Advances in geophysical exploration of ice sheets and glaciers

Abstract

ABSTRACT. T h e techniques of geoph ysica l expl oratio n can be used in a number of ways to investigate the interna l c h a rac teristics of ice b odi es. The a pplicatio n of radio-echo sounding is discussed else where by Robin (197s[a ] ) so is not consider ed h ere except fo r a few specia l cases. A n isotropy resulting fro m nonra ndom ordering of crysta l axes produces effects on seismi c wave propa gatio n sp eeds measura ble b y both refrac tion a nd re fl ection techniques. Th e effects on sh ear w aves a re pa rticu la rly strong, so the recen t d evelopm ent of an e ffecti ve shea r-wave gen e rator should pro ve very usefu l. M a rked e ffects on ra dio-wave p ola rizations have a lso b een noted, but are n o t readily interpre tabl e because the a niso tropic diel ectric ch a racteristics o f single-crys ta l ice in the radio-ech o frequency ra nge a re n ot yet known . El astic interna l fric tio n can be d e termined b y careful m easurem e nts of seismic-wave a mplitudes . Dielectric properties of t h e ice can be fo und from VLF propaga tion experime nts a nd electrical logging in drill h o les as well as from th e radio-echo sounding techniq u es discussed by R o bin ( 197s [a] ) . A n ac ti vation energy can b e estimated from d. c . resistivity measurem e nts w h ere th e temper a ture profil e is kn own . R ecent d evelopm ents h ave led to muc h improved m ethods of d e te rmining density-d epth variations a nd lo ng-term mean a nnual snow accumul a tio n ra tes from seismi c refrac tio n shoo ting. Inte rn a l discontinuities which can be studied include a proba ble m ora ina l layer d etec ted b y seismic reAectio ns, a brine-soaked zon e in shelf ice observed fr om radio-echo sounding a nd electrical resistivity surveying, a nd in verted crevasses p e n e tra ting upwa rd from the base of sh elf ice, d e tected by ra dio echoes. Secular cha nges in g ravi ty offer a sen sitive means of d e te rmin ing long-te rm c h a nges in surface elevatio n . R ESUME. Progres dans l'exploration geophysiqlle des caloUes glaciaires et des glaciers. Les techniques d e I'exploration geophys iqu e p euvent etre uti lisees d e nombreu ses m a ni eres pour e tud ie r les caracteristiques internes d es masses d e g lace. On discute p a r a illeurs I'a pplica tio n du sondage p a r radio-echo (Robin , 19 7s[a]l, a ussi, ne prend ro ns-nous p as en con side ra tion ce tte metho d e sauf pour quel ques cas specia ux. L ' anisotropie resultan t d ' un e distribution non-alea toire d es axes optiques d es crista ux a des consequences sur les v itesses de propaga tion des o ndes sismiq ues, m es urables pa r d es techniques de reAexio ns e t de refrac tion. Les effets sur les ondes d e cisaillem ent son t particulierement forts, d e sorte que le d eveio ppe ment recent d ' un ge n erateu r d 'ondes d e cisaille m ent s'avere tres u t ile. D es effets m a rques sur la pol arisa tio n d es ondes ra dio ont egal ement e te d eceles, m a is n e sont pas fa cile m ent interpreta bl es parce que les carac te ristique...

Seismic Short-Refraction Studies on the Ross Ice Shelf, Antarctica

“…As already discussed, the P-wave results show that the firn layers are essentially horizontal. It follows that the S-wave velocities reveal anisotropy, to which they are more sensitive than P-wave velocities (Bentley, 1975). More specifically, the results indicate that the firn in the upper 30 or 40 m of the ice shelf is not only anisotropic (from the differences between SV and SH), but not even transversely isotropic (as shown by the azimuthal variation).…”

Multi-channel and multi-polarization radar measurements around the NEEM site

“…Many previous studies show that ice properties, such as the birefringence and crystal orientation fabric (COF), could be inferred from multi-polarization radar measurements. The strength and polarization of radar echoes are affected by ice COF with respect to antenna orientations (Campbell and Orange, 1974;Bentley, 1975). Ice COF is closely related to ice flow history and has a strong effect on ice deformation rate (Azuma and Higashi, 1985); it is one of three reasons for radio echo reflections from internal ice layers (Fujita and Mae, 1994;Fujita et al, 1999;Eisen et al, 2007).…”

Satellite image atlas of glaciers of the world

“…Precision gravimeters can also be used to achieve the same results (Bentley, 1975). The latter technique has been used at the top of Devon Ice Cap (Winter, oral commun., 1976).…”