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Space Exploration (Oilfield Technology Article)

3D view of WorldDEM for Hejaz landscape in Saudi Arabia

3D view of WorldDEM for Hejaz landscape in Saudi Arabia. Pipeline planning over such varied terrain requires an accurate assessment of topography.

Increasingly satellites are making an important contribution to efficient oil and gas exploration and production. Systems such as TerraSAR-X have the ability to acquire both high resolution and wide-area radar images, independent of weather conditions and with a high geometric accuracy. Application areas are wide ranging and include elevation model generation, topographic mapping, offshore oil seep detection, geological interpretation and seismic planning, oil pollution, sea ice monitoring and ground movement assessment. A significant advantage of radar Earth observation satellites over optical satellites is the ability to acquire imagery independent of cloud or lighting conditions.

An advanced imaging system

TerraSAR-X is a commercial synthetic aperture radar (SAR) Earth observation satellite developed by Airbus Defence and Space, and launched in June 2007. In 2014, the lifetime expectancy of TerraSAR-X satellites was extended by a further five years, enabling the delivery of SAR data well beyond 2018.

The satellite carries a high frequency X-band SAR sensor,which can be operated in flexible imaging modes in order to meet the application requirements. The TerraSAR-X mission is a public private partnership between the German Aerospace Centre (DLR) and Airbus Defence and Space. Commercial distribution of TerraSAR-X data, value-adding, service development, and user training are the tasks of the Geo-Intelligence Programme Line within Airbus Defence and Space. The scientific use of TerraSAR-X is co-ordinate via DLR’s TerraSAR-X Science Service System. TerraSAR-X can acquire data in a range of resolutions from 0.25 m to 40 m according to the application, with individual scenes varying in size from ~4 x 4 km to ~270 x 200 km depending on the imaging mode. If suitable data is not available in archive for a particular application, the capability is in place to task the satellite with the appropriate parameters.

An application toolbox for serving the oil and gas industry’s needs

Oil and gas is a major application area for TerraSAR-X data. Often oil and gas companies have operations in remote areas, with difficult or dangerous access. Earth observation gives an insight into these areas and is increasingly going beyond just providing pixels and moving towards adding value with a range of derived products and services.

Imagery and digital elevation models

Earth observation data provides a rich information source for efficiently undertaking onshore geological interpretation from areas ranging from individual licence blocks, through basins to entire regions. In many areas of active exploration, existing geological mapping can be lacking in detail or accuracy. Radar imagery and derived elevation models are particularly valuable for geological interpretation in areas where there is persistent cloud cover or subtle topographic features. Features are typically captured in a GIS environment and include structural information such as relative bedding dip and orientation, fault identification and classification. Assessment of surface stratigraphy is commonly undertaken in combination with spectral content from optical sensors, with TerraSAR-X providing textural information. An understanding of the regional setting provided by satellite data, gives important context not achievable from ground based methods and at a more local scale, individual licence blocks can be assessed to guide fieldwork or seismic acquisition. Any existing mapping or field based observations are referred to and integrated into the interpretation process. Elevation datasets are enhanced to maximise the contained geological information. Subtle structurally controlled landform features can be identified by artificially shading the elevation model from multiple angles and by colour slicing at narrow elevation intervals.

Global digital elevation model

TanDEM-X, a virtually identical twin satellite to TerraSAR-X, was launched in June 2010. The two satellites are operated in a close formation flight with separation distances down to only a few hundred metres. Together they acquire the data, which is a basis for the recently launched global WorldDEM digital elevation model (DEM). WorldDEM has an accuracy level, which exceeds that of any global satellite based elevation model currently available, with 4 m absolute vertical accuracy and covering the Earth’s entire land mass of 150 million km2

The source data was acquired over a three year period with the processed elevation data, at 12 m grid spacing, being made available throughout 2015.

WorldDEM has a number of advantages including no break lines at regional or national borders and no heterogeneities caused by differing measurement procedures or data collection campaigns staggered in time. Elevation data is valuable in evaluating terrain variation, which can assist with seismic planning and for the assessment of geomorphological and structural features for geological interpretation. In addition infrastructure and facility planning applications during development often relies on an accurate understanding of terrain features. Accurate elevation data can also be a basis for orthorectifying other imagery datasets, correcting relief and elevation related distortions.

Global seeps database – reducing frontier exploration risk

Technology for reducing risk in the assessment of frontier areas is valuable during the early stages of oil and gas exploration. In the offshore environment, oil slicks on the sea surface sourced from oil seeps on the sea bed give an indication of a working hydrocarbon system. Radar imagery provides a reliable method for identifying and monitoring sea surface slicks by observing the dampening effect oil has on amplitude of any surface waves. Using TerraSAR-X and other sensors, Airbus Defence and Space has produced a global database of oil slicks over the last 20 years, containing over 19 000 radar scenes and over 22 000 potential oil seeps. Critical to the identification of surface slicks is the repeat coverage offered by the radar data. By observing a slick on multiple dates, over a period of a number of years, greater confidence can be assigned to the feature as being a natural oil seep, as opposed to transient oil pollution from shipping. Slicks are given a confidence level by experienced analysts according to the morphological characteristics, context and repeatability.

