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With the growth of industrialisation and mass consumerism in both the developed and developing world energy needs are in high demand. This demand has led to the growth and success of the trillion dollar oil and gas industries and many countries have devoted billions of dollars to the exploration and refining of these energy sources. This, however, has led to negative impacts on the environment.
Over the years oil and gas have leaked into the environment at various stages in their production. Hydrocarbon leakages have occurred at the exploration and drilling stage; storage and the transportation stage (through both tankers and pipelines); and even before the crude oil is refined or the oil and natural gas is put to various uses.
While carrying out oil and gas explorations blowouts of hydrocarbons have occurred when areas of very high pressure have been encountered during the drilling process. Leakages have also occurred while storing oil and gas underground reservoirs before being transported by tankers. However, the transportation of oil and gas remains the leading cause of environmental damage due to the oil and gas industries. Tanker collisions are quite frequent and can have severe consequences. Tankers have run aground and into coral reefs; they have collided with other vessels and fires leading to explosions have also occurred. Hydrocarbons have also escaped during the loading of oil unto the vessel.
Pipeline damage is also quite common and has severe impacts since it affects both land and sea. Pipelines running below the sea have been damaged for various reasons: defects in the material and construction of the pipeline; natural wear and tear resulting in corrosion; tectonic movements; and ship anchors and bottom trawls. Pipelines can either be a "source of small and long-term leakage or an abrupt (even explosive) blowout of hydrocarbons near the bottom" of the ocean (Patin 1999). Pipeline damage can also occur while transporting oil and gas below the land surface.
Part B: Discuss the effects that hydrocarbon may have on the environment and which of these effects might by detected by Remote Sensing.
Werff (2007) has assessed that hydrocardon leakage has caused major problems and have impacted countries economically and environmentally. The detection of the leakage using abrasive methods such as drilling can be destructive and expensive, remote sensing methods have proven to be much more efficient and cost effective.
One of the most noted and documented effect of hydrocarbon is its impact on vegetation. Werff (2007) has determined that when hydrocarbon gas comes in contact with the roots of vegetation it causes stress to crops which may lead to chlorosis. Everett (2002) has commented that the effect of the seepage to be detected is dependent on soil, climate, vegetation type and location specific, additional it is relative to the lifespan of the vegetation and that tolerance level of plants varies based on species. Oyundari (2008) stated that at high concentrations of hydrocarbon contaminations can either kill or severely hamper the ability of a plant to grow or germinate. Research conducted has surmised that the effect on plants and seeds is a result of the disruption of the water balance and the metabolism and toxicity of the plant and seed. Soil contaminated with natural gas will stunt the growth of vegetation.
Balks and Paetzold (2002) investigated the effects of hyrdrocarbon on the temperature and moisture content of soil in Antarctica and determined that when hydrocardon materials are added to soil it could affect the soil moisture regime and may increase the soil's hydrophobicity thereby decreasing the soil's moisture holding capacity. It was also determined that an increase in the carbon content of the soil may lead to an increase in the water holding capacity of the soil. Noomen (2003) found that changes in the soil composition due to hydrocarbon seepage results in microbiological changes and the formation of new minerals such as calcite. The research also point out that changes the soil mineralogy, where gas that seep to the surface displaces the oxygen in the soil air. Oxygen that is left in the soil is used by a bacterium that converts methane into water and carbon dioxide. Plants use oxygen, an environment that is deprived of oxygen produces stressed vegetation.
The stress on vegetation that is caused by hydrocarbon leakage may be detected by remote sensing methods. To detect vegetation, the near infrared band of the multi spectrum is used Werff (2007) applied a Normalized Difference Vegetation Index (NDVI) to express the presence of vegetation in a scale of 0 to 1. The index identified the position of the wavelength of the red to near-infrared intensity difference caused by chlorophyll, which he pointed out typifies vegetation spectra.
