Seismic investigation of the Larsen Ice Shelf, Antarctica: in search of the Larsen Basin

Abstract

Seismic reflection surveys were carried out over the Larsen Ice Shelf to examine the extent of the observed sedimentary sequences of the Larsen Basin as suggested by aeromagnetic and gravity data. The surveys were carried out with a small team of six, working from Skidoo motor toboggans and Nansen sledges. Charges of up to 8 kg were fired in hot-water drilled holes up to 9 m deep and 6 sec records made by a 48 channel TI DFS V system with a 4 ms sample interval. By towing a 2.4 km cable behind a Skidoo it was possible to obtain 2.4 km of 24 fold data per day. The reflection data were supplemented by shallow refraction surveys using a 12 channel Nimbus seismograph and by a 12 km expanding spread experiment. The refraction data gave velocities of 1305 ± 20 m s⁻¹ for surface snow and 3154 m s⁻¹ for the top 100 m of shelf ice. The 24 km of reflection data showed high seismic velocities with weak shallow reflectors, characteristics which are quite different from the known basin fill on James Ross Island. It is concluded that the surveys were done outside the basin and that the depth to basement estimates made from the aeromagnetic data do not provide a reliable guide to the extent of the basin.