Ice-internal and sedimentary structures in the Ekströmisen grounding line region detected with multi-offset seismics

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Hofstede, C. , Uenzelmann-Neben, G. , Nixdorf, U. , Eisen, O. and Kulessa, B. (2009): Ice-internal and sedimentary structures in the Ekströmisen grounding line region detected with multi-offset seismics , 69. Jahrestagung der Deutschen Geophysikalischen Gesellschaft, 23.-26.3.2009, Kiel. .
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Ekströmisen is a small catchment area in coastal Dronning Maud Land, Antarctica,terminating in the Ekström ice shelf, which is bounded by a narrow embaymentformed by two ice ridges. A seismic survey has been performed alonga flow line on Ekströmisen over about 22 km, crossing the grounding line betweenicesheet and shelf approximately midway of the profile. The measurementswere performed with explosives in shallow firn holes as seismic sources and a60 channel 1.5 km snow streamer for data recording. The data has been resortedto form a virtual 120 channel 3 km streamer, consisting of 150 shots. Themaximumshot-receiver offset is thus about three times larger then the ice thickness,yielding wide angle information for intra-ice and bedrock reflections. Standardseismic data processing yields 862 common depth points in total, with an incrementof 25 m. This provides a 20-fold coverage of each commondepth point. Inaddition to yielding the distribution of seismic velocity within the firn, ice andsediment, the data clearly images ice and sedimentary layers. Within the bottompart of the ice, a number of continuous internal layers are visible upstream of thegrounding line. Currently, our favorite explanation is abrupt changes in the crystalorientation fabric caused by a combination of laterally compressional flow andvertical shear, as observed with radio-echo sounding at other places in Antarctica.Upstream of and at the grounding line, structures are visible within the bedrock,which we interpret as sedimentary deposits related to glacial activity. The dataprovide the base for interpretations of the ice-dynamic and sedimentary processesoccurring in the basal ice layer and at the ice-bedrock boundary, of relevance forfurther understanding details of the ice sheet-to-shelf transition area.

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