The central part of the Filchner-Ronne Ice Shelf, Antarctica: internal structures revealed by 40MHz monopulse RES

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Abstract
About 1200 km of surface-based radio-echo sounding (RES) profiles were measured during the German Antarctic field season 1989-90 on the Filchner Ronne Ice Shelf (FRIS), Antarctica, with a 40 MHz monopulse sounder developed in Münster. In the area investigated, downstream of Doake Ice Rumples and Henry Ice Rise, a basal layer of marine ice up to 400 m thick was found below the meteoric ice, which is less than 100 m thick in some places. High absorption losses and low bottom reflectivity of the basal layer prevented sounding of the total ice thickness, except for areas near the ice edge with strong melting. From common mid-point (CMP) measurements, the velocities of electromagnetic waves in the ice shelf were determined, to obtain precise depths from travel times for airborne and surface-based soundings. The continuous profiling yielded the total ice thickness in the northern part and a variety of internal structures, including lavers in the meteoric and the marine ice, remnants of crevasses, varying reflections of the meteoric-marine interface reflection (MMR). From these data, it is shown that the most-disturbed MMR signatures originate at grounding zones at Doake lee Rumples and at both sides of Henry Ice Rise. The status of the meteoric ice bottom is preserved by the formation of marine ice. Hence, structures remain widely unchanged over 400 km of flow. Internal features that can be correlated on parallel profiles perpendicular to the flow provide an independent means for the construction of flow lines. Even in areas with a smooth MMR, reflection amplitudes show variations by a factor of 2. Their spatial frequency is maximal on profiles prependicular to the now direction. A possible explanation for the amplitude variations is the varying salinity of the top of the marine ice.