As repeat coverage is critical in increasing the confidence level of previously identified slicks, and for identifying discontinuous seepage events, emphasis is placed on adding new scenes to the database. Over 5000 new scenes were added to the database during 2013 – 2014 as part of a major update and updates are set to continue in the coming years as new areas are assessed and existing areas of coverage are revisited. TerraSAR-X is also being leveraged to acquire imagery in areas of the world lacking radar coverage or for specific customer studies.

Seep locations are commonly integrated with a variety of other geological and geophysical datasets to aid the understanding of potential exploration targets. Comparisons with seismic data have shown a strong correspondence between higher confidence slicks with structures and direct hydrocarbon indicators. Surface slick locations can be an important input into seismic planning processes, and clients have adjusted their planned seismic lines to correspond to locations of known seeps.

Geochemical sampling of surface slicks can be limited by difficulties in locating slicks if they are episodic or influenced by strong currents. TerraSAR-X radar imagery can be used to guide geochemical sampling in near-real time by tasking and rapidly providing imagery and interpretation layers directly to sampling teams on boats within the survey area. This approach increases the likelihood of a successful sampling programme and has been shown to be particularly valuable when combined with historical archive data of likely seepage locations.

Up-to-date topographic mapping and environmental monitoring

In areas with frequent cloud cover TerraSAR-X derived imagery and terrain models are used for topographic mapping and the capture of existing exploration and production infrastructure, particularly valuable where rapid development is occurring. Features identified include land cover types, pipelines, facilities and the calculation of contours.

The approach for identifying natural oil slicks on the sea surface using radar can be equally applied to oil spill monitoring, including routinely monitoring a target area to identify possible leakage from pipelines or facilities and to assist with larger scale pollution events to focus clean-up efforts. For pollution monitoring the Airbus Defence and Space’s global seeps database provides a baseline of natural seeps to assist with the assessment of pollution sources. TerraSAR-X currently has a revisit time of 11 days, but this will reduce with the launch of the Spanish PAZ satellite (owned and operated by Hisdesat) later this year, working in constellation with TerraSAR-X and TanDEM-X. The PAZ satellite has similar capabilities to TerraSAR-X and TanDEM-X, but with enhanced capabilities including the ability to receive automatic identification system (AIS) signals from ships to assist with the identification of polluting vessels.

Effective seismic planning

Controls on positioning seismic relate to the nature and distribution of geology, terrain, infrastructure and land use – features which are readily assessed using TerraSAR-X. In order to understand the spatial variation in slope, DEMs are analysed to assess potentially accessible areas and for visualisation purposes. Land cover information is required to evaluate areas, which are environmentally sensitive, or potentially hazardous, with the aim of reducing risk and minimising the impact of seismic collection. Hazardous areas include areas prone to flooding, subsidence or instability such as sand dune migration or mass movement. An understanding of the distribution and extent of habitation and agricultural activity is vital in survey planning to minimise the impact on local populations. In addition, poor coupling with the ground and a seismic source and receiver can be related to a number of surface or near surface factors, including lithology and features such as Karst, rocky terrain and soft sediment. Understanding the distribution and extent of these factors is critical during planning.

Precise ground control points

Oil and gas exploration requires large volumes of geo-information from different sources and acquisition dates. This information  needs to be combined to understand the full picture before making decisions on the development of oil and gas fields. For this purpose, geolocation accuracy needs to be precise, which very often turns out to be a challenge, especially if older data need to be incorporated into the database.

The ground control point product derived from TerraSAR-X can support this. Based on TerraSAR-X stereo imagery or multiple image datasets, ground control points (GCPs) are obtained at an outstanding quality, fully independent of weather conditions and daylight and with an accuracy of better than 1 m. GCPs are essential for precise orthorectification, derivation of exact location of objects, referencing of geodatasets and also the support of versatile planning purposes. The GCPs are generated using multiple TerraSAR-X images of different angles and orbits as well as applying stereogrammetric techniques. For GCP Extraction the highest accuracy and resolution staring spotlight data is collected. To cover a larger area, stripmap data can be used as an input, but at a reduced accuracy.

Reliable detection and tracking of sea ice

Timely information on sea ice is essential for the safety of any type of marine activities in the polar region, an area of increasing activity for oil and gas organisations.

TerraSAR-X Iceberg Detection Service provides information on detected vessels and ice objects in near-real time down to ~15 minutes after image acquisition.

High-resolution SAR data, which can be reliably acquired independent of weather and lighting conditions, facilitates detection and tracking of even small ice objects such as icebergs and floes. Traffic-hindering objects such as ice ridges can be identified and detailed information on ice properties provided.

TerraSAR-X acts as a tool box for supporting oil and gas operations throughout the upstream project lifecycle, with flexible radar imaging modes ranging from very high resolution for site specific assessment, to regional imaging for broad area evaluation. Application areas are equally wide ranging and are assisting with reducing risk whether environmental, financial or to personnel and assets.

Read this article in Oilfield Technology

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