Mapping Layers of Clay in a Vertical Geological Surface Using Hyperspectral Imagery: Variability in Parameters of SWIR Absorption Features under Different Conditions of Illumination

Richard Murphy,Sven Schneider,Sildomar Monteiro

doi:10.3390/rs6099104

Abstract

Hyperspectral imagery of a vertical mine face acquired from a field-based platform is used to evaluate the effects of different conditions of illumination on absorption feature parameters wavelength position, depth and width. Imagery was acquired at different times of the day under direct solar illumination and under diffuse illumination imposed by cloud cover. Imagery acquired under direct solar illumination did not show large amounts of variability in any absorption feature parameter; however, imagery acquired under cloud caused changes in absorption feature parameters. These included the introduction of a spurious absorption feature at wavelengths > 2250 nm and a shifting of the wavelength position of specific clay absorption features to longer or shorter wavelengths. Absorption feature depth increased. The spatial patterns of clay absorption in imagery acquired under similar conditions of direct illumination were preserved but not in imagery acquired under cloud. Kaolinite, ferruginous smectite and nontronite were identified and mapped on the mine face. Results were validated by comparing them with predictions from x-ray diffraction and laboratory hyperspectral imagery of samples acquired from the mine face. These results have implications for the collection of hyperspectral data from field-based platforms.

Highlights

Identifying and mapping clay minerals on vertical outcrops of geology or on mine faces in open-pit mines is important for economic reasons and safety
Using the automated feature extraction (AFE) method, the distribution, identity and abundance of clay minerals could be mapped on the mine face without the use of a spectral library, enabling thin layers of clay, to be distinguished and mapped
The ability to rapidly quantify aspects of mineralogy from short-wave infrared (SWIR) absorption features, without the use of a spectral library and without a priori knowledge is of significance to mining corporations and government organizations interested in mitigating risks of slope failure

Summary

Introduction

Identifying and mapping clay minerals on vertical outcrops of geology or on mine faces in open-pit mines is important for economic reasons and safety. In the context of remote sensing, the position of clay layers within the stratigraphy could potentially be used as a basis for the contextual classification of geological units in cases where they could not be distinguished on the basis of their spectral signature alone. Even relatively thin layers of clay can potentially increase the risk of slippage [5]. 2–6 cm thick layers of clay in coal seams—so called clay mylonites—represent planes of weakness which can significantly increase the risk of slippage or failure along their length [6,7]. Identifying the types and abundance of clay minerals in these layers provides information that can be incorporated into assessments of risks or models of potential slope failure in road cuttings or in open pit mines, e.g., [12]

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