Radio-Frequency Interferometry—A new technique For studying glaciers

Abstract

ABSTRACT. A new method of electromagnetic sounding in resistive electrical environments has been developed for use in lunar exploration. It is applicable to the study of terrestrial glaciers and ice sheets. A horizontal electric dipole antenna on the ground is used to transmit power at frequencies of 1,2,4,8, 16 and 32 MHz. A set of orthogonal receiving coils is mounted on a vehicle which traverses away from the transmitter. Field strength is recorded as a function of distance. Waves which travel above the surface interfere with waves from the subsurface, generating interference patterns which can be used to determine the dielectric cons tant, th e loss tangent, and depth to reflecting horizons.The technique was tested on the Athabasca Glacier in western Canada. At I, 2 and 4 MHz the ice was found to have a dielectric constant of about 3.3, a loss tangent (tan Il) which is roughly inversely proportional to frequency giving va lues ofJtan Il in the range of 0.25 to 0.35 (whereJis in MHz ). These values correspond well with the known properties of ice near 0° C, which is a temperature typical of temperate glaciers. It has been possible to determine the depth of the ice but results are not always cons istent with previous seismic and gravity surveys and with drilling. At frequencies of 16 and 32 MHz, scattering is the dominant feature of the results. At 8 MHz there is a transition from clear-cut interference patterns to the scattering patterns. From these findings, we suggest that the Athabasca Glacier has a large number of dielectric scatterers with dimensions less than about 35 m, probably due in large part to crevasses. RESUME. Interferometrie de Jriq·,ences radio -lIIle nOllvelle techniqlle /Jollr l'itllde des glaciers. Une nouvellc methode de sondages electromagnetique a travers des milieux elec triques resistants, a ete imagine a I'usage des explorations lunaires. EIIe pft!sente des possibilites d'application dans I'etude des glaciers terrestres et des calottes glaciaires. Une antenne electrique dipolaire horizon tale sur le sol est utilisee pour transmettrc de l'energie sur des frequences de I, 2,4, 8, 16 et 32 MHz. Un ensemble de bobines receptrices disposees orthogonalement est monte sur un vehicule qui circule a distance de l'emetteur. L'intensite du champ est mesuree en fonction de la distance. Les ondes qui se propagent au-dessus de la surface se combinent avec celles venues de dessous la surface, engendrant des interferences dont on peut se servir pour calculer la constan te dielectrique, la perte en tangente et la profondeur des horizons reflecteurs.On a essaye cette technique dans le glacier de I'Athabasca dans l'Ouest Canadien. A 1,2 et 4 MHz, on a trouve pour la glace une constante dielectrique de 3,3, une perte de tangente (tg Il) qui est grossierement inversement proportionnelle a la frequence, donnant des valeurs deJtg Il de l'ordre de 0,25 a 0,35 (ouJest en MHz). Ces valeurs correspondent bien avec les proprietes connues de la glace au voisinage de 0° C, temperature typique des glaciers tempere...

Advances in geophysical exploration of ice sheets and glaciers

“…The interference method can also be used to measure ice thickness, since a secondary interference pattern will be formed upon arrival of the reflection from the bottom of the ice. The application of this technique to the Athabasca Glacier yielded results which are in general agreement with other thickness de terminations (Strangway and others, 1974). It is not apparent, however, that this method has any particular advantage over standard pulse echo sounding.…”

Advances in geophysical exploration of ice sheets and glaciers

“…The application of this technique to the Athabasca Glacier yielded results which are in general agreement with other thickness de terminations (Strangway and others, 1974). It is not apparent, however, that this method has any particular advantage over standard pulse echo sounding.…”

The Electrical Properties of Snow and Ice

“…This should lead to a higher absorption of radio waves, nevertheless radio-echo sounding is generally easier on polar glaciers. This is due to the larger amounts of scattering in temperate glaciers, a process which tends to dominate over dielectric absorption above 8 MHz (Strangway and others, 1974). The radioecho technique can also be used to detect discontinuiti es in the ice which give significant changes in the dielectric parame ters over distances small compared with the pulse length (Harrison,197 3).…”