Continuum removed band depth analysis for detecting the effects of natural gas, methane and ethane on maize reflectance
Introduction
Hydrocarbon gases in the soil may be sourced from a leaking gas pipeline or natural hydrocarbon seepage. Gas pipelines that are buried in the soil can start leaking for several reasons, of which aging and third party interference are the most common. If the leak is large or undiscovered for a long time, substantial volumes of explosive gases in the soil can develop into dangerous situations and a loss of revenue will be incurred (in the United States alone, the National Transportation Safety Board (NTSB) reported millions of dollars in losses and several casualties due to gas pipeline leaks in recent years (National Transportation Safety Board, 2001, National Transportation Safety Board, 2003)). Natural hydrocarbon seepages are vents of gases that escape from an underlying gas-bearing rock. The locations of long-term natural hydrocarbon seepages are generally known and if they are located in populated areas care can be taken to avoid dangerous situations, but after earthquakes their location can shift (Jones & Burtell, 1996). Early detection of gas seepage could therefore help to prevent dangerous situations.
It is known that natural gas in the soil affects vegetation health and reflectance, even though the precise mechanisms have not been identified. Vegetation growing in soil contaminated with natural gas shows reduced growth (Hoeks, 1972b, Pysek and Pysek, 1989, Smith et al., 2004b) as well as changes in spectral reflectance, particularly in the ‘red edge’ region (the slope between red and near infrared reflectance) commonly seen to shift towards shorter wavelengths in stressed vegetation (Boochs et al., 1990, Horler et al., 1983). In most studies, the red edge of vegetation canopies shifted towards shorter wavelengths when exposed to natural hydrocarbon seepage (Bammel and Birnie, 1994, Reid et al., 1988) or to simulated gas seepage (Oliveira et al., 1997, Smith et al., 2004b). Yang et al. (1999) however found a shift of the red edge towards longer wavelengths, which was suggested to be a result of fertilization caused by the hydrocarbons. Hydrocarbon gas itself is invisible, so changes in vegetation growth or reflectance could serve as an indicator of gas leakage. The response of plants to gas in the soil is believed to arise from oxygen shortage in the root zone, caused by a displacement of soil air by the gas (Braverman et al., 1962, Hoeks, 1972a). The oxygen concentration in the soil may decrease further due to methanotrophic bacteria that oxidize the methane present in natural gas thereby generating carbon dioxide and water (Hanson & Hanson, 1996). A study by Steven et al. (2006) showed that in addition to oxygen displacement by methane, bacterial oxygen depletion ranged from 1.8% to 4.7% (as percentage of total soil air), depending on temperature and crop type. Most plants need oxygen for respiration of the roots and for proper uptake of water and nutrients (Drew, 1983). High carbon dioxide concentrations may be an additional factor affecting plant growth (Boru et al., 2003). Moreover, low oxygen and high carbon dioxide concentrations can result in changes in soil pH and redox potential (Eh) (Schumacher, 1996), possibly causing mobilization of phytotoxic nutrients such as manganese and aluminium. Smith et al. (2004a) demonstrated that soil oxygen displacement by natural gas, argon, nitrogen and waterlogging caused an increase in reflectance in the visible region and a change in the shape and position of the red edge. These results suggested that the changes in reflectance due to gas leakage are caused by oxygen shortage and not by the natural gas itself (Smith et al., 2004a). Detecting gas leakage using vegetation reflectance is therefore similar to detecting the effects of soil oxygen shortage, making the use of additional data such as gas pipeline maps necessary for reliable gas detection (Smith et al., 2004a).
