Remote Sensing Today and Tomorrow: Applications in the Gulf Region
Dr. Farouk El-Baz, Director, Centre for Remote Sensing, Boston University, Boston MA 02215, U.S.A.
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INTRODUCTION
Much has been learned during the past three decades about photographing the Earth from space through a series of American, Russian and European space missions. Starting in 1965, many useful photographs were acquired by astronauts of the American Gemini, Apollo, Skylab, Apollo-Soyuz, and the Russian Mir missions. In addition, the Landsat program introduced in 1972 digital imaging from unmanned spacecraft, where data were transmitted to ground receiving stations. The technology of these systems provides an advanced new tool for detailed study of the natural and cultural resources and the changing environment of the Earth. This is particularly true in desert regions where the land features are not masked by vegetation (El-Baz, 1988).
Unmanned and the manned spacecraft systems are planned to fly in high, medium, or low orbits. The highest orbits are left to the unmanned weather satellites, such as Meteosat, which are propelled to a height of 36,000 kilometers above the Earth. At this altitude, their motion is equivalent in speed to the rotation of the Earth about its axis. These satellites are termed geostationary, and remain above the same point on the Earth to acquire and transmit repetitive images as frequently as hourly. Due to their high altitude, the images they collect cover most of one hemisphere of the Earth at low spatial resolution, which is ideal for studying global weather phenomena.
Intermediate orbits are those from 5,000 to 2,000 kilometers above the Earth, the region where most unmanned imaging satellites are placed. For example, the polar-orbiting satellites of the National Oceanic and Atmospheric Administration (NOAA) fly at altitudes of 835 to 870 km; and the near-polar orbits of the American Landsat and the French Systeme Pour l'Observation de la Terre (SPOT) reach a maximum altitude of 920 km above the Earth. Images collected from these altitudes provide greater local detail than is possible from the high altitude satellites, but the area covered by individual images is significantly reduced. One result is that these satellites provide less frequent coverage of the same area than weather satellites (once every 16 days for Landsat, and 28 days for SPOT).
On the lower end, most manned missions are placed in orbits below 500 km, to a minimum of 150 kilometers above the Earth. For example, the Space Shuttle operational altitude is about 300 kilometers. From this altitude, images show greater detail such as those of the Space-borne Imaging Radar (SIR).
DIGITAL IMAGING SYSTEMS
Landsat: the Earth Resources Technology Satellite (ERTS-1), the first unmanned digital imaging satellite, was launched on July 23, 1972. Four other satellites from the same series, later named Landsat, were launched at intervals of a few years. The Landsat spacecraft carried a Multi-Spectral Scanner (MSS), a Return Beam Vidicon (RBV), and later, a Thematic Mapper (TM) imaging system.
Multi-Spectral Scanner: the MSS produce images representing four different bands of the electromagnetic spectrum. The four bands are designated band 4 for the green spectral region (0.5 to 0.6 microns); band 5 for the red spectral region (0.6 to 0.7 microns); band 6 for the near-infrared region (0.7 to 0.8 microns); and band 7 for another near-infrared region (0.8 to 1.1 microns). Radiation reflectance data from the four scanner channels are converted first into electronic signals, then into digital form for transmission to receiving stations on Earth. The recorded digital data are reformatted into computer compatible tapes for processing and analysis. For example, the green band (band 4) most clearly shows underwater features, because of the ability of green radiation to penetrate shallow water, and is useful in coastal studies. The two near-infrared bands, which measure the reflectance of the Sun's rays outside the sensitivity of the human eye (visible range) are useful in the study of vegetation cover (El-Baz, 1978).
Return Beam Vidicon: the RBV was flown on Landsat 1, 2, and 3 in the interest of photogrammetry to acquire better geometric accuracy and ground resolution (40m) than was available from the Multi-Spectral Scanner (80m resolution). The system contained three cameras that operated in different spectral bands: blue-green, green-yellow, and red-infrared. The three RBV cameras were aligned in the spacecraft to view the same 185 km square ground scene as the MSS. Although the RBV is not in operation today, images are available and can be utilized in mapping.
