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).
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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 the 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.
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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 Âµm, 1.73 Âµm, 2.33 Âµm (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).
DIAL Principle (Zirnig and Ulbricht 2003)
-Twin engine planes (Hymap, AVRIS)
-Helicopters (DIAL, LIDAR)
-Satellite (ASTER, Landsat)
From: (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