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Electromagnetic Sounding of Bottom Crevasses on the Ross Ice Shelf, Antarctica *

  • Kenneth C. Jezek (a1), Charles R. Bentley (a1) and John W. Clough (a2)

Abstract

During the 1976—77 season of the Ross Ice Shelf Geophysical and Glaciological Survey, a series of vertical electromagnetic sounding profiles of subsurface features was completed at station J-9. The survey comprised three five-kilometer north-west-south-east profiles separated by one kilometer and six two-kilometer north-east-south-west profiles, and was carried out on the surface using 35 MHz and 50 MHz radar systems. Folded-dipole antennae were used and oriented to detect reflectors both along and perpendicular to the profile path. This was done to facilitate the interpretation of the data, which indicated a complex system of bottom crevasses. Measurements of the positions, heights, and shapes of these crevasses showed at least two sets of crevasses varying in both strike and size. The larger crevasses, about 120 m high and oriented more or less normal to the flow direction, are probably associated with the movement of ice stream B across the grounding line between the West Antarctic ice sheet and the Ross Ice Shelf. A satisfactory explanation for the secondary set of crevasses, about 60 m high and forming an angle of 60° ±10° with the first set, has not yet been found.

Résumé

Au cours de la saison 1976—77 de l’Expédition glaciologique et géophysique du Ross Ice Shelf, on a mené à bien à la station J-9 une série de sondages électromagnétiques verticaux pour explorer la zone sous la surface. Le programme comprenait trois profils de cinq kilometres de direction Nord-Ouest Sud-Est espacés d’un kilometre et six profils Nord-Est Sud-Ouest de 2 km, qui ont été exécutés depuis la surface avec des radars de 35 MHz et de 50 MHz. On a utilisé des antennes dipoles repliées et orientées de manière à détecter les réflecteurs aussi bien perpendiculaires que parallèles à la direction des profils. Ceci afin de faciliter l’interprétation des résultats qui indiquent un système complexe de crevasses de fond. Les mesures des positions hauteur et forme de ces crevasses ont montré au moins deux catégories de crevasses différentes à la fois par la direction et par la dimension. Les plus grandes crevasses d’environ 120 m de haut et orientées plus ou moins perpendiculairement à la direction de l'écoulement sont probablement associées au mouvement du courant de glace B à travers la ligne au sol séparant la calotte Ouest Antarctique et la Ross Ice Shelf. On n’a pas encore trouvé une explication satisfaisante pour la seconde catégorie de crevasses qui ont environ 60 m de haut et forment un angle de 60° ± 10° avec les crevasses du premier type.

Zusammenfassung

Während der Sommerkampagne 1976—77 des Ross Ice Shelf Geophysical and Glaciological Survey wurde eine Serie von elektromagnetischen Tiefenprofilen zur Sondierung von Unter-grundserscheinungen bei Station J-9 aufgenommen. Die Aufnahme umfasste drei, 5 km lange, NW–SE-gerichtete Profile im Abstand von je 1 km und sechs, 2 km lange Profile von NE nach SW; sie wurde an der Oberfläche mit 35 MHz- und 50 MHz-Radargeräten durchgeführt. Es wurden gekreuzte Dipolantennen benutzt und so ausgerichtet, dass Reflexionen sowohl in Profilrichtung wie senkrecht dazu empfangen werden konnten. Dies sollte die Interpretation der Daten erleichtern, die ein komplexes System von Spalten an der Unterseite vermuten liessen. Die Bestimmung der Lage, der Höhe und der Form dieser Spalten ergab zumindest 2 Systeme von Spalten, die sich sowohl in der Richtung wie in der Crösse unterscheiden. Die grösseren Spalten, die 120 m hoch und mehr oder weniger genau senkrecht zur Fliessrichtung orientiert sind, sind vermutlich mit der Bewegung des Eisstroms B verbunden, der die Aufsctzlinie zwischen dem westantarktischen Eis und dem Ross Ice Shelf überquert. Für das System sekundärer Spalten, die 60 m hoch sind und mit dem ersten System einen Winkel von 60°±10° bilden, wurde noch keine befriedigende Erklärung gefunden.

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References

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