Experimental Study On The Effect Of Roughness And Observation Angle On Thermal Infrared Spectra Characteristics And Unmixing

The mineral component and content determination belongs to a fundamental work in mine production.Considering the relatively low sampling density,long period and low efficiency of the conventional field sampling and chemical examination method,the requirement of rapid delineation,grade division,and efficient ore blending of ore bodies can not be satisfied.In recent years,quantitative inversion methods based on spectral features recognition have been developed and the spectral unmixing method was one of the typical methods.However,the inversion accuracy based on the spectral unmixing method is seriously affected by multiple influence factors and complex mechanism,such as the surface roughness and the observation angles.In order to reveal the influence of roughness and observation angle on the thermal infrared spectrum and spectral unmixing of rock,the experimental studies focused on the effect caused by these two factors were performed which was funded by the National Natural Science Foundation's projects‘Experimental study on the effect characteristic of roughness and observation angle on thermal infared spectral unmixing of massive rock'.This study yielded the following main contents and results:(1)Study on the surface morphology of natural massive rock.Surface morphology of natural rock was measured with 3D laser scanning to 3 kinds of typical samples.The surface morphology characteristic of rock was analysised by wavelet analysis method and the surface roughness of natural massive rocks was obtained.The roughness parameters of Ra and Rq of all the samples were calculated so the roughness ranges of the natural massive rocks were obtained.The results show that,the roughness parameters of Ra and Rq for all the samples are in the range of 0.8-8.8?m and 1.1-11.5?m,respectively.All the rough surfaces are basically according with the normal distribution and the fractal distribution.(2)Study on the effect of roughness on thermal infrared spectral feature of mineral.The typical minerals(quartz,orthoclase,anorthose etc.)were made into the samples with a series of roughness based on the observation results of natural massive rock roughness and Becker criterion.Then the thermal infrared spectra of these samples were acquired and the spectral features of the sample caused by different roughness were analyzed.The results show that:The roughness has obvious influence on the spectral characteristics of the three minerals,and the influence characteristics are different at different bands.There is no significant effect in the CF bands,and here is an obviously variation in the RF range.The emissivity increase obviously with the increasment of roughness till reaching to a certain threshold.The inflection point of the emissivity threshold of quartz,orthoclase and plagioclase are about 4.11?m,3.74?m and 4.04?m,respectively,which is consistent with the results based on the Becker criterion.There is no significant regular emissivity change on the value in the TF band.Further quantitative analysis shows that the effect of roughness on the emissivity spectrum is obvious at the wavelength of 8.27?9.69?m for quartz,8.67-10.28?m for orthoclase,8.51?8.74?m and 9.05?10.22?m for plagioclase,in other words,the emissivity spectrum is sensitive to the roughness change.(3)Study on the effect of roughness on thermal infrared spectrum unmixing.The monzonitic granites are simulated by means of mineral block mosaicking.The influence of roughness on rock spectral unmixing was studied by cross unmxing mode based on the spectrum of mineral endmembers and simulated rock at different roughness.The results show that the spectral unmixing error is less than 5%when the mineral endmember and rock share the same roughness and the unmixing precision is high.Otherwise,the spectral unmixing error is greater than 5%(with the maximum 16.80%)when the roughness of the endmember and simulated rock are at different levels.Therefore,the effect of roughness should be taken into account in the quantitative inversion of rock mineral components by spectral unmxing for high precision.(4)Study on the effect of observation angle on thermal infrared spectral feature and unmixing.The thermal infrared spectra of quartz,orthoclase and plagioclase samples with general roughness are acquired at 9 inclined angles,and then the effects of observation angles on the spectral characteristics are studied.The results show that:the position of CF shifts towards the shortwave direction significantly when the observation angle is lager than 50°.The same phenomenon of shifts towards shortwave is observed in the RF range as well when the angle is increased from 0° to 77°.The depth of the absorption valley in the RF band increases when the angle is larger than 20°.The emissivity values in the TF range decrease obviously when the angle is larger than 40°.Further quantitative analysis shows that the sensitive wavebands for the spectrum to the inclined angle change are 7.52?8.71?m of quartz,7.97?9.21 ?m of orthoclase,and 7.96-9.10?m of plagioclase.The observation angles are selected as the factors to affect unmixing accuracy of massive rocks.The spectrum of the simulated monzonitic granites at different inclined angles were unmixed by the spectrum of minerals endmembers by vertical observation.The results show that:the effect of observation angle on spectral unmixing is not obvious in 0?20°,with the unmixing error less than 5%.Then,the observed angle has a significant effect on the spectral unmixing precision when the observation angle is larger than 20°.The unmixing error is greater than 5%and the average error is 17.2%when the angle is in the range of 30?77°.Therefore,the influence of observation angle should be taken into account in the quantitative inversion of rock mineral components with high accuracy using spectral unmixing as well.(5)Mechanism analysis of experimental results and its guiding significance for quantitative inversion of rock and ore.According to the theory of light scattering,multiple reflections,Hapke model and Kirchhoff approximation method,the mechanism on the effect of mineral sample roughness and observation angle on spectral emissivity were explored and analyzed.The results show that the effect of roughness on emissivity is caused by multiple factors,such as the surface scattering,volume scattering and cavity effect etc.The directional emissivity of mineral samples under general roughness is affected by smooth surface emissivity,multiple scattering,and obscuration of features etc.Based on the aforementioned studies,the guiding significance of the experimental results for quantitative inversion of rock and ore in actual mines is discussed.