Thematic Mapper: the TM is a sensor that was carried on Landsat 4 and 5 (as well as Landsat 6, which was destroyed upon launch) with seven spectral bands covering the visible, near infrared, and thermal infrared regions of the spectrum. It was designed to satisfy more demanding performance parameters from experience gained in the operation of the MSS with better ground resolution (30 m). At this ground resolution, many small agricultural fields may be accurately characterized (Szekielda, 1988). The seven spectral bands were selected for their band passes and radiometric resolutions. For example, band 1 of the Thematic Mapper coincides with the maximum transmissivity of water and demonstrates coastal water mapping capabilities superior to those of the MSS. It also has beneficial features for the differentiation of vegetation, along with bands 2 to 4. Vegetation and soil moisture may be estimated from band 5 readings, and plant transpiration rates may be estimated from the thermal mapping in band 6. Band 7 is primarily motivated by geological applications, including the identification of rocks altered by percolating fluids during mineralization (Hord, 1986).
SPOT: this system was designed by the Centre Nationale d'Etudes Spaciales (CNES) and built by the French industry in association with partners in Belgium and Sweden. Like the American Landsat it consists of remote sensing satellites and ground receiving stations. The imaging is accomplished by two High Resolution Visible (HRV) instruments that operate in either a panchromatic (black-and-white) mode for observation over a broad spectrum, or a multi-spectral (color) mode for sensing in narrow spectral bands. The ground resolutions are 10 and 20 meters respectively. For viewing directly beneath the spacecraft, the two instruments can be pointed to cover adjacent areas. By pointing a mirror that directs ground radiation to the sensors, it is possible to observe any regions within 450 km from the nadir, thus allowing the acquisition of stereo photographs for three-dimensional viewing and imaging of scenes as frequently as every four days.
Shuttle Imaging Radar: imaging radar instruments transmit waves towards the surface and record the returned echoes. The latter are stronger (bright) from rocky terrain and weaker (dark) from smooth surfaces. On its maiden flight in November 1981 (Elachi et al., 1982), the Shuttle Imaging Radar experiment (SIR-A) provided the first indication of the ability of radar to penetrate through sand and reveal buried courses of ancient rivers in the Eastern Sahara (Figure 1). In this part of the Western Desert of Egypt, drilled wells in "East Oweinat" produced water for an experimental, 5,000 acre farm. It is estimated that the groundwater resources in this region are capable of supporting agriculture in nearly 200,000 acres for 200 years (El-Baz, 1984) The next flight, three years later (SIR-B), obtained images of the Arabian Peninsula, among other regions. In addition, the renamed Spaceborne Imaging Radar (SIR-C) experiment, returned 25-meter resolution images from flights in April and October 1994. Unlike previous orbital radar systems, SIR-C acquired digital images using a 24-centimeter L-band and a 6-cm C-band, with each wavelength, recording both vertically and horizontally polarized waves. Different combinations of wavelengths and polarizations can be used to produce color images to emphasize particular features. SIR-C images were used by the author to study the groundwater potential of the Emirate of Dubai, as discussed below.
RADARSAT: this satellite is a project led by the Canadian Space Agency (CSA) in partnership with NASA and NOAA. It was launched in a polar, Sun-synchronous orbit by a Delta II rocket on 4 November 1995. It features a C-band, horizontally polarized, synthetic aperature radar (SAR), with flexible beam steering and resolution characteristics (Raney et al., 1991). Thus, it is the first SAR to operate in a variety of resolutions (down to 10m), swath widths, and incidence angles. This flexibility is designed to maximize potential applications of the data and the frequency of coverage required for operational use.
Declassified Images: in February 1995 the United States Government announced the declassification of the Corona Program "spy satellite" photographs obtained between 1960 and 1972 of various parts of the world. Shortly thereafter, Russia followed suit by declassifying similar products. The DD-5, very high resolution, black-and-white images, are among those declassified; the original images with 0.7 meter resolution are degraded by digitization of photographic prints to approximately two meter resolution prior to selling outside of Russia for civilian uses.