Research by Clever (2004) and Rosso (2005) have shown that spectral difference in reflectance of leaves and canopy surfaces that are derived from the leaf optical properties and related to the physiological and biophysical status of the vegetation. It was highlighted that optical properties of the leaf is attributed to the cellular make up, water content and biochemical composition and pigment concentrations. From the electromagnetic spectrum remote sensing uses the visible region of the spectrum to determine the chlorophyll content, the near infrared for the cell structure and the short wave infrared for the water content all of which to determine the spectral signature of the leaf of the vegetation.
Changes in the biochemistry and cellular composition of vegetation under stress will have a difference in their reflectance. Stressed plants reflect more red but have a low reflectance in the near infrared when compared with healthy plants. A larger reflectance in the red can be attributed to pigment loss Oyundari (2008).
Leakage of hydrocarbon from pipelines is of great concern as it affects both humans, causing respiratory problems and the environment when they leak into the soil and affect vegetation growth. Careful observation of the pipelines is of utmost importance in preventing long term contamination of the environment and agricultural yields.
This paper mentioned that detection of hydrocarbon leakage is possible, however most methods are expensive, abrasive and destructive, remote sensing offers a more less destructive and abrasive means of identifying leakages and its span. The use of remote sensing can identify early signs of plant stress all with an aim of presenting any further damage to the environment.
To a great extent the study of hydrocarbon (specifically oil and gas) seepages has been focused on the marine environment, mainly as a preventative tool against oil spills and to a lesser extent oil exploration. The onshore study of seepages has taken a similar tack in focusing on oil/gas exploration (Ellis, Davis, and Zamudio 2001; Hörig et al. 2001). Increasingly remote sensing is being used to detect incursions in pipeline right-of-way and to detect leaks.
Pipeline leaks results in unusually high concentrations of ethane, propane and methane which can lead to mineral alterations to the surround soil and rock as well as temperature, radiometric and geobotanical anomalies (van der Meer et al. 2002).
In the study of the earth's surface for micro seeps two methods of detection may be used, direct or indirect detection. Direct detection measures hydrocarbon accumulations in the form of oil pools or the build up of hydrocarbon vapours while indirect methods focus on the effects of the seepage. These secondary effects include mineralogical changes, bleaching, clay mineral alternation, electrochemical changes and microbial anomalies (Khan and Jacobson 2008).
There are several remote sensing sensors and platforms that are widely in hydrocarbon exploration and detection. These are radar, Landsat Multispectral Scanner (MSS), Landsat Thematic Mapper (TM) and airborne multispectral scanners (van der Meer et al. 2002).
The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) captures multispectral imagery. It is the first satellite borne multispectral thermal infrared remote sensing system with spectral, spatial and radiometric resolutions suitable for geological applications (Bihong et al. 2007).
Bihong et al. showed in their study of the Dushanzi oilfield area that ASTER band ratio 2/1 unbleached red bed displayed high values because of their high ferric oxide content while bleached soils and rocks showed high values on an ASTER greyscale image band ratio 4/8. It is possible to separate bleached and unbleached beds; bleached beds appeared blue to light blue and unbleached beds were brownish red and blue. This was derived using an ASTER false colour composite of ratio 2/1 in the red channel, band 3 in the green channel and ration 4/8 in the blue channel. They concluded that the mineralogical variance was due the presence of carbonate minerals present in the bleached red beds that was not present in unbleached sections and that ASTER could be used as a potential tool in hydrocarbon detection in arid to semi arid regions.
By using spectral enhancement method principle component analysis on some Landsat +ETM bands it is possible to detect the mineral alterations (clay and ferric iron). Band ratios of Landsat TM has also been used to locate ferrous iron, bleached red beds and clay mineralization (Shi, Fu, and Ninomiya 2010). Specifically using Landsat TM2 and TM3 (Almeida-Filho, Miranda, and Yamakawa 1999) it is possible to map bleached materials by including a vegetation index (Landsat TM4/3) and calculating the difference between TM2/3 and TM4/3 it is possible to enhance tonal variations seen in areas of hydrocarbon seepages (figure 1).
