Investigation of landslide susceptible terrain using landsat TM Imagery - Preliminary results
Site description
The study site, as shown in
Fig.1, includes the poker Flats mining area and the entire Lignite Cree Basin. Lignite creek Flows westward into the Nenana River near the town of Healy, Alaska. The study area leis along the northern flank of the Alaska Range.
The climate in the study area indicates an average annual high of 34° F and a low of 19° F on the basis of the last 10 years of weather records. Total annual precipitation in the area ranges from 15 to 18 inches with most arriving during late summer rain storms. Spring thaw in late May also contributes much runoff.
Much of the natural ground in the Lignite Creek Basin is unstable, either presently or has been in the past. The Coal-bearing formation is prone to slope movement. Where slope spare undercut by streams, failure often result even on gentle slopes. Other slope failures are associated with solifluction and thermal degradation of underlying permafrost.
Fig.1 Image showing the Lignite Creek Basin.
Approach
Pre-mining aerial photographs of usibelli coal mine reveal substantial evidence of landslides both inside and outside of the current mine site (Corser and Usibelli, 1989). Many of the slope failure prone areas in the study site cannot be easily identified by field survey due commonly accessibility and economic reasons.
In this study, a Landsat-5 thematic Mapper (TM) image, acquired on September 22, 1984, (Scene ID Y5020520430X0) was utilized as a primary source for gathering the needed information. Landsat-5 TM sensors provide a coverage of 185 by 185km with a ground resolution of approximately 30 m in reflective spectrum. Six images in reflective bands (0,45 to 2.35mm) and one image thermal band (10.4 to 12.5 mm) were used to investigate the stud area for surface cover characteristics, such as vegetation, soils, water bodies, drainage patterns, geologic conditions, and permafrost distribution. Information related to terrain elevation, permafrost distribution, existing landslide deposits and site geology was collected and digitized. The location of existing slope failures were also digitized to serve as a reference for cross-examination of the validity of the landslide susceptibility algorithm which will be developed in the second phase of the study.
Landsat image processing was mainframe of the study, which included tasks of image rectification, enhancement and information extraction.
Image processing and digitization
The Landsat TM image was processed by a Comtal/3M image processor,
with a VAX 11/750 as the host computer. A Land Analysis System (LAS)
software package was utilized for image processing and Analytical Mapping System (AMS) was used for geo-data digitization.
Thirteen ground control points in the study area were selected from a
topographic map to create a reference image and the TM-5 image was
used as the search image. Pixel size of the search image was
reformatted to 25x25m, and pixel values were resampled using the
nearest neighbor (NN) method.
The same thirteen ground control points for geometric registration were also used in AMS registration to create both the reference file (Latitude and Longitude unit) and the search file (UTM unit). Landslide deposits from the US geological survey geologic map of the Healy D-4 quadrangle 22 polygons. In addition, landslides surveyed by Wilbur and Beget (1988) were digitized and converted into a file containing eight geological units. A digital elevation model of the site was obtained from topographic map.
Fig.2 Digital geological map showing coal-bearing unit.