Future Systems: in addition to data already available, four commercial entities are presently planning to launch digital imaging satellites. These will be the first ventures with no federal support, proving that satellite images are commercially viable products. The new system will be able to reveal small features on the Earth's surface. These digital imaging systems will be designed to produce multi-spectral data in the range of 5 to 15-meter resolution, and panchromatic, stereo images from five to one meter in resolution. These characteristics would allow use of the data in such fields as city planning and archaeological investigations.
APPLICATIONS IN THE GULF REGION
Existing as well as future satellites images have wide applications in the Gulf region. This is particularly true since arid conditions prevail and the landscape features are clearly exposed and not covered by vegetation. In particular, the land and the coastal zones of the Gulf have not been thoroughly studied in modern times, and there is much to be learned about them. Furthermore, because of the production of oil throughout the Gulf region, constant monitoring of the environment from space becomes a necessity.
There are numerous applications of satellite images in every field of specialization. It is not the scope of this paper to list all the various applications. Suffice it to exemplify geologic applications including the study of: (a) relationship of sand seas to groundwater; (b) distribution pattern of paleo-fan deltas; (c) environmental impacts of the Gulf War.
A. Relationship of Sand Seas to Groundwater
Dune accumulations in the Arabian Peninsula occur within or near topographic depressions (El-Baz, 1979). This must be explained in any theory regarding the origin of the sand and the evolution of the dune forms in space and time. Two other important observations must also be taken into account. The first is that wind in this desert moves from the north toward the south during most of the year. As in the Eastern Sahara, such wind patterns are measured by meteorological stations (Manent and El-Baz, 1980), deduced from dune forms, or estimated by measuring the direction and rate of motion of sand dunes (Wolfe and El-Baz, 1982). The second observation is that sands in these dune fields are composed mostly of well rounded quartz grains. The exposed rocks to the north of the sand seas are mostly limestones, which could not have been the source of the vast amounts of quartz sand (El-Baz, 1992a).
The two observations discount the possibility of the origin of the majority of the sand by wind erosion and transportation from the north. Therefore, it is more likely that the areas covered by dune sand were relatively low areas that received sediments from northward and eastward flowing stream channels in the geological past. When the conditions of climate changed and dry conditions prevailed, the wind sculptured these sand accumulations into various dune forms and sand ridges.
The patterns of dunes in Ar-Rub Al-Khali in particular support this theory (El-Baz, 1982). The largest linear forms in the Western Desert of Egypt were named "whaleback" dunes by Bagnold (1941). He theorized that they grew so large that they no longer could move. However, dunes move because individual sand grains are dislodged by the wind. Furthermore, cross-sections made into similar dunes in the Western Desert of Egypt show that the sand is horizontally layered rather than curved parallel to dune profiles (El-Baz et al., 1979). This suggests that what Bagnold named whaleback dunes are residual sand ridges, left behind as the wind eroded sand from what are today sand-free interdune corridors.
This hypothesis has a far reaching implication: because the sand was transported by paleo-rivers, the depositional basins would have received vast amounts of fresh water. Some of that water would seep, through primary and/or secondary porosity, into the rocks beneath the sands. Thus, areas that encompass large sand dune accumulations may host vast groundwater resources. In addition to the comparatively well-known horizontal aquifers in porous sediments, it is believed that more of that water exists in fracture zone aquifers (Koch and El-Baz, 1992), which are extensive, nearly vertical zones in the rock, such as Wadi Al-Batin (Figure 2), that may contain large amounts of yet untapped resources. In addition, fan deltas of the past are indicative of the paths of water during the humid phases of the past.
B. Paleo-Fan Deltas of Dubai
The source of groundwater in the Emirate of Dubai begins at the Oman Mountains to the east. This is probably true throughout much of geologic time since the late Paleozoic, when repeated uplift and faulting resulted from the collision of the northeastern corner of the Arabian Plate with the Eurasian plate. It is, therefore, plausible that wadi systems and their delta fans began to form in Mesozoic time.
The western margin of the Oman Mountains is characterized by numerous delta fans. The present outlines of these fans terminate at the eastern border of the sand deposits of the desert plain.