ASTER data (very near infrared and short wave infrared) is seen as more capable in detecting mineral alternations because it has 6 bands in the short wave infrared region compared to the 2 bands Landsat TM and +ETM have in that region (Shi, Fu, and Ninomiya 2010).
Figure 1(Almeida-Filho, Miranda, and Yamakawa 1999)
Hydrocarbon effects on vegetation can be detected in humid climates, there are challenges however. Vegetation's response to hydrocarbon seepage is location specific and depends heavily on the climate, drainage, soil type and vegetation type (Everett, Staskowski, and Jengo 2002). Since the hydrocarbon seepage occurs over a long time frame relative to the vegetation lifespan. It produces changes in leaf structure, crown density, plant vigour, and plant distribution (Everett, Staskowski, and Jengo 2002).
Hyperspectral data is very useful in detecting changes in the vegetation reflectivity from red to infrared - the 'red edge'. It is also useful in monitoring changes in the slope, inflection point and height of the vegetation spectral curve (Everett, Staskowski, and Jengo 2002).
It is important to note that changes in vegetation can also be monitored with TM data. This is usually most effective in areas where there is heavy vegetation cover and distinct vegetation communities can be observed.Conversely, the anomalies that cause vegetation stress due to hydrocarbons are not unique to pipeline leaks. These anomalies may arise due to other types of pollution (van der Meijde et al. 2009).
Reflectance spectroscopy is also a tool that can be used to identify anomalous spectral features in vegetation. In their study van der Meijde et al. conclusively identified the correlation of hydrocarbon pollution (benzene) and vegetation irregularities. The red edge was consistently lower on the pipeline than further away for polluted areas. Using vegetation stress as an indicator needs to be done in conjunction with drilling to validate findings.
Differential Absorption Lidar (DIAL) and Lidar (light detecting and ranging) techniques together can be used to detect natural gas leaks. DIAL can successfully analyse trace gases and Lidar involves analysing backscattered light emitted from a laser in the ultraviolet, visible or infrared portion of the spectrum.
The DIAL technique uses light pulses of two wavelengths to eliminate the backscatter effects on the measurement signal. One is absorbed by the gas while the other is used as a reference.
The HyMap hyperspectral sensor is an across track scanner. This sensor has 126 bands and the spectrometers are usually mounted in a small twin engine plane. There are 4 spectrometers that take readings in the visible, near infrared, short wave infrared 1 and 2 regions. Each produces 32 spectral bands of imagery.
Hydrocarbons have characteristic absorption features in at 1.72 µm, 1.73 µm, 2.33 µm (Hörig et al. 2001; Cloutis 1989; Ellis, Davis, and Zamudio 2001). Using HyMap, the visible and near infrared regions of the spectrum are useful in recognizing the absorption features. Landsat TM data is too coarse to detect the absorption features (Hörig et al. 2001).
Figure 2: DIAL Principle (Zirnig and Ulbricht 2003)
-Twin engine planes (Hymap, AVRIS)
-Helicopters (DIAL, LIDAR)
-Satellite (ASTER, Landsat)
Table 1: (Zirnig et al. 2001)
Estimate of the amount of pollution
There are studies of seepages - manmade and natural - that have indicated that from a point source the gaseous extent (horizontally) is four meters for sandy soils and one meter in clay soils (van der Werff et al. 2008).
Using the DIAL method with an open measurement path, it can be used to "measure trace gas concentrations with a spatial resolution" (Zirnig and Ulbricht 2003). A closed measurement path - which is used with helicopter borne systems do not allow for the measuring of gas concentrations, however work has been done (Zirnig and Ulbricht 2003), that allows for the limited use of the open measurement system in helicopters given certain specifications. These include:
-Travelling speed: 10 - 40m/sec
-Inspection altitude of 50 - 250 m
-Entry of leak position reported