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1.
Previous studies have shown that spectral signatures of hydrocarbon-bearing materials are characterized by prominent absorption features at 1.73 and 2.31?µm. Many other materials also show absorption features at wavelengths in the interval from 2.0 to 2.5?µm, yielding a mixed response in spectral signatures. In contrast to this wavelength range, most materials show similar spectral characteristics in the 1.73?µm range. Mainly hydrocarbon-bearing materials produce an absorption feature which is unique and prominent at 1.73?µm. A Hydrocarbon Index (HI) was developed and tested for the direct detection of hydrocarbons. The HI transforms multispectral data into a single image band that shows the distribution of hydrocarbons on the ground surface. The HI takes advantage of reflection differences around the 1.73?µm feature in hydrocarbon spectra. The HI indicates the presence of the 1.73?µm hydrocarbon absorption feature in a pixel spectrum. HI values can be easily calculated from radiance and reflectance data recorded by high signal-to-noise ratio hyperspectral scanners.  相似文献   

2.
A recently-launched high-resolution commercial satellite, DigitalGlobe’s WorldView-3, has 8 bands in the shortwave infrared (SWIR) wavelength region, which may be capable of estimating canopy water content at 3.7-m spatial resolution. WorldView-3 also has 8 multispectral bands at 1.24-m resolution with two bands in the near-infrared (NIR). The relative spectral response functions for WorldView-3 were provided by DigitalGlobe, Inc., and band reflectances were determined for reflectance spectra of PROSPECT model simulations and leaf data from maize, trees, grasses, and broadleaf herbaceous eudicots. For laboratory measurements, the range of leaf water contents was extended by including drying leaves and leaf stacks of corn, soybean, oaks, and maples. Correlations between leaf water content and spectral indices from model simulations suggested that indices using SWIR band 1 (center wavelength 1210 nm) had low variability with respect to leaf water content, but also low sensitivity. Other indices using SWIR band 5 (2165 nm) had the highest sensitivity, but also had high variability caused by different values of the leaf structure parameter in PROSPECT. Indices using SWIR bands 2, 3 and 4 (1570, 1660, and 1730 nm, respectively) had high correlations and intermediate variability from the leaf structure parameter. Spectral indices calculated from the leaf data had the same overall patterns as the simulations for variation and sensitivity; however, indices using SWIR band 1 had low correlations, and the best correlations were from indices that used SWIR bands 2, 3 and 4. Spectral indices for maize, grasses, and herbaceous crops and weeds had similar responses to leaf water content; tree leaves had higher index values and saturated at lower leaf water contents. The specified width of NIR band 2 (860–1040 nm) overlaps the water absorption feature at 970 nm wavelength; however, the normalized difference of NIR band 1 and 2 was insensitive to water content because NIR band 2’s spectral response was most heavily weighted to wavelengths less than 930 nm. The high spatial resolution of the WorldView-3 SWIR data will help analyze how variation among plant species and functional groups affects spectral responses to differences in canopy water content.  相似文献   

3.
This Letter presents field‐based evidence of the perturbing effects of surface anisotropy on the remote sensing of burned savannah. The analysis is based on bidirectional spectral reflectance data collected at different solar illumination angles and convolved to Moderate‐resolution Imaging Spectroradiometer (MODIS) reflective bands. Results from a grass savannah site show that burning reduces the anisotropy of the surface compared to its pre‐burn state. In contrast, at a shrub savannah site, burning reduces or increases surface anisotropy. Spectral indices defined from 1.240 µm and 2.130 µm reflectance, and 1.640 µm and 2.130 µm reflectance, provided stronger diurnal separation between burned and unburned areas than individual reflectance bands but do not eliminate anisotropic effects. The Normalized Difference Vegetation Index (NDVI) provides weak diurnal separation relative to these near‐ and mid‐infrared based indices. Implications of the findings are discussed for burned area mapping.  相似文献   