These fans were formed by the deposition of sediments after occasional rainfall. For this reason, the fans exhibit a plethora of fine drainage lines, each resulting from the erosion of previous fluvial deposits. In such a pattern, coarser deposits usually abound at the apex of the delta-shaped fan, while finer silts would occur at the outer edge of the fan. Although the coarser deposits would have higher water storage capacity, farmed areas are usually located at the delta edges because of the finer, more fertile soils.
SIR-C and Landsat TM image interpretations reveal that the present day fans are superimposed on older fluvial deposits. The desert plain of Dubai is underlain by dark, fine-grained material. This material, usually consisting of gravel, sand and silt, must have been deposited by former stream channels. Some of these stream channels would have reached the western coastline of this part of the Arabian Peninsula.
Today's coastline exhibits promontories that may be interpreted as delta fans. It is possible to extrapolate linear features indicated by lowlands in the desert plain, that may have been passageways for stream channels, all the way to the coastline (Figure 3).
Streams that formed these wadis may be partially active in today's climate, but their courses are interrupted by younger, Quaternary age, wind-blown sand dunes. These dunes appear to have been responsible for the northward shift of the main wadis. As eastward-moving sand accumulates in high ridges (some are over 50 meters in height; UAE Univ., 1993), it blocks the westward-moving water courses and forces them to move northward.
This process appears to have been assisted by regional structural movements. The Dubai region was uplifted during regional tectonics, as evidenced by the receding shoreline features in the western part of the Emirate. Furthermore, there appears to have been a regional, northward downwarp of the terrain, as evidenced by the "drowned" coastline of the region north of Dubai.
C. Environmental Impact of the Gulf War
An assessment of the geologic impact of Iraq's invasion of Kuwait and the war that ensued was conducted by the Boston University Center for Remote Sensing jointly with the Kuwait Institute for Scientific Research, under the sponsorship of the Kuwait Foundation for the Advancement of Sciences (El-Baz and Al-Ajmi, 1993).
Interpretation of multi-temporal satellite images followed by field investigations were used to establish the nature of changes to the desert features of Kuwait. Pre- and post-war Landsat TM images were digitally compared to produce environmental change maps. Panchromatic SPOT images were enhanced to identify causes of the changes. Image classification techniques were used to distinguish classes of change, and geographic information system (GIS) methodologies allowed the production of thematic maps.
The first major change resulted from the removal of lag deposits from the desert pavement by the heavy traffic of military vehicles, planting of landmines, digging of trenches, and building of berms to hide military personnel and hardware. The exposure of fine-grained soil, to the action of the prevailing wind from the northwest, mobilized vast amounts of dust and sand (El-Baz, 1992b). The finest grains were carried by the wind and increased dust storms, and the sand-sized particles accumulated by saltation into mobile dunes. In one area along the northern shore of Kuwait Bay, a field of 22 dunes, each over two meters in height, were formed in eight months (Misak, 1994). In other localities to the west, numerous sandsheets and new sand dunes were formed by aeolian action. These deposits have disturbed installations in the desert and encroached on roads and agricultural farms down wind of the newly exposed soil horizons.
The second major change resulted from the formation of over 240 oil lakes, and the deposition on the desert surface of oil droplets and soot from the fire plumes of over 700 exploded oil wells. As the oil droplets mixed with surface sand and gravel, they hardened into a layer of "tarcrete," up to 12 centimeters in thickness (El-Baz, 1994). This layer formed broad strips, up to 15 kilometers wide, southeast of the oil fields (Figure 4). The soil in these strips was poisoned, resulting in the eradication of natural vegetation. These areas are no longer utilizable as rangelands for desert animals. Furthermore, it is possible that poisonous elements from the oil, such as nickel and vanadium, may seep downward to pollute groundwater resources.
Satellite image data and thematic maps made it possible to estimate the surface areas affected by the various military activities. Over 936 square kilometers of the desert surface were damaged by military vehicle traffic and earth movements. About 3,530 square kilometers of the desert pavement were destroyed by planting landmines and their clearance after the end of the military conflict. In addition, over 978 square kilometers of the surface were damaged by the formation of tarcrete layers and oil lakes. In total, the results of the war-related environmental degradation affected approximately 30% of the land area of Kuwait (El-Baz et al., 1994).
Literature Cited
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