Figure 31 shows part of a West-to-East flight line as seen in two 
thermal IR wavelength bands. The ratio of radiances from this particular 
pair of wavelength bands is generally sensitive to Si0, content of the 
rocks. The physical basis for the method depends on two properties of 
silicates. First, all silicate rocks characteristically display broad 
emissivity minimums (reststrahlen bands) in the 8 to 12 um wavelength 
region. This minimum is caused primarily by the silicon-oxygen stretching 
modes in silicate lattices. Secondly, some relationship exists between 
the position of the silicate emissivity minimum and rock type. The 
minimum tends to shift to longer wavelengths as the silica content de- 
creases. From left to right is a dacite porphyry mountain range, 
alluvium, basaltic lava flow, more alluvium, playa deposits and more 
basaltic lava. The darkest parts of the ratio image have been found to 
have the largest SiO, content, as expected. Note the difference between 
2 
the dacite mountains and basaltic lava on the ratio map; this is caused 
by the difference in their chemical make-up; in this case their SiO, 
content. Figure 32 shows the same region, but in different wavelength 
regions. The two bands used for ratioing in this case were a visible 
green band and a reflective IR band. This ratio is sensitive to the 
presence of iron oxides, dark corresponding to greater contents of iron 
oxide. Note the striking enhancement of the andesite dike in the 
dacite porphyry mountain; this dike is hardly discernible in the single 
channel images. Part of this dike was not even recorded on the best 
geological maps of the area. Also note the greater iron oxide content 
in the basaltic lava as compared to the alluvium. Figure 33 shows 
another flight line (North to South), once again in the thermal IR 
region. Pisgah Crater is located on the right-hand side of the images. 
The SiO, differences are greatly enhanced in the ratio image, where the 
alluvium appears dark and the basaltic lava flow appears bright. 
It seems likely that these developments will lead to better methods 
for rapid geologic mapping and mineralogical exploration in previously 
inaccessible areas. 
5. Conclusions 
Despite making substantial progress in achieving improved processing 
techniques, improved throughput parallel processing systems, and showing 
the practical use of multispectral sensing in various earth resources 
applications under limited conditions, there is still much to be 
accomplished before multispectral scanner surveys provide successful, 
operational user information systems for earth resources and land use 
surveys. Potential benefits of such information will remain largely 
"potential" until this work is done. 
The most critical need is for processing systems capable of better 
keeping pace with the sensors so that unprocessed data does not have 
to be accumulated and stored. Greatly reduced costs of processing are 
also likely to result with an attendant increase in interest in multi- 
spectral scanner surveys on the part of potential users.