Mineral Mapping Using Spectroscopy

Mineral Mapping Using Spectroscopy: From Field Measurements to Airborne and Satellite-Based Imaging Spectrometry

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).