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

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. Ill 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. All improved scanning capacitor for high-resolution DEP and a gamma-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 ill 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. Dcnsity-bascd velocities differ front the DEP velocities by <1%. 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 oil these comparisons, we evaluate the accuracy with which the depth of electromagnetic reflectors observed in common-offset profiles call be determined, and discuss reasons for the observed differences between CMP- and core-based profiles. Moreover, We compare the errors determined front 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 1 useful technique to determine the depth of radar reflectors in combination with common-offset measurements, especially oil a region-wide basis.