Synthetic aperture radar
A technique for precise monitoring of movement over large areas is to use radar satellite remote sensing techniques. This technique is known as Interferometric Synthetic Aperture Radar (SAR) and involves capturing a radar image of the ground surface, which is then compared with images taken at a different time to obtain the relative ground movement. Significant features of this technique are that the image can cover an area as large as 2500 km2 and relative movements can be measured in the range of 5–25 mm. The measurements are independent of the weather, cloud cover and daylight condition.
Every material on or off the earth’s surface reflects light in a characteristic pattern; the manner in which light of different wavelengths is reflected or absorbed from each material is known as its reflectance spectrum. By filtering reflected light to specific wavelengths (colors in the visible part of the spectrum), images can be created that enhance our ability to differentiate materials. Multispectral imaging makes use of a few broad wavelength bands in the electromagnetic spectrum, primarily visible and infrared. Hyperspectral imaging techniques makes use of this characteristics to obtain reflectance spectra for the region being imaged over a large number of discrete, contiguous spectral bands. A contributing factor in many highwall failures is the presence and distribution of mechanically incompetent, clayrich altered rock in pit walls. Most alteration minerals have characteristic absorption features that can be recognized with hyperspectral imaging and thus can be used to help identify these weakened, altered rocks. Synthetic aperture radar (SAR) uses electronic techniques to create a very long virtual antenna by processing pulsed signals from a real antenna as it moves past a target area. Because the width of the radar beam is inversely proportional to the length of the transmitting antenna, this long virtual antenna allows high-resolution imagery. Interferometric synthetic aperture radar is a relatively low-cost system that can be deployed in light aircraft and operated at low altitudes. Displacement measurements can be used to track mass movement of failing slopes in surface mines and possibly to warn of imminent catastrophic collapse.