A hole was drilled through the Ross Ice Shelf 450 kilometers from the barrier. Scientific sampling through this hole revealed a sparse population of crustaceans, fish, and microbial biomass. The seabed consists of mid-Miocene glaciomarine mud. Geothermal heat flow is average. Oceanographic data indicate an active circulation and melting at the base of the ice.

ABSTRACT. The OrIgm and nature of radio-echo in terna l reflections is the subjec t of much debate. Calculations of sin gle-and multipl e-l ayer refl ection coefficients for dielectric changes in the ice sh eet indicate that the observed refl ec tion strength may result from reflec ti ons from many thin layers but the zone of thin layers may bes t be approx imated by re placing the zone with a singl e thi cke r layer with average dielectric properties. Calculation of refle ctions fro m density changes, dust and dirt bands and an isotropy of. indica te d e nsity variations as the most likel y cause of the internal layering in the uppe rmost kilometer of ice sheets.Comparison of the d epth of internal rcfl ec tions at " Byrd" station , Antarctica, with the physical properties of the ice meas ured in a deep core hol e at that loca tion support this conclusion.R ESUME. Sondages par echo radio: R ejfexiolls sur les niveaux illtern es dall s les caloUes glaciaires. L 'origine et la na ture des refl exions internes de I'echo radio sont le suj e t d e nombreux debats. Les calculs de coefficients de reflexion d'un niveau unique ou d e niveaux multiples de changement dielectriques dans la calotte glaciaire montrent que I'intensite des n:flexion s observees peut res ulte r de n:fl ex ions sur un grand nombre d e couches minces mais que la zone de ces cou ch es minces peut etre simulee au mi eux en r e mpla~ant cette zone par un seul niveau plus e pais avec des proprietes di elec triques moyennes. Le calcul des reflexions provenant des ch a ngements d e d e nsite, de lits d e pouss ierc ou de boue e t d'a nisotropie de • indique que les variation~ de d e nsite sont la cause la plus proba ble d e la stra tifica tion interne dans le kilometre superieur des calottes glaciaires. La comparaison de la profondeur des refl exi o ns internes a la Station "Byrd" dans I' Antarctique avec les proprie tes physiques de la g lace mesuree par un caro ttage profond dans les memes parages confirme ces conclusions.ZUSAMM ENFASSUNG. Radar-Echololllllg: R ejfexiollCll an illneren Schichten in Eisdecken. Der Ursprung und Charakter inn eren R efl ex ion en beim R ad a r-Echo wird h a ufig diskutiert. Berechnungen der R eflexionskoeffizi enten fur diel ektrische Schwan kungen in del' Eisd ecke bei einfach e r und mehrfacher Schichtung zeigen, dass die beobachtete Refl ex ionss tarke durch R e Aexione n a n vielen dunnen Schichten erzeugt werden ka nn , dass aber di e Zone dunner Schi chte n am bes ten durch Ersatz der Zone mit einer einzelnen dickeren Schicht von mittl ere n dielektrischen E igenschaften anzunahern ist. Die Berechnung der R e Aexion an Dichtesprungen , Sta ub-und Schmutzban dern und di e Aniso tropic von • e rweisen Dichteschwankungen als die wa hrscheinli chste Ursache fur die innere Schi chtun g in dem obersten Kilom eter von Eisd ecke n. Ein Ve rgl eich zwischen d el' Tiefe inn erer R e Aex ionen a n del' "Byrd" Sta tion, Antarktika, mit den physikalischen Eigenschaften d es Eises, gemessen in e inem tiefen Bohrloch an diesel' ...

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.

The origin and nature of radio-echo internal reflections is the subject of much debate. Calculations of single- and multiple-layer reflection coefficients for dielectric changes in the ice sheet indicate that the observed reflection strength may result from reflections from many thin layers but the zone of thin layers may best be approximated by replacing the zone with a single thicker layer with average dielectric properties. Calculation of reflections from density changes, dust and dirt bands and anisotropy of ϵ indicate density variations as the most likely cause of the internal layering in the uppermost kilometer of ice sheets. Comparison of the depth of internal reflections at “Byrd” station, Antarctica, with the physical properties of the ice measured in a deep core hole at that location support this conclusion.

An abrupt change in radio echo sounding travel time was observed on the ice shelf near McMurdo station, Antarctica, and was mapped by a zig-zag traverse. This boundary corresponds to the horizontal extent of brine penetration into the edge of the ice shelf.