Advances in geophysical exploration of ice sheets and glaciers

Abstract

The techniques of geophysical exploration can be used in a number of ways to investigate the internal characteristics of ice bodies. The application of radio-echo sounding is discussed elsewhere by Robin (1975[a]) so is not considered here except for a few special cases. Anisotropy resulting from non-random ordering of crystal axes produces effects on seismic wave propagation speeds measurable by both refraction and reflection techniques. The effects on shear waves are particularly strong, so the recent development of an effective shear-wave generator should prove very useful. Marked effects on radio-wave polarizations have also been noted, but are not readily interpretable because the anisotropic dielectric characteristics of single-crystal ice in the radio-echo frequency range are not yet known. Elastic internal friction can be determined by careful measurements of seismic-wave amplitudes. Dielectric properties of the ice can be found from VLF propagation experiments and electrical logging in drill holes as well as from the radio-echo sounding techniques discussed by Robin (1975[a]). An activation energy can be estimated from d.c. resistivity measurements where the temperature profile is known. Recent developments have led to much improved methods of determining density-depth variations and long-term mean annual snow accumulation rates from seismic refraction shooting. Internal discontinuities which can be studied include a probable morainal layer detected by seismic reflections, a brine-soaked zone in shelf ice observed from radio-echo. sounding and electrical resistivity surveying, and inverted crevasses penetrating upward from the base of shelf ice, detected by radio echoes. Secular changes in gravity offer a sensitive means of determining long-term changes in surface elevation.

Polarization of radio waves transmitted through Antarctic ice shelves

“…Avariationin echo strength with antenna orientation has been known for a long time (Jiracek, 1967; Bentley, 1975). The difference in power can exceed 20 dB for values of birefringence approaching 180˚ and r near unity, or for a highly anisotropic reflecting surface (see equation (7a) in Hargreaves (1977), or equation (17a) in Doake (1981)).…”

“…Since ice sheets were first sounded by radar, it has been known that they affect the polarization of radio waves (Jiracek, 1967; Bentley, 1975). However, the few systematic studies that have been carried out have analyzed only echo strength, not phase (Hargreaves, 1978; Woodruff and Doake, 1979; Yoshida and others, 1987; Fujita and Mae, 1993).…”

An improved coherent radar depth sounder

“…In additio n to m eas urements of ice thickness a nd layers, other features a nd char acteri stics obser ved include region s of bottom m elting a nd freez ing (Neal, 1979), ice-bottom sliding velocit y (D oa ke, 1975), glacier velocity (Doake a nd others, 1976), sub -i ce la kes form ed by pressure m elting (O swald, 1975), a nd bottom creyasses Oezek a nd oth ers, 1979). Informatio n o n oth er ice para meters, including sign a l abso rpti on (Nea l, 1976), sig na l fading p atterns (H arriso n, 1971;Berry, 1975), p rop agati on veloc it y (R obin, 1975 b ) a nd birefringen ce (Bentley, 1975;H arg reaves, 1977;\ VoodrufT and D oake, 1979), h a\'C been obta ined from RES d ata.…”

An improved coherent radar depth sounder

“…glacier velocity (Doake and others, 1976), sub-ice lakes formed by pressure melting (Oswald, 1975), and bottom crevasses (Jezek and others, 1979). Information on other ice parameters, including signal absorption (Neal, 1976), signal fading patterns (Harrison, 1971; Berry, 1975), propagation velocity (Robin, 1975b) and birefringence (Bentley, 1975; Hargreaves, 1977; Woodruff and Doake, 1979), have been obtained from RES data.…”