Although in large gas leaks oxygen shortage may be the cause of reflectance changes, in small leaks there may be no oxygen shortage. Since all work so far has been done on large leaks–e.g. 100 l/h (Smith et al., 2004b), 250 l/h (Pysek & Pysek, 1989)–and natural hydrocarbon seepages, it is not known if natural gas itself affects leaf reflectance. It is assumed that methane is not toxic to plant roots (Arthur et al., 1985) or aboveground parts of plants (Gustafson, 1944). Natural gas however comprises not only methane (CH4; 70% to 95%) but also ethane (C2H6; 0% to 20%), higher hydrocarbons such as propane and butane (< 1%), and small concentrations of other components such as nitrogen, carbon dioxide and oxygen (Hoeks, 1972a, Olah and Molnar, 2003). If natural gas itself affects vegetation reflectance, small leaks that do not cause oxygen shortage could then be detected in addition to larger leaks that can be detected using the existing methods. The objective of this study was to test whether natural gas and two of its components methane and ethane affect plant leaf reflectance. A greenhouse experiment was designed in which leaf reflectance and growth characteristics such as height and chlorophyll were measured of maize plants growing in soil polluted with natural gas, methane and ethane. The gas was confined to the root zone of the plants so that any effect by the gases is likely to be noticed in the root functioning first. Since a major function of roots is supplying plants with water and nutrients (Lynch, 1995), we expected that malfunctioning of the roots would result in reflectance changes commonly related to plant stress, such as increased reflectance in the chlorophyll and water absorption regions (Carter, 1993, Lichtenthaler, 1996). Chlorophyll has strong absorption in the visible region, while leaf water absorbs mainly in the near and shortwave infrared (Curran, 1989, Tucker, 1980). Absorption feature analysis using continuum removal has been shown to enhance the differences in shape between the absorption features of interest (Kokaly & Clark, 1999). In this study, continuum removal was applied to two chlorophyll absorption features and two water absorption features to study whether natural gas, methane and ethane in the soil increase the reflectance in these absorption features. In addition, the physiological reflectance index (PRI; Gamon et al., 1992) was calculated to test whether the photosynthetic efficiency was affected by any of the gases.
Section snippets
Plant species and experimental design
The experiment was carried out in a greenhouse in Wageningen, the Netherlands from January 8, 2004, to March 22, 2004. The experiment was designed such that the influence of natural gas, methane and ethane on maize (Zea mays cv Nescio) plants could be measured without causing oxygen shortage in the soil. Each gas was mixed with air before use by a gas company (Hoek Loos B.V., Schiedam, the Netherlands) and put in cylinders, after which each gas–air mixture was delivered through plastic pipes of
Plant morphology
The plants growing in natural gas, methane and ethane were respectively 7%, 5% and 17% shorter than the control plants (Table 3). Only the difference between ethane and control was significant (p ≤ 0.05). Dry weight and chlorophyll values varied at most 6% and were not significantly different (p ≤ 0.05).
Comparison of band depths
On average, the control plants had the deepest band depths in all absorption regions while ethane-treated plants had the shallowest band depths. The band depths of the plants treated with natural
Discussion
Only ethane caused a significant decrease in absorption in the red region (550–750 nm). In particular the 560–600 nm region of the red absorption feature was affected with a decreased band depth of up to 14% compared to the control (Fig. 4). This is the slope between the green reflectance peak and the red absorption trough: the plants growing in ethane should therefore be more orange in colour than the control plants, but this was not visible by eye. An increased yellow leaf reflectance has
Conclusion
The objective of this study was to test whether natural gas, methane and ethane in the soil cause an increase in continuum removed reflectance in the chlorophyll and water absorption regions. Although methane and natural gas caused a minor decrease in band depth in the chlorophyll absorption regions, only ethane caused a significant decrease in band depth in the 550–750 nm absorption feature, in particular in the yellow region (560–600 nm). Normalization of the band depths resulted in an
References (60)
- et al.
Detecting vegetation leaf water content using reflectance in the optical domain
Remote Sensing of Environment
(2001) Remote sensing of foliar chemistry
Remote Sensing of Environment
(1989)- et al.
Estimating the foliar biochemical concentration of leaves with reflectance spectrometry; testing the Kokaly and Clark methodologies
Remote Sensing of Environment
(2001) - et al.
High resolution derivative spectra in remote sensing
Remote Sensing of Environment
(1990) - et al.
