1994 年 14 巻 3 号 p. 220-229
This paper describes the C-band backscattering variations and the development of a numerical model of backscatter from lake ice growing on shallow tundra lakes in northern Alaska. This research was prompted by earlier observations of unusual X- and L-band backscatter variations in side-looking airborne radar (SLAR) data, and the recent availability of a well-calibrated time-series of SAR data and concurrent ground observations and measurements.
The North Slope of Alaska is a large expanse of low-lying tundra with many shallow lakes which comprise more than 40% of the surface area. During the 1970s, in late winter, SLAR images of these lakes showed interesting variations in backscatter intensity : areas of low backscatter at lake margins were believed to represent ice that was frozen to the lake bed while areas of high backscatter represented floating ice that contained tubular bubbles which acted as forward scatterers.
From September 1991 to April 1992, backscatter intensity variations from shallow tundra lakes near Barrow, NW Alaska, were studied using C-band SAR data from the ESA Remote-Sensing Satellite-1 (ERS-1). The SAR data were processed at the Alaska SAR Facility (ASF) and the backscattering coefficient was derived for a number of lakes. Field measurements in April 1992 confirmed that the highest values were associated with floating ice containing tubular bubbles, while the lowest backscatter values were associated with ice frozen to the lake bed. The ice frozen to the bottom of the lakes also contained tubular bubbles.
Ice core measurements indicated that the lake ice comprised three layers : 1) a surface layer of granular ice with roughly spherical bubbles with radii smaller than a wavelength : 2) a layer of clear bubble-free ice, and ; 3) a bottom layer containing tubular bubbles resembling thin cylinders with lengths ranging from 15 mm to 91 mm and a radius much smaller than a wavelength. The number of tubular bubbles per square meter, i.e. bubble density or ice porosity, was quite variable.
The backscatter model that has been developed comprises the following elements : 1) an ice layer of variable thickness ; 2) ice sub-layers with air inclusions of variable density, size and shape, including cylinders of finite length, prolate spheroids and spheres : 3) air-ice boundary is specular surface, while ice-water and icefrozen soil boundaries are specular or slightly rough.
The model results confirm that backscatter is a sensitive function of the presence of an ice-water interface, with a roughly 30-times greater reflectivity than an ice-frozen soil interface. The model also shows that backscatter increases as the ice grows. The model has also been tested using bubble data derived from ice cores in April 1992. The modelled backscatter is compared with backscatter derived from ERS-1 SAR images obtained at the same time as the fieldwork. It is found that the modelled and actual backscatter have high correlation.