ABSTRACT. Seismic short-refraction studi es were carri ed out at five stations on the Ross I ce Shelf during the 1976-77 summer season as part of the compreh ensive Ross I ce Shelf Geophysical and Glaciological Survey. Measurements of the velocities of compressional waves were made at each locatio n. Compressional wave velociti es were measured a long 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 appa rent velocities are estimated to be about ± 50 m S-I at short dista nces, diminishing to about ± 10 m s-! near the ends of the profiles. Compressional-wave velocities show only slight variations with azimuth and only over certain depth intervals, sh owing that constant-veloci ty surfaces are essentially horizontal. Shear-wave velocities, h owever, exhibit large variations according to azimuth and polarization, indicating that transverse isotropy is violated at leas t in the upper 30-40 m of the ice shelf. It is believed that the anisotropy is caused by stru ctural 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. RESUME.

No 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–1at short distances, diminishing to about ±10 m s–1near 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.

ABSTRACT. Seismi c compressional-wave data from sh ort refraction shooting carried out during the 19 74-75 a nd 1976-77 seasons at J-9, the site of the R oss I ce Shelf Drilling Proj ect, h ave been compared. Significant dissimila rities were found to exist between the two sets of data. The m easurements were made at locations about 2 km a p a rt, with three unreversed profil es 60° a pa rt recorded during the 1976-77 season and on e unreversed profil e during the 1974-75 season. T he resulting velocity-d epth profil es, and h en ce the d eri ved density-depth p rofil es, differ by as mu ch as 8% , with the 1976-77 results indicating a m aximum velocity, corresponding to solid ice, at a sh a llower depth tha n the 1974-75 data. Both profil es were subj ected to the same analysis, and a comparison of tra vel-time curves shows the differen ces to be real. Densities m easured on cores from a 100 m bore hol e d rilled in 1974-75 a bout 50 m from the cent er of the 1974-75 profil e agree well with d ensities computed from that profil e. The density differen ce is believed to be due to the p assage of the ice through the high-stress system associated with the interaction between I ce Stream B, fl owing in from the W est Antarctic ice sh eet, and the R oss I ce Shelf. A reversed r efraction profil e carried o ut a t station B. C . about 30 km up-stream , sh ows evidence of dipping layers th a t m ay be similarly caused. REsuME.

Seismic compressional-wave data from short refraction shooting carried out during the 1974–75 and 1976–77 seasons at J-9, the site of the Ross Ice Shelf Drilling Project, have been compared. Significant dissimilarities were found to exist between the two sets of data. The measurements were made at locations about 2 km apart, with three unreversed profiles 60° apart recorded during the 1976–77 season and one unreversed profile during the 1974–75 season. The resulting velocity–depth profiles, and hence the derived density–depth profiles, differ by as much as 8%, with the 1976–77 results indicating a maximum velocity, corresponding to solid ice, at a shallower depth than the 1974–75 data. Both profiles were subjected to the same analysis, and a comparison of travel-time curves shows the differences to be real. Densities measured on cores from a 100 m bore hole drilled in 1974–75 about 50 m from the center of the 1974–75 profile agree well with densities computed from that profile. The density difference is believed to be due to the passage of the ice through the high-stress system associated with the interaction between Ice Stream B, flowing in from the West Antarctic ice sheet, and the Ross Ice Shelf. A reversed refraction profile carried out at station B.C. about 30 km up-stream, shows evidence of dipping layers that may be similarly caused.