Seismic Studies on the Grid Western Half of the Ross Ice Shelf: RIGGS I and RIGGS II

  1. Charles R. Bentley and
  2. Dennis E. Hayes
  1. James D. Robertson1 and
  2. Charles R. Bentley2

Published Online: 16 MAR 2013

DOI: 10.1029/AR042p0055

The Ross Ice Shelf: Glaciology and Geophysics

The Ross Ice Shelf: Glaciology and Geophysics

How to Cite

Robertson, J. D. and Bentley, C. R. (1990) Seismic Studies on the Grid Western Half of the Ross Ice Shelf: RIGGS I and RIGGS II, in The Ross Ice Shelf: Glaciology and Geophysics (eds C. R. Bentley and D. E. Hayes), American Geophysical Union, Washington, D. C.. doi: 10.1029/AR042p0055

Author Information

  1. 1

    ARCO Oil and Gas Company, Dallas, Texas 75221

  2. 2

    Geophysical and Polar Research Center, University of Wisconsin, Madison, Wisconsin 53706

Publication History

  1. Published Online: 16 MAR 2013
  2. Published Print: 1 JAN 1990

ISBN Information

Print ISBN: 9780875901954

Online ISBN: 9781118664735

SEARCH

Keywords:

  • Anisotropy;
  • Average compressional wave velocity;
  • Density and elastic moduli;
  • Ice thickness and sea-bottom topography;
  • Sea-bottom reflection coefficients and acoustic impedances;
  • Sea bottom slopes;
  • Ultrasonic velocity measurements;
  • Velocities of compressional (P) and shear (S) waves

Summary

Airlifted geophysical surveys were carried out on the grid western half of the Ross Ice Shelf, Antarctica, during the austral summers of 1973–1974 and 1974–1975, as part of the Ross Ice Shelf Geophysical and Glaciological Survey (RIGGS). Seismic reflection records were obtained at 76 stations, seismic short-refraction records at nine stations, seismic long-refraction records at four stations, radar-sounding reflection records at 93 stations, and gravity measurements at 89 stations. The seismic results, supplemented by radar-sounding measurements of ice thickness, are discussed here. The P wave velocity increases from about 500 m s-1 at the surface to 3811±7 m s−1 at depths of 70–80 m in the ice, and the S wave velocity increases from about 300 m s−1 at the surface to about 1970 m s−1 at 60 m. The maximum P wave velocity is significantly lower than the maximum velocity (3850 m s−1) in grounded ice sheets at the same mean annual surface temperature. The average P wave velocity through the ice shelf is 3688±15 m s−1. Densities and elastic moduli computed from seismic velocities are consistent with densities measured on ice cores and elastic moduli determined in laboratory experiments on ice. Significant depths in the densification process of the firn have been located by analysis of the seismic velocity gradients at 11±2 m (the “critical depth”), 25±10 m (significance uncertain), and 46±8 m (the firn-ice boundary). There is S wave velocity anisotropy in the firn that probably is caused by layered structure, but comparison between seismic and radar echo times shows no evidence of an average preferred orientation of crystallographic c axes in the body of the ice shelf. A complete listing of ice and water layer thicknesses and ocean bottom elevations is given. These results have already been discussed elsewhere. Sea bottom slopes are locally similar to regional slopes, which suggests that the seabed is relatively smooth at wavelengths of a few kilometers. Interval velocities and acoustic impedances in the layer of sediment at the seafloor match those expected for unconsolidated glacial marine till. A seismic reflector at a depth of 50–150 m within the till probably correlates with a glacial erosional surface previously discovered in sediments in the Ross Sea. The best estimate of the P wave velocity in seismic basement at long-refraction seismic stations is 5.5–5.7 km s−1 One or two kilometers of lower-velocity rocks and sediments overlie basement beneath three floating stations; on Crary Ice Rise basement lies about three quarters of a kilometer beneath the ice.