Prospecting by Satellite
In frontier areas, satellite data are often brought into play during the earliest stages of exploration, well before seismic survey plan- ning and layout. There, satellite imagery is used to prioritize areas likely to contain oil and gas prospects. Employing a variety of sen- sors, satellites are especially suited for gross reconnaissance of remote regions and large survey areas. The data from these different types of sensors are useful far beyond their capability to map topography, regional geology, lineaments and structural trends. Satellite data acquired over land are ana- lyzed to infer the presence of hydrocarbons through indirect signs, such as chemical, physical or microbiological changes in soil and vegetation. For example, when gas seeps to the surface, it partially displaces oxygen within the soil to create an oxygen-poor envi- ronment. This also affects the reduction- oxidation potential and pH of the soil. These changes are manifested as alterations in soil mineralogy such as the formation of new min- erals (calcite, pyrite and uranium), by bleaching of red-bed outcrops or by electro- chemical changes.1
Such permutations are, in turn, reflected in the health or type of vegetation surrounding a gas seep. Not only is oxygen depleted from the soil, but accompanying changes in soil-nutrient solubility result in a deficiency or excess of nutrients taken up by plants. These effects may register in the plant’s spectral response detected by satellite optical sensors. The reflectance of stressed plants is often higher in the visible region and lower in the near- infrared.2
The pattern and intensity of such indicators may be important for delineating
1. Red beds are reddish sedimentary strata, such as sandstone, siltstone or shale, which have accumulated under oxidizing conditions; the red color comes from specks of iron oxide minerals.
2. Noomen MF, Skidmore AK and van der Meer FD: “Detecting the Influence of Gas Seepage on Vegeta- tion, Using Hyperspectral Remote Sensing,” in Habermeyer M, Mülle A and Holzwarth S (eds): Pro- ceedings, The 3rd EARSeL Workshop on Imaging Spectroscopy.Herrsching, Germany: ERSeL (2003): 252–255.
3. Jones VT, Matthews MD and Richers DM: “Light Hydrocarbons for Petroleum and Gas Prospecting,” in Hale M (ed): Handbook of Exploration Geochemistry:
fractures or other characteristics of subsurface accumulations, and gas has been detected along certain linear features seen on satellite images.3 Offshore, satellite imagery is useful for developing exploration leads through identifi- cation of possible oil seeps. Oil, emanating from natural seeps on the seafloor, rises to the surface of the ocean where it may be detectable through visible, near-infrared and radar imagery. Synthetic aperture radar (SAR), in particular, is highly successful in detecting oil on the sea surface. This side- looking radar transmits signals at an oblique angle to the Earth, and thus it is sensitive to backscatter produced by tiny capillary waves on the ocean’s surface.4
Oil tends to dampen waves on the ocean sur- face, producing a smooth surface that reflects most of the signal away from the SAR receiver. The backscatter intensity is anomalously low over a smooth surface compared to the sur- rounding area. However, numerous factors affect the interpretation and location of surface slicks relative to source vents on the seafloor. Factors that can move or obscure the presence of a smooth ocean surface include wind velocity and direction, currents, cloud cover, meteoro- logical conditions and marine vegetation.5 More importantly, the damping of surface waves may be attributed to numerous processes that require further investiga- tion—many slicks have nothing to do with the presence of oil. Rain cells, wind shadows and current flow can smooth local areas of the sea surface. Algal mats and even coral spawn also affect sea motion. Bathymetric slicks are generated by localized acceleration of currents flowing over submarine channels.
Geochemical Remote Sensing of the Sub-Surface, vol. 7. Amsterdam: Elsevier (2000): 133–212.
4. A capillary wave is a ripple or small surface-water wave with a maximum wavelength of 1.73 cm [0.68 in]. This wavelength is so short that the surface tension of the water itself exerts a restoring force to its motion.
5. Hood KC, Wenger LM, Gross OP and Harrison SC: “Hydrocarbon Systems Analysis of the Northern Gulf of Mexico: Delineation of Hydrocarbon Migration Path- ways Using Seeps and Seismic Imaging,” in Schumacher D and LeSchack LA (eds): Surface Explo- ration Case Histories: Applications of Geochemistry, Magnetics, and Remote Sensing, AAPG Studies in Geology no. 48 and SEG Geophysical References Series no. 11. Tulsa: AAPG (2002): 25–40.
These slicks have suggested the presence of uncharted channels that were subsequently verified by high-resolution multibeam swath bathymetric surveys.6 Off the North West Shelf of Australia, SAR has detected slicks during the ebb of nocturnal neap tides, five nights after a full moon occur- ring between March and April and between October and November. These annular- to cres- cent-shaped areas of low backscatter, found over coral reefs and carbonate shoals in the southern Timor Sea, have been interpreted as coral spawn slicks.7
Restricting SAR acquisi-
tion to predictable nonspawning times has avoided the misinterpretation of slicks caused by coral spawn as those caused by oil. This application highlights further potential for SAR as a tool for biological research. The identification of natural oil seeps is instrumental in revealing undiscovered resources. However, the ability to determine which SAR slicks are caused by oil requires careful analysis of ancillary data. Recognizing links between SAR slicks and oceanographic or biological processes makes it possible to improve the assessment of potential explo- ration targets.
Overall, satellite remote-sensing techniques are valuable in rapidly screening large or inac- cessible areas. They can be used to prioritize prospects for further investigation by other technologies, such as coring, airborne laser fluorescence and seismic surveys.8
As with all
sensing from a distance, this approach is best used selectively and proves its worth when verified by ground-truth measurements. —MV
6. Jones AT, Thankappan M, Logan GA, Kennard JM, Smith CJ, Williams AK and Lawrence GM: “Coral Spawn and Bathymetric Slicks in Synthetic Aperture Radar (SAR) Data from the Timor Sea, North-West Australia,” International Journal of Remote Sensing 27, no. 10 (May 2006): 2063–2069.
7. Jones et al, reference 6.
8. The airborne laser fluorescence (ALF) technique measures fluorescence of aromatic hydrocarbons that have been excited by a laser fired at the sea surface. ALF surveys can detect the presence of micron-thick hydrocarbon accumulations.
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