A new instrument for passive remote sensing: 2. Measurement of leaf and canopy reflectance changes at 531 nm and their relationship with photosynthesis and chlorophyll fluorescence
Remote Sensing of Environment
(2004) - et al.
A narrow-waveband spectral index that tracks diurnal changes in photosynthetic efficiency
Remote Sensing of Environment
(1992) - et al.
Detection of the change in leaf water content using near- and middle-infrared reflectance
Remote Sensing of Environment
(1989) - et al.
Spectroscopic determination of leaf biochemistry using band-depth analysis of absorption features and stepwise multiple linear regression
Remote Sensing of Environment
(1999) Vegetation stress: An introduction to the stress concept in plants
Journal of Plant Physiology
(1996)- et al.
Predicting in situ pasture quality in the Kruger National Park, South Africa, using continuum-removed absorption features
Remote Sensing of Environment
(2004)
Structure analysis and classification of boreal forests using airborne hyperspectral BRDF data from ASAS
Remote Sensing of Environment
Estimation of vegetation water content and photosynthetic tissue area from spectral reflectance: A comparison of indices based on liquid water and chlorophyll absorption features
Remote Sensing of Environment
Use of hyperspectral derivative ratios in the red-edge region to identify plant stress responses to gas leaks
Remote Sensing of Environment
Impact of nitrogen and environmental conditions on corn as detected by hyperspectral reflectance
Remote Sensing of Environment
Remote sensing of leaf water content in the near infrared
Remote Sensing of Environment
The spectral determination of chlorophylls a and b, as well as total carotenoids, using various solvents with spectrophotometers of different resolution
Journal of Plant Physiology
Yellowness index: An application of spectral second derivatives to estimate chlorosis of leaves in stressed vegetation
International Journal of Remote Sensing
The response of tomato plants to simulated landfill gas mixtures
Journal of Environmental Science and Health
Spectral reflectance response of big sagebrush to hydrocarbon-induced stress in the Bighorn basin, Wyoming
Photogrammetric Engineering and Remote Sensing
Phenological changes in the red edge shift of Norway spruce needles and their relationship to needle chlorophyll content
Physical Measurements and Signatures in Remote Sensing
Non-destructive method for estimating chlorophyll content of leaves
Science
Shape of the red edge as vitality indicator for plants
International Journal of Remote Sensing
Responses of soybean to oxygen deficiency and elevated root-zone carbon dioxide concentration
Annals of Botany
Determining the cause of death of vegetation by analysis of soil gases
Gas Age
Primary and secondary effects of water content on the spectral reflectance of leaves
American Journal of Botany
Responses of leaf spectral reflectance to plant stress
American Journal of Botany
Response of thematic mapper bands to plant water stress
International Journal of Remote Sensing
Reflectance spectroscopy: Quantitative analysis techniques for remote sensing applications
Journal of Geophysical Research
Cited by (91)
A multi-temporal method for detection of underground natural gas leakage using hyperspectral imaging
2022, International Journal of Greenhouse Gas ControlIdentifying plants under natural gas micro-leakage stress using hyperspectral remote sensing
2022, Ecological InformaticsMonitoring natural and anthropogenic plant stressors by hyperspectral remote sensing: Recommendations and guidelines based on a meta-review
2021, Science of the Total EnvironmentCitation Excerpt :In the Near-Infrared (NIR, 750–1300 nm) region, a lower reflectance is observed on plants undergoing gas stress, but this observation cannot be generalized to all species. Likewise, the modifications of reflectance in the Short-Wave Infrared (SWIR, 1300–2500 nm) region remain unclear, as contrasted responses were observed among species exposed to the same gas, and for a single species exposed to several gases (Noomen et al., 2006; Ustin and Curtiss, 1990). In many cases, a concentric ring-like pattern of plant stress is observed at canopy scale under gas accumulation or oxygen depletion in soils (Jiang et al., 2020).
Hydrocarbon micro-seepage detection from airborne hyper-spectral images by plant stress spectra based on the PROSPECT model
2019, International Journal of Applied Earth Observation and Geoinformation