 km further south than previously thought. New mass-flux calculations result in an estimate or 51 km^3 a^-1 for the ice-stream transport from the ice sheet into the eastern ice shelf. The Mollereisstrom (MES), west of Foundation Ice Stream, shows a maximum ice thickness of 1100-1200 m in the grounding line area and a mass flux of 23 km^3 a^-1.Assuming steady-state conditions, mass-balance calculations based on the new data result in a mean melt rate of about 1 m a^-1 at the ice-shelf base for the entire southeastern part of the RIS. The melt rate in the grounding-line area of Foundation Ice Stream exceeds 9 m a^-1. In contrast, other ice streams draining into the Filchner-Ronne Ice shelf show maximum melt rates from 1-2 m a^-1 (MES) to 4 m a^-1 (Rutford Ice Stream). Our calculations indicate that nearly all of the ice deposited in the drainage area of the eastern RIS on the ice sheet does not reach the ice-shelf front as original meteoric ice, but is melted at the ice-shelf base.

Internal structure of the ice sheet between Kohnen station and Dome Fuji, Antarctica, revealed by airborne radio-echo sounding

Abstract This study aims to demonstrate that deep ice cores can be synchronized using internal horizons in the ice between the drill sites revealed by airborne radio-echo sounding (RES) over a distance of >1000km, despite significant variations in glaciological parameters, such as accumulation rate between the sites. In 2002/03 a profile between the Kohnen station and Dome Fuji deep ice-core drill sites, Antarctica, was completed using airborne RES. The survey reveals several continuous internal horizons in the RES section over a length of 1217 km. The layers allow direct comparison of the deep ice cores drilled at the two stations. In particular, the counterpart of a visible layer observed in the Kohnen station (EDML) ice core at 1054 m depth has been identified in the Dome Fuji ice core at 575 m depth using internal RES horizons. Thus the two ice cores can be synchronized, i.e. the ice at 1560 m depth (at the bottom of the 2003 EDML drilling) is ∼49ka old according to the Dome Fuji age/depth scale, using the traced internal layers presented in this study.

Spatial depth distribution of the subglacial bed and internal layers in the ice around NGRIP, Greenland, derived with airborne RES

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.