USGS Spectroscopy Lab - Mapping Introduction

http://speclab.cr.usgs.gov

Derived From: Clark and Swayze, Summaries of the 6th Annual JPL Airborne Earth Science Workshop March 4-8, 1996

Imaging Spectroscopy Material Maps: Cuprite Introduction

Remote Sensing has a long history. Originally, black and white, then color photography was obdained from aircraft to image large areas. Color photography can be used to distinguish materials on the ground, but the spectral range of color photography, similar to the spectral range of our eyes, is limited, so material discrimination is also limited.

319K GIF The image shown here is a pseudo true color composite derived from AVIRIS daat obtained over Cuprite, Nevada, in 1993. The scene is 10.5 km wide and 17 km high and north is up.

Next came satellite systems giving digital imagery from the UV to near-infrared. The Landsat TM system has 6 broad filters from 0.4 to 2.5 microns. By digital manipulation of combinations of images from each filter, many materials can be distinguished. Note, that while materials can be distinguished, the filters are broad band, meaning that individual absorption features can't be resolved. Thus the colors indicate a spectral trend but can't be used to uniquely identify a specific material. In fact, if you compare specific colors to the mineral maps below, you will see that a color in this TM ratio map may be indicating many minerals and thus different geologic formations. For example, trace the cyan (light blue) in this image to the vibrational absorptions mineral maps below. Cyan corresponds to calcites, muscovites, and even buddingtonites.

355K GIF The image shown here is a color composite derived from synthesized TM bands from AVIRIS data obtained over Cuprite, Nevada, in 1993. The scene is 10.5 km wide and 17 km high and north is up.

Imaging spectroscopy has many spectral channels compared to previous systems. An imaging spectrometer has enough spectral channels to resolve absorption bands in materials of interest. The system illustrated here, the NASA/JPL Airborne Visual and Infra-Red Imaging Spectrometer (AVIRIS), has 224 spectral channels from 0.4 to 2.5 microns (your eye covers only about 0.4 to 0.68 microns). The spatial resolution is about 17 meters pixel spacing. (see link to the AVIRIS data facility on the home page)

273K GIF The mineral map shown here is derived from analyzing the vibrational absorption features in minerals (typically in the 2 to 2.5 micron spectral region) common to OH-, CO3-, and SO4-bearing minerals. Each mineral has a specific crystal structure and subtle changes in that structure change absorption bands, even with the same ion (such as OH). Thus specific mineralogy can be identified. In fact, the absorption is so sensitive that small changes in chemistry of a mineral make identifiable changes in the absorption bands, so that solid solution series, or element substitutions can be mapped (for example the high, medium, and low aluminum content of muscovites, or the K versus Na alunites).

The image shown here is a mineral map derived from AVIRIS data obtained over Cuprite, Nevada, in 1995. The scene is 10.5 km wide and 17 km high and north is up.

221K GIF The mineral map shown here is derived from analyzing the electronic absorption features in minerals (typically in the 0.4 to 1.2 micron spectral region) common of Fe2+ and Fe3+ bearing minerals. Each mineral has a specific crystal structure and subtle changes in that structure change absorption bands, even with the same ion (such as Fe2+). Thus specific mineralogy can be identified. In fact, the absorption is so sensitive that small changes in chemistry of a mineral make identifiable changes in the absorption bands, so that solid solution series, or element substitutions can be mapped. Electronic absorption bands are often quite strong, and in reflectance, the bands can become flattened on the bottom when the grain size is large (this is called saturation), thus grain size can sometimes be mapped. Unfortunately, electronic absorptions are often broad, and it is sometimes difficult to assign unique mineralogy (for example, some Fe2+ bearing minerals have similar absorptions).

The image shown here is a mineral map derived from AVIRIS data obtained over Cuprite, Nevada, in 1995. The scene is 10.5 km wide and 17 km high and north is up.

U.S. Geological Survey, a bureau of the U.S. Department of the Interior
This page URL= http://speclab.cr.usgs.gov/map.intro.html
This site is maintained by: Dr. Roger N. Clark rclark@usgs.gov
Last modified November 13, 1998.