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Author: Hengqian Zhao
Supervisor: Qingxi Tong;Lifu Zhang
The retrieval of quantitative information of minerals has strong economic and scientific meanings, and is essential for the long term development of human beings. Hyperspectral remote sensing has unique advantage and great potential in quantitative analysis of minerals, but has not been fully exploited in practical applications. However, the primary cause is due to the uncertain factors in quantitative analysis of minerals using hyperspectral remote sensing, which includes the complex mechanism of mineral spectra, the accuracy of spectral unmixing, and the accuract extraction of mineral absorption features. To address this issue, this paper firstly systematically analyzed the mechanism of mineral spectra, and then investigated the uncertain factors in quantitative retrieval algorithms, which can be generally grouped into two classes: spectral unmixing and absorption feature extraction. As for spectral unmixing, the effects of spectral unmixing models and band position on mineral retrival accuracy were fully investigated. In the field of absorption feature extraction, a new absorption feature extraction method was proposed to retrieve more accurate absorption parameters. This dissertation will further serve as a guide for improved quantitative retrievals of minerals from spaceborne, airborne, or other hyperspectral remote sensing platforms, and pave the way to explore some important subjects using remotely sensed mineral contents, including the metallogenic prognosis, planetary geology evolution, mine environmental restoration, etc. The main conlusions and results are as follows.
(1) The mechanism of mineral spectra were systematically analyzed from five different aspects, including the mineral types, the chemical change, the physical propoties, spectral measurement conditions, and spectral mixing effect, and discussed about the correlations between these factors. Among them, spectral mixing effect is the main source and theory basis of quantitative analysis of minerals using hyperspectral remote sensing data.
(2) This dissertation summarized the concept of mixing reflectance reconstruction (MRR), and proposed a thorough method to determine the accuracies of spectral unmixing models based on MRR. The MRR error can be analyzed from 3 different dimensions, including the spectral dimension, the spatial dimension, and the total dimension. By measuring the spectra of proportionally mixed mineral powders, we were able to investigate and verify the relationship between MRR accuracy and spectral unmixing accuracy. This finding validates that when the actual fractions are not available, it is possible to estimate the spectral unmixing accuracy based on an MRR accuracy analysis.
(3) A newly developed LCR model was developed based on investigating the existing spectral unmixing algorithms and physical mechanism analysis. Existing models having typical applications related to mineral analysis, including the Linear model, the NL model, the CR model and the SH model, were also summarized. Experiments on the well-known AVIRIS data for Cuprite allowed us to evaluate the effects of the five unmixing models based on the MRR accuracy analysis. The results revealed that the LCR model yielded good results in nearly all aspects, and had considerable potential for practical application. By comparing the total MRR error of ATREM dataset and Flat Field dataset, the effects of atmospheric correction on spectral unmixing accuracy were verified, but the level of influence was different for different unmixing models. LCR model achieved the most outstanding results, which verified its great robustness.
(4) This dissertation proposed a new spectral unmixing model based on derivative of ratio spectroscopy (DRS), which can directly interpret the correspondence between the target content and variations of mixing spectra, eliminate the influence of other substances in the mixture, and extract the bands which are more sensitive to the target information. Based on this model, the effects of spectral position on spectral unmixing accuracy were investigated using mineral powder mixtures. The results indicated that the absorption features have strong influence on the spectral unmixing accuracy, and the bands near the slopes of absorption feature valleys tend to have higher accuracy.
(5) Based on the study of the mechanism of background removal methods, represented by continuum removal, a new spectral fitting method was presented to obtain the background curve, and a novel background removal method named reference spectral background removal (RSBR) was given. RSBR retains the advantages of continuum removal, and when given the reference spectral background, RSBR can eliminate the influence of unwanted contribution factor, and extract the absorption feature of target contribution factor.
(6) Based experiments on both mineral powder mixtures and airborne hyperspectral data, RSBR was demonstrated to have the following advantages in extraction of absorption feature parameters: 1) RSBR can extract accurate absorption centers and absorption widths from mixing spectrum, independent of the variation in abundance; 2) absorption depths calculated from the RSBR spectra are strongly linearly correlated to the fractions of the component of interest; 3) the spectral waveform of the specific absorption factor can be well extracted by RSBR, and by using spectral matching methods, such as SAM, the mineral composition can be identified.