典型煤的近红外反射光谱特征的方向性研究

高光谱遥感是煤矿区探测的有效方法，对于煤炭资源调查、矿区环境监测等具有重要意义，其中煤、矸石、植被、水体等被遥测物各个方向的反射光谱特征是煤矿高光谱遥感的基础，为此有必要针对典型煤的方向反射光谱特征进行研究。从我国不同矿区收集了无烟煤、烟煤、褐煤三大类煤中的4种典型煤样，4种煤样按煤阶由高到低顺序包括无烟煤一号、贫煤、气煤、褐煤二号，在实验室利用方向反射测量球坐标实验装置测定了每种煤样半球空间各反射方向的近红外波段(1 000~2 500 nm)反射光谱曲线。通过对反射光谱曲线波形分析，发现同一种煤不同反射方向的近红外反射光谱波形基本相似，但在整体反射率大小和局部波形特征上具有差异性，光谱曲线整体反射率越大，吸收谷越明显。随反射角增大，4种煤在前向反射方向(180°探测方位角)反射光谱曲线均整体上升，在后向反射方向(0°探测方位角)反射光谱曲线高度变化相对较小。在每种煤半球空间各反射方向的反射光谱曲线中，选取了1 400，1 700，1 900，2 200和2 300 nm 5个特征波长，通过分析此5个特征波长处的反射率空间分布极坐标云图，发现4种煤在5个特征波长点处均具有一定的双向反射特征，均表现出较明显的前向反射热点特征和相对较弱的后向反射热点特征，无烟煤一号后向反射热点特征比贫煤、气煤、褐煤二号更明显，贫煤、气煤、褐煤二号随煤阶降低后向反射热点特征逐渐增强。每种煤前、后向反射方向的反射光谱中，通过对5个特征波长处反射率与反射角进行相关性分析，发现前向反射特征波长反射率与反射角近似呈线性函数关系，后向反射特征波长反射率与反射角近似呈高斯函数关系，且高斯函数拟合曲线波峰随煤阶降低向较大反射角度移动。该研究为矿区煤炭高光谱遥感最优探测几何的选择提供了依据，也为矿区煤炭资源精准探测提供了参考。

Study on Directional Near-Infrared Reflectance Spectra of Typical Types of Coal

Hyperspectral remote sensing is an effective method for coal mining area detection, and it is of great significance for coal resource surveys and environmental monitoring in the mining area. At the same time, reflectance spectrum characteristics of remotely measured objects such as coal, gangue, vegetation and water body in all directions are the basis of hyperspectral remote sensing in the coal mine. In this paper, the directional reflectance spectra of typical types of coal were studied. Four typical types of coal in the three major coal types anthracite, bituminous and lignite were collected from different mining areas in China. According to the increasing rank, these coals included No.1 anthracite, meager coal, gas coal and No.2 lignite. Spectral reflectance curves of each type of coal in all directions in hemispheric space were measured in the near-infrared band (1 000~2 500 nm) using the spherical coordinate device for directional reflection measurement in the laboratory. By waveforms of spectral reflectance curves acquired, it was found that near-infrared reflectance spectrum curves of the same coal in different reflection directions show similar waveforms. However, there are some differences in overall reflectance and local waveforms, and the rule is that the absorption valleys become more obvious with increasing overall reflectance. With increasing reflection angle, reflectance spectrum curves of all these four types of coal rise on the whole in the forward direction (180° azimuth), but the change is relatively small in the backward direction (0° azimuth). In each directional reflectance spectrum curve in the hemispheric space of each coal, five characteristic wavelength points, including 1 400, 1 700, 1 900, 2 200 and 2 300 nm were selected. By polar nephograms of the spatial distribution of reflectance at the five wavelength points, it was found that all these four types of coal show bidirectional reflection and prominent hot spots in the forward direction and relatively weaker hot spots in the backward direction. The hot spots in the backward direction of No.1 anthracite appear relatively more obvious than those of meager coal, gas coal and No.2 lignite. With decreasing coal rank, meager coal, gas coal, and No.2 lignite show the rule of relatively enhanced hot spots in the backward direction. The correlation between reflectance and reflection angle of backward and forward direction at the five wavelength points of each type of coal were analyzed. It was found that the correlations between reflectance and reflection angle are approximately linear and Gaussian functions in forwarding and backward direction respectively. Moreover, with decreasing coal rank, the peak of the Gaussian fitting curve moves to a larger reflection angle. This study provides the basis for the selection of the optimal detection geometry and reference for precise detection of coal resources in hyperspectral remote sensing of mining areas.