| Summary: | Introduction This report describes the use of Landsat data in exploration for limonitic anomalies in a heavily vegetated, sub-Arctic terrain, the Circle 1° x 3° quadrangle, Alaska. Landsat data have been used successfully in sparsely vegetated, arid and semi-arid regions to locate limonitic alteration associated with sulfide mineralization (for example, Rowan and others, 1974; , Two different methods were used to extract spectral information from the Landsat data for limonite detection 1) a color-ratio composite image, and 2) a MSS 4/5 grayscale dotprint. Both procedures used ratios of Landsat digital numbers to subdue the effects of surface topography and to enhance the presentation of spectral information in the data. The color-ratio composite (CRC) image was created at 1:800,000 scale with the 4/5, 4/6, and 6/7 ratios in red, blue, and green, respectively. This is apparently similar to the procedure used by In the second processing method, the individual, path radiance corrected 4/5 ratio was printed in a gray scale rendition at l:63,360-scale, the scale of the inch-to-a-mile maps in the Circle quadrangle. The same path radiance correction as was used in bands 4 -and 5 on the CRC had a very apparent effect of subduing the tendency for anomalies to occur preferentially on well- On the 4/5 dot prints, any limonitic rocks would,likely have larger 4/5 ratios than the reddish blueberry bushes or the brownish grass (table 2). A search of the 4/5 dotprints for the lowest 4/5 ratios was thus a useless methodology to follow for this data set. If restricted to one ratio and this Landsat data, a better method would be to find the pixels having the largest 6/7 ratio, because absorption in the .85-1.00 ym region (band 7) caused by ferric and ferrous iron Conclusions and recommendations The heavy vegetation cover in the Circle quadrangle and the use of Landsat data from after the autumn color change contributed significantly to the failure of this exploration effort. Landsat data from midsummer, prior to the vegetation ...
|
|---|