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Seismic Short-Refraction Studies on the Ross Ice Shelf, Antarctica *

  • Joseph F. Kirchner (a1) and Charles R. Bentley (a1)

Abstract

Seismic short-refraction studies were carried out at five stations on the Ross Ice Shelf during the 1976–77 summer season as part of the comprehensive Ross Ice Shelf Geophysical and Glaciological Survey. Measurements of the velocities of compressional waves were made at each location. Compressional wave velocities were measured along more than one azimuth at three sites, and shear wave velocities (both components) at two. Travel-time curves were fitted to an exponential expression by means of a non-linear least-squares regression technique. The errors in the apparent velocities are estimated to be about ±50 m s–1 at short distances, diminishing to about ±10 m s–1 near the ends of the profiles. Compressional-wave velocities show only slight variations with azimuth and only over certain depth intervals, showing that constant-velocity surfaces are essentially horizontal. Shear-wave velocities, however, exhibit large variations according to azimuth and polarization, indicating that transverse isotropy is violated at least in the upper 30–40 m of the ice shelf. It is believed that the anisotropy is caused by structural details in the firn perhaps modified by preferred crystal orientation and that it may arise at least partly from anisotropic stresses in the ice shelf.

Résumé

On a exécuté des études de réfraction sismiques courtes en cinq stations du Ross Ice Shelf au cours de la saison d’été 1976–77 dans le cadre des expéditions glaciologiques et géophysiques. En chaque point on a mesuré les vitesses des ondes de compression. On a mesuré ces vitesses des ondes de compression selon plusieurs azimuths en trois sites et les vitesses des ondes de cisaillement (deux composantes) en deux sites. Les courbes distance parcourue-temps ont été ramenées à une expression exponentielle par une technique de régression aux moindres carrés non linéaire. Les erreurs dans les vitesses apparentes sont estimées à ±50 ms–1 à de courtes distances, diminuant jusqu’ à ±10 m s–1 près de l’extrémité des profils. Les vitesses des ondes de compression ne montrent que de faibles variations avec l’azimuth et seulement pour certains intervalles de profondeurs, ce qui montre que les surfaces de vitesse constante sont essentiellement horizontales. Les vitesses des ondes de cisaillement cependant, montrent de larges variations selon l’azimuth et la polarisation, ce qui indique que l’isotropie transversale n’existe pas au moins dans les 30 ou 40 m supérieurs de la calotte. On pense que cette anisotropic provient de détails de structure dans le névé qui peuvent être modifiés par une orientation préférentielle des cristaux et que ceci peut se produire au moins partiellement à cause d’efforts anisotropes dans la calotte glaciaire.

Zusammenfassung

Während der Sommerkampagne 1976–77 des umfassenden Ross Ice Shelf Geophysical and Glaciological Survey wurden seismische Refraktionsstudien auf kurze Distanzen an fünf Stationen durchgeführt. An jedem Punkt wurde die Geschwindigkeit der Kompressionswellen gemessen. Auf drei Stationen wurden diese Geschwindigkeiten in mehr als einem Azimut bestimmt, die Geschwindigkeit der Scherwellen in beiden Komponenten auf zwei Stationen. Die Laufzeitkurven wurden mit Hilfe einer nichtlinearen Regressionstechnik nach der Methode der kleinsten Quadratsumme einer Exponentialfunktion angepasst. Die Genauigkeit der scheinbaren Geschwindigkeiten kann bei kurzen Distanzen auf etwa ±50 m s–1 abgeschätzt werden; sie steigt nahe den Profilenden auf ca. ±10 m s–1. Die Geschwindigkeit der Kompressionswellen schwankt mit dem Azimut nur geringfügig und nur bei gewissen Tiefenintervallen, womit sich erweist, dass die Flächen konstanter Geschwindigkeit im wesentlichen horizontal sind. Dagegen zeigen die Geschwindigkeiten der Schwellen beträchtliche Schwankungen je nach Azimut und Polarisation, was auf eine transversale Anisotropie zumindest in den oberen 30–40 m des Schelfeises hinweist. Sie wird auf strukturelle Einzelheiten im Firm, vielleicht in Form von bevorzugten Kristallorientierungen, und zumindest teilweise auf anisotropen Druck im Schelfeis zurückgeführt.

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References

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