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Investigation of Polar Snow Using Seismic Velocity Gradients *

  • James D. Robertson (a1) and Charles R. Bentley (a1)

Abstract

Compressional wave velocity gradients at 43 of 50 Antarctic traverse stations plot as sequences of straight lines on semilogarithmic graph paper. Intersections of the lines appear to correlate with depths at which the predominant metamorphic mechanism in polar snow changes. The seismie pattern supports a three-layer interpretation of snow densification. The base of the upper layer (8.4±12.3 m) corresponds to the “critical depth” of Anderson and Benson (1963) at which snow grains settle into a “random closepacked” arrangement. The base of the lower layer may correspond to the firn: ice transition depth, but more data are needed to confirm this conclusion. It is unclear what densification phenomenon is marked by the base of the middle layer (27.7±4.4 m). The distinction between the middle and lower layers tends to disappear and the velocity gradient at a fixed depth increases as mean annual accumulation decreases.

Les gradients de vitesse de propagation des ondes de pression dans 43 des 50 stations de la traversée de l’Antarctique, dessinent des suites de lignes droites sur un papier semilogarithmique. Les intersections des lignes semblent être liées aux profondeurs auxquelles change le mécanisme prédominant de métamorphose dans la neige polaire. Le comportement sismique suggére une interprétation du tassement de la neige comme un phénomène à trois niveaux. La base du niveau supérieur (8,4±2,3 m) correspond à la profondeur critique de Anderson et Benson (1963) à laquelle les grains de neige se tassent dans l’arrangement aléatoire le plus compact. La base du niveau inférieur correspond peut-être à la limite névé-glace, mais on doit avoir plus de données pour confirmer cette conclusion. On ne peut dire definitivement à quel processus de densification correspond la base du niveau médian (27,7±4,4 m). La distinction entre les milieux median et inférieur tend à disparaître et le gradient de vitesse à une profondeur donnée s’accroît lorsque l’accumulation annuelle moyenne diminue.

Zusammenfassung

Die Geschwindigkeitsgradienten von Kompressionswellen an 43 von 50 Stationen in antarktischen Profilen ergeben, auf halblogarithmischem Papier aufgetragen, eine Folge von geraden Linien. Die Schnittpunkte der Geraden scheinen mit den Tiefen, in denen sich der vorherrschende metamorphische Mechanismus im polaren Schnee ändert, korreliert zu sein. Das seismische Muster legt eine Drei-Sehichten-Interpretation bei der Schneeverdichtung nahe. Die Basis der oberen Schicht (8,4±2,3 m) entspricht der „kritischen Tiefe” von Anderson und Benson (1963), in der Schneekörner sich in dichtester Packung anordnen. Die Basis der unteren Schicht könnte der Tiefe des Überganges von Firn zu Eis entsprechen, doch muss diese Annahme durch weitere Daten erhärtet werden. Es bleibt unklar, welches Verdichtungsphänomen sich in der Basis der mittleren Schicht (27,7±4,4 m) aüssert. Mit abnehmender mittlerer Jahresakkumulation neigt der Unterschied zwischen der mittleren und unteren Schicht zum Verschwinden, während der Geschwindigkeitsgradient in einer bestimmten Tiefe zunimmt.

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Copyright

References

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