4.
This study investigated the usability of different multitemporal compositing methods for burnt area detection purposes in the humid tropical conditions of insular Southeast Asia, characterised by persisting cloud cover, varying fire‐induced spectral changes and large amount of small burn scars. Six monthly composites of the Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance images (MOD09) were built using six different algorithms. The performance and usability of the compositing algorithms were evaluated using three criteria: separability between burnt and unburnt areas, homogeneity and average sensor zenith angle. Maximum surface temperature method (band 31, 11.0 µm) produced the most homogeneous composites with preference to close‐to‐nadir observations. However, these composites showed unexpectedly low separability between burnt and unburnt areas, mainly due to low spatial resolution of band 31 (1000 m). Overall, taking into account the performance of the compositing methods and the large amount of small burnt areas in insular Southeast Asia, a minimum NIR (band 2, 0.86 µm) method, combined with pre‐compositing cloud and cloud shadow removal, was seen as the most suitable compositing method for burnt area detection in this region. Its strengths were 250 m spatial resolution in pixel selection and high burnt/unburnt separability combined with reasonably good performance on homogeneity and average sensor zenith angle.  相似文献   

5.
A summary of the spectral characteristics of the LANDSAT-4 and LANDSAT-5 Thematic Mapper instruments, the protoflight (TM/PF) and flight (TM/F) models, respectively, is presented. Data collected by the Hughes/Santa Barbara Research Center on the instruments and their components to determine compliance with the spectral coverage and spectral matching specifications served as the basis for the characterization. Compliance with the spectral coverage specifications (e.g. band locations) was determined by deriving band-by-band relative spectral response (RSR) curves from spectral measurements on the individual components contributing to the overall spectral response: filters, detectors and optical surfaces. The integrated system RSRs were not measured. The derived RSRs for the reflective bands were similar between TM/PF and the TM/F. The bandpasses between 50 per cent RSR points varied by only 2nm at most between the sensors and were: band 1, 452-518 nm; band 2, 528-609 nm; band 3, 625-693 nm; band 4, 776-904 nm; band 5, 1568-1784 nm and band 7, 2097-2348 nm. The upper-band edge of band 5 was outside the desired and specified range of 1750 ±20 nm; this implies that there will be more contribution from variable atmospheric water vapour absorption. In the emissive thermal band 6, the TM/PF and TM/F showed fundamentally different spectral responses. Though the lower band edges were both at approximately 104/mi, the TM/PF upper-band edge was detector limited at a temperature-dependent value of about 11·7μm, whereas the TM/F upper-band edge was filter limited at 12·4μm. Despite the TM/PF's band 6 narrow bandwidth, its radiometric performance exceeded requirements, so the band's narrowness was not a serious concern. Spectral matching measures the spectral differences between channels within a band by calibrating them all with one source and comparing their outputs to a spectrally different source. Satisfactory TM/PF spectral matching data were never obtained. TM/F channels were shown to have comparable or better spectral matching than past and existing MSS sensors.  相似文献   

6.
This study explores the detectability of atmospheric carbon dioxide (CO2) absorption at the short wave infrared bands of Hyperion EO-1 hyperspectral images. Two CO2 absorption bands at around 2.002–2.012 and 2.052 µm, respectively, were identified on the reflectance spectra derived from the images. Considering the principle of differential optical absorption spectroscopy (DOAS) and assuming the O2-A band around 0.762 µm as reference, the spatial variation of the column density was estimated for Kolkata, a region around 22° 31′ N 88° 21′ E. The ratio of the individual absorption depth of CO2 to that of O2, instead of the ratio of their column density, was found to be more suitable to express the spatial distribution of CO2 concentration over short distance, of the order of metres. The methodology was validated with Orbiting Carbon Observatory-2 (OCO-2) data. The work suggests the inclusion of 2–3 narrowband channels corresponding to O2-A and CO2 absorptions in future moderate resolution multispectral sensors in order to facilitate the monitoring of atmospheric CO2.  相似文献   

7.
This study investigates fire‐induced spectral changes detected by the Moderate Resolution Imaging Spectroradiometer (MODIS) in different land‐cover types in Borneo. Linear discriminant analysis is used to determine the most powerful band combinations among the MODIS reflective bands for discrimination between burnt and unburnt areas in each land‐cover type. The results show that the nature of fire‐induced changes is dependent on pre‐fire vegetation characteristics in this region. Bands 1 (0.64 µm), 2 (0.86 µm), and 7 (2.14 µm) are found to be the most sensitive bands in land‐cover types dominated by green vegetation, and consequently indices or combinations of indices using these three bands are potentially effective for burnt‐area detection in the majority of areas. In land‐cover types dominated by dry vegetation and soil, MODIS band 5 (1.24 µm) alone showed the greatest statistical separability and could not be significantly improved by any multiband index.  相似文献   

