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.

Internal characteristics of ice-marginal sediments deduced from georadar profiling and sediment properties (Brøgger Peninsula, Svalbard)

“…For completeness the snow layer has been added to the model. The ice layer is expressed by a commonly measured value of ε r = 2.8 (Eisen et al, 2002). The trace comparison shows a fairly good matching of the +-+ amplitude event at 50 ns in both the field and the synthetic wavelet at the relevant depth (Fig.…”

Constraining variable density of ice shelves using wide-angle radar measurements

“…The disadvantage of this low frequency is that fewer reflectors above the firn-ice transition can be picked at this low resolution, relative to higher-frequency data sets (cf. Eisen et al, 2002 who derived an 8 % velocity error with a 25 MHz radar versus a 2 % error with 200 MHz radar). We found that the method applied here can cope with the picking uncertainties at 10 MHz, whereas using Dix inversion frequently resulted in interval densities much larger than the pure ice density.…”

“…Murray et al, 2000;Winebrenner et al, 2003;Hempel et al, 2000;Eisen et al, 2002;Bradford et al, 2009;Blindow et al, 2010). Alternatively, only the receiver can be moved ( Fig.…”

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

“…Comparison of wave speed-depth profiles derived from CMP survey data and ice-core properties from polar regions show that the agreement in a flat area can be within a couple of percent (Eisen et al, 2002). The larger discrepancies in our case, which are on the order of 10%, are probably related to dipping of internal reflectors with respect to the surface, and to curved ray paths, which are neglected in the small spread approximation used here (Yilmaz, 1987).…”

“…As wave speed profiles derived from different physical properties of ice cores (e.g. γ -ray absorption or permittivity) are more consistent than those based on CMP surveys (Eisen et al, 2002), we use the TWTdepth relations from KCS and KCH for migration and conversion of the radargrams to the depth domain.…”