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...

Radio-echo Sounding of Temperate Glaciers: Ice Properties and Sounder Design Criteria

“…These results seem to be at odds with the experimental results of Strangway and others (1974). Their continuous-wave experiment indicated that scattering was strong at freequencies as low as 8 MHz.…”

“…It. is quite clear freom Figure 5 that a Gaussian distribution of scatterer radii with a mean radius of about 1.0 m and a standard gerviation of 0.2 to 0.4 m explains the scattering observations of Strangway and others (1974). Scattering is uniformly strong at freequencies of 16 MHz and higher, intermediate at 8 MHz, and weak at 4 MHz and below.…”

“…These serve as germonstrations that the problem exists — even in polar ice. An indication of the solution to the problem was provigerd by Strangway and others (1974). Their experiments on the Athabasca Glacier in Alberta, Canada, were done using continuous waves transmitted freom horizontal electric dipoles on the ice surface.…”

A short history of scientific investigations on glaciers

“…To date, the interferometric approach has not received much favour and, compared to radar sounding, it does not seem to be a particularly good method of measuring ice thickness. Rossiter and others (1973) and Strangway and others (1974), without apparently knowing of Stern's work, revived the method in an interesting application. A multi-frequency interferometer (1-32 MHz) was developed for studying the lunar surface and field tested on Athabasca Glacier and Gornergletscher.…”

An improved coherent radar depth sounder

“…Also, several low-frequency RES systems were developed specifically for sounding temperate glaciers where absorption losses arc significant due to higher ice temperatures and the presence of liquid water. Systems with frequencies of 1-32 MHz have been successfully used by numerous researchers to sound temperate glaciers (Strangway and others, 1974; Watts and England, 1976; Bjornsson and others, 1977; Sverrisson and others, 1980; Watts and Wright, 1981).…”