What is Remote Sensing? (A hint: it is not rocket science!)
Remote sensing is a very efficient and effective tool for oil and gas exploration, since it enables the quick acquisition of relevant information and target definition in areas of considerable extent or difficult access, providing an appropriate context for decision making as to further exploration efforts.
Using its classic definition, remote sensing is the science and technology of gathering information about an object through the analysis of data acquired by a device that is not in physical contact with it. Such an object, usually an area of interest in an oil and gas exploration program, can be identified, characterized and defined from sensors mounted on board satellites, airplanes or helicopters.
Optical remote sensing sensors acquire data through visible, near and shortwave and thermal infrared, which form images of the earth’s surface by detecting the solar radiation reflected or absorbed and emitted from targets on the ground. Different materials reflect, absorb and emit differently at different wavelengths; thus, targets can be differentiated by their spectral reflectance signatures in the remotely sensed images.
The reflectance spectrum of a material, being the fraction of radiation reflected as a function of the incident wavelength, serves as a unique signature for the material. This material can therefore be identified from its spectral reflectance signature if the remote sensing sensor has sufficient spectral resolution to distinguish its spectrum from those of other materials. A multispectral imaging sensor is a multichannel detector with several spectral bands, while hyperspectral imaging sensors acquire images in a hundred or more contiguous spectral bands.
Furthermore, some remote sensing satellites and airborne active sensors emit pulses of microwave radiation which illuminate the areas to be imaged; these are called Synthetic Aperture Radar (SAR) Sensors. Images of the earth’s surface are formed by measuring the microwave energy scattered by the ground or sea back to the sensors. These satellites carry their own ‘flashlight’ emitting microwaves to illuminate their targets. The images can, therefore, be acquired day and night and have the additional advantage that they can penetrate clouds, so images can be acquired even when the earth’s surface is shrouded in mist.
With a Little Help From Space
The rationale behind the application of this technology in oil and gas exploration is that the migration of lightweight hydrocarbons to the subsurface can generate local anomalous areas. These are characterized by reduction conditions that facilitate the development of a variety of chemical and mineralogical changes that can be detected through remote sensing techniques. Possible alterations include bleaching, the development of iron and clay minerals, the formation of carbonates and geobotanical anomalies, among others.
Surficial and near-subsurface thermal variations produced by hydrocarbon migration can also be recognized with specific sensors. Such variations measured by thermal devices could be a result of endogenous factors, such as anomalous thermal fluid flows, structural boundaries or lithological changes. These are morpho-structural alterations that have to be recognized and interpreted separately from the ones due to exogenous factors related to the presence, content and type of vegetation, humidity and topography.
Hence, the analysis of the presence and abundance of such minerals and soil or vegetation chemical anomalies, together with the existence of thermal anomalies combined with a comprehensive study of the structural geology and geomorphology of the area, facilitates the identification of hydrocarbon-related potential target areas.
Understanding the Big Picture
In 2012 HytecAltoAmericas S.A. was contracted by the Colombian Hydrocarbon National Agency (ANH) to conduct a remote sensing study in the Vaupés-Amazonas and Caguán- Putumayo Basins in southern Colombia, over an area of approximately 280,000 km2. The objective of the survey was to detect hydrocarbon prospective areas using satellite, airborne and field remote sensing technologies.
The remote sensing study comprised different phases that were all carried out during 2012. After the careful selection of imagery data during the acquisition phase, followed by the effective preparation, processing and interpretation of the spectral data, a number of prospective oil and gas exploration areas were identified.
The initial phase of the survey involved the selection and acquisition of all the ASTER, LandSat 7 ETM+, LandSat 5 TM, LandSat 4, MODIS, SRTM, and PALSAR imagery over the entire study area. RADARSAT images were also acquired over selected areas. A total of 767 images were finally acquired after the careful evaluation of over 13,000 images.
Preprocessing and processing techniques were applied to the acquired data during the second phase. These included atmospheric corrections, geometric adjustments and georeferencing procedures during the preprocessing stage, while geobotanical and geological enhancements, structural filters, thermal calculations, mineral, soil and vegetation indexes were calculated during the processing stage. Both the Stressed Vegetation Index and the Bleaching Index proved to be very useful in the identification of areas of interest.
All processes and indexes were applied independently on three previously defined spectral domains (each divided into two regions) in the survey project area that bear different soil and vegetation characteristics. This separation of domains and regions allowed their consistent application and the corresponding accuracy of the interpretation which was later performed.
Interpretation of such products allowed the identification of 156 spectral target areas, categorized according to the weight of their spectral, thermal and structural characteristics during the third phase of the survey. Oil seeps registered in regional databases were also taken into account after the interpretation process for validation purposes.
Spectral targets defined with satellite data were field validated during phase four, which mainly comprised an intense airborne hyperspectral survey over selected anomalies and a ground spectral and geochemical survey over some of them.
The airborne hyperspectral data was acquired by means of a radiometer mounted on a helicopter, flying approximately 330m above the ground. The decision to use a helicopter rather than an airplane was a result of the high and extremely dense cloud conditions of the region, which makes surveying with airplanes almost impossible. Data was registered in 1,200 narrow bands between the ultraviolet (UV), visible (V), near infrared (NIR) and short wave infrared (SWIR) portions of the electromagnetic spectrum, with a 15 to 30m pixel size, equivalent to the spatial resolution of the acquired satellite data.
The ground survey consisted of the acquisition of further spectral data with a field spectrometer, concentrating mainly on vegetation and soil samples. The latter were also collected at specific areas of interest for geochemical analysis such as Total Organic Content (TOC) evaluation. Pyrolisis was performed on samples with TOC values over 0.9.
At this stage it is very important to recall the physiographic characteristics and lack of logistics in the study area. Almost all of the surveyed area is covered by a thick impenetrable vegetation layer, roads are non-existent and gasoline and minimum lodging infrastructure can only be obtained in a very small number of towns or villages – not to mention the known security issues of the region. Consequently, the field validation phase can be considered a great success: almost half of the satellite spectral anomalies were directly or indirectly field validated by means of the hyperspectral helicopter and field survey program.
Satellite, airborne and field spectral data as well as the geochemical survey results were all integrated in a Geographic Information System (GIS) database for its final comprehension, analysis and interpretation. As a result, the location of prospective oil exploration targets within the studied region was defined.
The hydrocarbon prospective targets identified cover approximately 45,000 km2, which represents 16% of the whole area.
The final weighting process after field validation indicated that 12% of the targeted anomalies showed characteristics which were strong indicators of potential occurrences of hydrocarbon accumulations.