the visible spectrum and some of the VNIR. It has a higher resolution than the other bands, which helps sharpen final images. These sensors—for six spectral bands plus the pan band—detect sunlight reflected from the Earth’s surface. The last Landsat 7 sensor detects heat radiated in the thermal-infrared (TIR) band, which has a significantly longer wavelength than the other bands. The surface thermal properties from the TIR band distinguish mineralogy. Many rocks—and tar—that are black in the visible and SWIR bands are differentiated by their response in the TIR range because the minerals that compose the rocks radiate heat at different intensities. The TIR response from cool surface features such as ice and water is low. Similarly, cooling induced by evaporation in wadi beds, open faults and karst features is also characterized by a low-energy TIR response. Other remote-sensing satellites detect different bands; some have more bands than Landsat 7, and others have fewer. Thus, the specific methodology applied to distinguish surface and near-surface features is somewhat dependent on the satellite’s capabilities. The area included in an image and the resolution of the image also depend on the data source. For example, the Landsat 7 satellite has a large frame size of 185 km [115 mi] by 180 km [112 mi]. Its resolution in the thermal band is 60 m [197 ft]; in the visible and infrared bands, it is 30 m [98 ft]. The highest resolution comes from a pan band: 15 m [49 ft]. At the other extreme, a high-resolution satellite with a small viewing area, QuickBird, has a square frame size of 16.5 km [10.3 mi] and a resolution of 61 cm [2 ft] in its pan band and 2.4 m [8 ft] in an infrared band.3 A few satellites obtain radar images of the surface. Imaging radar uses an active illumi - nation system, in contrast to the passive optical- imaging systems just described that rely on illumination from the Sun. This mode of opera - tion gives radar systems the ability to image through clouds and at night, distinct advantages over systems relying on natural light. An antenna mounted on an airplane or spacecraft transmits the radar signal. Termed side-looking radar, it hits the Earth’s surface obliquely and scatters. The same antenna receives the reflected signal, known as the echo.


3. QuickBird is owned by DigitalGlobe. For additional information: http://www.digitalglobe.com/index.php/85/ QuickBird (accessed February 11, 2009).


4. The Shuttle Radar Topography Mission (SRTM) data are administered by the Jet Propulsion Laboratory of the California Institute of Technology, Pasadena, USA. See


Echo amplitude is recorded, and when used for coherent radar processing such as synthetic aperture radar (SAR), the phase of the received echo is also recorded.


The amplitude of the captured signal within each pixel represents the radar backscatter for that area on the ground, with bright areas indicating a significant amount of the radar energy reflected back to the antenna. This reflected energy depends on several conditions of the target area, such as its electrical proper ties, moisture content and perhaps most importantly, the physical size of the scatterers in the area. Generally, a brighter backscatter on the image indicates a rougher surface, while dark areas represent flat surfaces.


Radar imaging can also be used to obtain surface height using interferometric SAR. One method to obtain height generates parallax by using two separated antennas mounted on the same platform. The resulting stereoscopic image is used to create a digital elevation model (DEM). One common source of the parallax view used for topographic interpretation is a US National Aeronautics and Space Administration (NASA) space shuttle mission performed in 2000.4


Its SAR antenna obtained images with lateral resolution of 30 m in the USA and 90 m [295 ft] in the rest of the world. The nominal vertical resolution is 30 m, but it is strongly dependent on topography; in flat terrain, it can have an accuracy of about 1 m [about 3 ft]. Another source for DEMs is the ASTER package on the Terra satellite, which has two VNIR cameras that can be arranged to obtain a stereoscopic image.5


The resulting DEM has a 30-m


vertical resolution and a 15-m lateral resolution. A second mode uses a single antenna with images taken on separate passes of the airplane or spacecraft over the target. This method of determining small changes in elevation over a period of time detects surface movement as small as 1 cm [0.4 in.], which can be used to monitor subsidence over reservoirs.6 A different method uses a laser scanner mounted on an airplane, referred to as laser- induced detection and ranging (LiDAR). Since the plane’s altitude is much lower than that of a radar-equipped satellite, LiDAR yields a higher- resolution DEM; typical resolution is 10 cm


www2.jpl.nasa.gov/srtm/ (accessed February 11, 2009).


5. ASTER stands for Advanced Spaceborne Thermal Emission and Reflection Radiometer. Terra is the flagship satellite of the Earth Orbiting System, a series of NASA spacecraft. For more information: http://asterweb.jpl. nasa.gov/ (accessed February 11, 2009).


[4 in.] vertically and about 20 to 100 cm [8 to 39 in.] laterally. LiDAR service must be ordered specifically for the area of interest. The first steps in remote-sensing evalua tion are determining what information is required and what is available. Since its launch in 1999, the Landsat 7 satellite—with its multispectral capabilities—has scanned the planet on a 16-day cycle. Other satellite databases are also avail able. Knowledge of the topography of a region under study helps determine which combina - tions of spectra will be of greatest utility. In addition, accurate field surveys obtain detailed information at specific locations to provide ground truth for the remotely sensed data. Satellite images have a wide variety of applications. Map views and 3D surface modeling are important tools in designing infrastructure and assessing flood risks. Remote sensing discriminates some surface mineral deposits, provides input for planning and monitoring CO2 storage projects and enables reconstruction of glacial activity through evaluation of moraines. Comparison of older satellite images with new ones—the Landsat program began in 1972— reveals changes in land use or condition. Remote evaluation also helps determine and monitor groundwater levels—important input for seismic studies because the water table is often the first refractor encountered by the seismic signal. One objective for use of satellite images within the E&P industry is to determine the risks associated with conducting a seismic survey.


Seismic Survey Evaluation


Geologists select a seismic survey location because of what may be in the subsurface, for the most part irrespective of surface conditions. Therefore, the survey planners must cope with the challenges inherent in an area’s geography and topography to find the best specific locations for seismic source and receiver placement. In heavily forested areas, vibrators and other vehicles have limited access. The same is true of swampy or marshy ground. In desert climates, loose sand dunes limit access for vibrators. Steep slopes also prevent support-vehicle access. Other geographic features present their own logistical problems, which can be detected using combina - tions of remote-sensing methods (next page).


6. Van der Kooij M: “Land Subsidence Measurements at the Belridge Oil Fields from ERS InSAR Data,” presented at the 3rd ERS Symposium, Florence, Italy, March 14–21, 1997. See http://earth.esa.int/workshops/ers97/papers/ vanderkooij1/ (accessed February 5, 2009).


For more on subsidence: Doornhof D, Kristiansen TG, Nagel NB, Pattillo PD and Sayers C: “Compaction and Sub- sidence,” Oilfield Review18, no. 3 (Autumn 2006): 50–68.


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Oilfield Review


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