Mineral Mapping Using Spectroscopy

Abstract

Mineral physics dictate the appearance of rocks and soils across the electromagnetic spectrum. In the Visible/Near-Infrared (VNIR) and Short Wave Infrared (SWIR), many materials absorb radiation at specific wavelengths, allowing their identification by the position and character of absorption features. Electronic processes at wavelengths less ~1.0 micrometers allow (in addition to others) identification of minerals containing Fe⁺³. Molecular vibrational features at wavelengths between ~1.0 and 2.5 micrometers are diagnostic of minerals containing anion groups such as Al-OH, Mg-OH, Fe-OH, Si-OH, CO₃, NH₄, and SO₄. Small differences in absorption band position and shape can be correlated with mineral compositional differences and variability. Imaging spectrometry (also known as “hyperspectral imaging” or HSI) has been used since the early 1980s to perform 2-dimensional mapping of mineral distribution based on spectroscopic characteristics. Field spectroscopy plays a critical role in the calibration, analysis, and validation of imaging spectrometer data. Imaging spectrometer datasets have been acquired around the world using airborne platforms and recent satellite systems provide spectral measurements for selected areas. The case history presented shows an example of the progression of imaging spectrometer data to its current state-of-the-art and demonstrates the link between laboratory, field, and imaging spectrometer data.

Introduction

The physics of visible/near-infrared (VNIR) and short-wave-infrared (SWIR) spectroscopy are well known. Key spectral features in these regions allow identification of a variety of materials using laboratory and field spectroscopy, including minerals, vegetation, man-made materials, snow and ice, and water (Clark et al., 2003, 2007). In geology, electronic processes at wavelengths less ~1.0 micrometers allow (in addition to others) identification of minerals containing Fe⁺³, while molecular vibrational features at wavelengths between ~1.0 and 2.5 micrometers are diagnostic of minerals containing anion groups such as Al-OH, Mg-OH, Fe-OH, Si-OH, CO₃, NH₄, and SO₄. Small differences in absorption band position and shape can be correlated with mineral compositional differences and variability (Gaffey, 1986; Duke, 1994).Imaging spectrometry, “the acquisition of images in hundreds of registered, contiguous spectral bands such that for each picture element of an image it is possible to derive a complete reflectance spectrum”, is technology that has been available since the early 1980s (Goetz et al., 1985). It has also become known as “Hyperspectral Imaging” or ‘HSI". Its utility for detailed materials mapping has been demonstrated for a variety of scientific disciplines (Goetz et al., 1985, Kruse, 1988; Hamilton et al., 1993; Clark et al., 1996; Green and Dozier, 1996; Kruse et al., 2006, Kruse and Perry, 2009). Current airborne HSI sensors provide high-spatial resolution (2-20m), high-spectral resolution (10-20nm), and high SNR (>500:1) data for a variety of scientific disciplines. AVIRIS: The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) represents the current state of the art. AVIRIS, flown by NASA/Jet Propulsion Laboratory (JPL) is a 224-channel imaging spectrometer with approximately 10 nm spectral resolution covering the 0.4 – 2.5 micrometer spectral range (Green et al., 1998). The sensor is a whiskbroom system utilizing scanning foreoptics to acquire cross-track data. The IFOV is 1 milliradian. Four off-axis double-pass Schmidt spectrometers receive incoming illumination from the foreoptics using optical fibers. Four linear arrays, one for each spectrometer, provide high sensitivity in the 0.4 to 0.7 micrometer, 0.7 to 1.2 micrometer, 1.2 to 1.8 micrometer, and 1.8 to 2.5 micrometer regions respectively. AVIRIS is flown as a research instrument on the NASA ER-2 aircraft at an altitude of approximately 20 km, resulting in approximately 20-m pixels and a 10.5-km swath width. Since 1998, it has also been flown on a Twin Otter aircraft at low altitude, yielding 2 – 4m spatial resolution. There are also a number of other airborne instruments, both commercial and developed by various governments around the world (http://www.geo.unizh.ch/~schaep/research/apex/is_list.html). The launch of NASA’s EO-1 Hyperion sensor in November 2000 marked the establishment of VNIR/SWIR spaceborne imaging spectrometer mapping capabilities. Hyperion is a satellite sensor covering the 0.4 to 2.5 micrometer spectral range with 242 spectral bands at approximately 10nm spectral resolution and 30m spatial resolution from a 705km orbit (Pearlman et al., 2003). Hyperion is a pushbroom instrument, capturing 256 spectra each with 242 spectral bands over a 7.5km-wide swath perpendicular to the satellite motion along an up to 160km path length. The system has two grating spectrometers; one visible/near infrared (VNIR) spectrometer (approximately 0.4 – 1.0 micrometers) and one short-wave infrared (SWIR)) spectrometer (approximately 0.9 – 2.5 micrometers). Key Hyperion characteristics are discussed further in Green et al. (2003). Hyperion data are available for purchase from the U. S. Geological Survey (USGS E0-1 Website: http://eo1.usgs.gov/ ). Thousands of Hyperion scenes have been acquired for a variety of disciplines. The EO-1 Science Validation Team has evaluated and validated the instrument. Selected results have been published in various venues (Asner and Green, 2001; Hubbard and Crowley, 2001; Kruse et al., 2003). Also see Ungar (2003) for a summary along with associated papers.

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