CHAPTER 11 REMOTE SENSING
| Jurisdiction | United States |
(Apr 1980)
REMOTE SENSING
Consultant
Tucson, Arizona
TABLE OF CONTENTS
1.0 Introduction
2.0 Basic Principles
2.1 The Spectrum
3.0 Instruments
3.1 Cameras
3.2 Spectrometers
3.3 Scanners
3.4 Radar
4.0 Image Processing
4.1 Photographic Enhancement
4.2 Digital Computers
4.3 Optical Diffraction
4.4 Scanning Processors
5.0 Resource Examples
5.1 Base and Precious Metals
5.2 Rare Earths
5.3 Diamond
5.4 Placer Deposits
5.5 Sulfur
6.0 Some Legal Implications
7.0 The Future
7.1 Geosat
7.2 The Safford Site
8.0 Summary
9.0 Bibliography
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1.0 Introduction
The concept of remote sensing is that of obtaining views of the earth's surface through electromagnetic frequencies extending far beyond the visible spectrum. The technology sprang from aerial photography but began with the development of radar before World War II. During the war, color infrared film was developed to detect camouflage by extending the spectral range of color film beyond red into solar (non-thermal) infrared. In the last two decades optical mechanical scanners have begun to fill the wavelength spectrum between that covered by photographic and microwave instruments.
The use of images from space for regional geologic mapping began with photographs taken by the Gemini astronauts. Satellite photography increased in quantity and quality with the Apollo program and finally the Skylab series. In 1972 the first ERTS (earth resources test satellite) satellite sent back its telemetered scanner images (figure 1).
"Oversell" of the results of the NASA-funded ERTS application experiments may have temporarily set back the growing acceptance of remote sensing by mining explorationists. However, the ERTS experiments were seen by Congress to be a success and ERTS-2 was launched, followed by another ERTS now renamed Landsat C. The last two,Landsat B (ERTS-2) and Landsat C are still operational. Exploration departments of major companies now use computer enhanced Landsat images with airborne remote sensing to complement their ground reconnaissance programs.
A fairly large portion of the United States is covered by aerial photography taken by NASA's ultrahigh altitude U-2 and RB-57 research aircraft. These 1:125,000 scale photographs are useful as an intermediate step between satellite images and aerial photographs taken at conventional altitudes.
2.0 Basic Principles
The technology of remote sensing has grown outward from aerial photography in many directions; (a) the use of nonvisible parts of the electromagnetic spectrum, (b) the development of non-photographic sensors including active sensors such as lasers and radar, (c) the availability of new platforms to carry those sensors including satellites, hyperaltitude aircraft, and helicopters, and (d) the development of new data processing technology.
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The Spectrum2.1
The spectrum of electromagnetic frequencies that is visible to our eyes is nicely arranged in rainbows. Violet is the high frequency, short wavelength end and red marks the low frequency, long wavelength end of the less-than-one-octave 0.4 micron to 0.7 microns available to human eyes (figure 2) that we call "light". This band coincides with the spectral wavelength "window" of solar energy to which natural water is transparent. Thus we humans are still restricted (with the exception of our remote sensing instruments) to the spectrum useful to our fishy ancestors.
The ultraviolet sunlight is visible to bees and to all photographic film. The ultraviolet frequencies are generally avoided in aerial photography due to atmospheric scattering, which is why we use ultraviolet-absorbing "skylight" filters on cameras. Photographic film extends in short wavelength sensitivity far beyond the spectral limitations of glass lenses (about .35 microns) and the atmosphere into the X-ray region. At the extremely short wavelength end of the spectrum airborne gamma ray spectrometers are used in mineral exploration for the identification of radioactive emissions from potassium, thorium and uranium.
Let us now direct our attention to infrared radiation. The infrared region of the spectrum covers the entire span from the red end of vision at 0.7 microns through 10 octaves to microwaves at millimeter wavelengths. The region from about 14.0 microns to 1.0 millimeters is not available due to atmospheric opacity. For remote sensing the infrared region is divided into rather dissimilar spectral bands. Immediately adjacent to visible red at 0.7 microns is the solar photographic infrared which is useful in color infrared areal photography out to wavelength 0.8 microns. This near infrared photographic region cannot be used to directly sense the temperature of the ground, but is merely using reflected sunlight as in ordinary photography. Infrared aerial photography is superior to conventional photography in mining exploration in two ways: (a) better haze penetration, and (b) better differentiation of vegetation. However, the visual region is at least as effective for photographic discrimination of rock types and alteration zones.
To extend remote sensing imaging farther into longer solar infrared wavelengths, we must leave photography and use the more exotic optical mechanical scanners. These instruments use electronic sensors that can cover the entire optical region from ultraviolet, through the visible, the solar infrared, and the thermal infrared (far infrared). The non-photographic solar-reflective infrared region extends from 0.9 microns to about 5.0 microns. Especially important to the explorationist is the 1.0 to 3.0 micron region. Here nearly all altered rock sepctra exhibit an intense absorption band, associated O-H bond stretching and A1-O-H bond bending vibrations in clay minerals, pyrophyllite, dioctahedral micas, and alunite.
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Scanners allow us to remotelysense surface temperatures in the 3.0 to 14.0 micron region and to create thermal images. In the 8.0 to 14.0 micron band solar radiation is effectively absent, but there is an overlapping spectral region from 3.0 to...
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