8.
Understanding the reflectance spectral variations associated with oxidation of iron sulphide materials is highly important to the detection of environmental changes affected by the detrimental acid mining drainage and acid sulphate soil (ASS) using hyperspectral sensing. An incubation experiment with iron-sulphide-rich sediments was set up to simulate the formation and evolution of ASS. The sediments were shaped into different micro-landforms. Spectral changes and mineral transformation were observed by comparing weekly measurements of the materials in the different landforms using an ASD (Analytical Spectral Devices) reflectance spectrometer, as well as comparing the measurements of the initial materials and the residual materials after oxidation. We found that the spectral changes usually occur at the spectral absorptions near 0.9 μm, which relate to ferric iron contents, near 1.17 μm, which relate to ferrous iron content, and near 2.265 μm, which relate to jarosite. Spectral variations are detected both in specific absorption position and depth. We also found that the causes of the spectral variations are mainly due to the variations in mineral transformation and consequent mineral composition, owing to lying in different oxidation stages and drainage conditions in the different landforms. This article shows not only oxidation of sulphides to different mineral phases spectrally but also the link of the phases with landforms and drainage conditions and the link between oxidation degree and oxidation progress, all of which have not been sufficiently studied before.  相似文献   

9.
Separation of multiple microparticles at high throughput is highly required in different applications such as diagnostics and immunomagnetic detection. We present a microfluidic device for multiplex (i.e., duplex to fourplex) fractionation of magnetic and non-magnetic microparticles using a novel hybrid technique based on interactions between flow-induced inertial forces and countering magnetic forces in a simple expansion microchannel with a side permanent magnet. Separation of more than two types of particles solely by inertia or magnetic forces in a straight microchannel is challenging due to the inherent limitations of each technique. By combining inertial and magnetic forces in a straight microchannel and addition of a downstream expansion hydrodynamic separator, we overcame these limitations and achieved duplex to fourplex fractionation of magnetic and non-magnetic microparticles with high throughput and efficiency. Particle fractionation performance in our device was first optimized with respect to parameters such as flow rate and aspect ratio of the channel to attain coexistence of inertial and magnetic focusing of particles. Using this scheme, we achieved duplex fractionation of particles at high throughput of 109 particles per hour. Further, we conducted experiments with three magnetic particles (5, 11 and 35 µm) to establish their size-dependent ordering in the device under combined effects of magnetic and inertial forces. We then used the findings for fourplex fractionation of 5, 11 and 35 µm magnetic particles from non-magnetic particles of various sizes (10–19 µm). This Multiplex Inertio-Magnetic Fractionation (MIMF) technique offers a simple tool to handle complex and heterogeneous samples and can be used for affinity-based immunomagnetic separation of multiple biological substances in fluidic specimens in the future.  相似文献   

10.
Measurement of liquid film thickness is essential for understanding the dynamics of two-phase flow in microchannels. In this work, a miniaturized sensor matrix with impedance measurement and MEMS technology to measure the thin liquid film underneath a bubble in the air–water flow in a horizontal microchannel has been developed. This miniaturized sensor matrix consists of 5 × 5 sensors where each sensor is comprised of a transmitter and a receiver electrode concentrically. The dimension and performance of the sensor electrodes were optimized with simulation results. The maximum diameter of the sensor ring is 310 µm, allowing a measurable range of liquid film thickness up to 83 µm. These sensors were distributed on the surface of a wafer with photolithography technology, covering a total length of 8 mm and a width of 2 mm. A spatial resolution of 0.5 × 2.0 mm2 and a temporal resolution of 5 kHz were achieved for this sensor matrix with a measurement accuracy of 0.5 µm. A series of microchannels with different heights were used in the calibration in order to achieve the signal-to-thickness characteristics of each sensor. This delicate sensor matrix can provide detailed information on the variation of film thickness underneath gas–water slug directly, accurately and dynamically.  相似文献   

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