Figure 1. Index map of SIR-C radar strips across Saudi Arabia.
Figure 2. Key SIR-C parameters.
Figure 3. Sand dune trends and prevailing wind directions of the Arabian Peninsula.
Figure 4. Generalized geologic map of the Arabian Peninsula.
Figure 5. Saudi Arabian stratigraphy.
Figure 6. Map of the Arabian tectonic plate.
Figure 7. Index map of SIR-C radar images.
Figure 8. Remote Sensing Center facilities, the Research Institute King Fahd University of Petroleum and Minerals.
Figures 9-18. Interpreted SIR-C images.
The Research Institute, King Fahd University of Petroleum and Minerals, has participated in the NASA/JPL science plan to evaluate Shuttle Imaging Radar for paleodrainage and geologic mapping purposes. The Space Shuttle Endeavour, carrying Shuttle Imaging Radar (SIR-C), has imaged selected parts of the Earth during two missions in April and October 1994. The SIR-C/X-SAR instruments acquired remote sensing data in L, C, and X bands. Using data provided by NASA/JPL, geologic interpretation of L-band image strips over the Arabian Peninsula has revealed faults, folds, joints, formation contacts, unconformities, karst topography, and paleodrainage systems beneath thin sand cover. These features are not visible on other remote sensing images (Landsat and SPOT) and are not shown on existing geologic maps. By virtue of its highlight/shadow and sand penetration capability and sensitivity to terrain textural factors, the L-band radar images complement and enhance interpretation of other geological remote sensing data and provide a new and unique view of the Earth's surface.
Remote Sensing technology has been continuously improved over the past several decades for applications in earth observation. The National Aeronautics and Space Administration (NASA) of the USA has been playing a leading role in the development and testing of new remote sensing instrumentation. In this regard NASA has been investigating the use of radar for remote sensing using the space shuttle as the platform.
The first Shuttle Imaging Radar (SIR-A) launched in 1981 and the second Shuttle Imaging Radar SIR-B launched in 1984. Ford et al. (1983 and 1986) illustrate some results of these previous Columbia SIR-A and Challenger SIR-B missions. Those missions were followed by two SIR-C/X-SAR missions in April and October 1994. The SIR-C/X-SAR instrument has been developed by cooperation among NASA, the German Space Agency (DARA), and Italian Space Agency (ASI).
The SIR-C/X-SAR provides increased capability over the previous radar missions by acquiring digital images at three microwave wavelengths, viz., L-band (23.5 cm), C-band (5.8 cm), and X-band (3.1 cm). L and C bands can have four polarization modes, HH, VV, HV and VH, whereas X band has only VV polarization. This allows derivation of the complete scattering matrix of a scene on a pixel by pixel basis for L and C bands. From this scattering matrix, every polarization configuration (linear, circular, or elliptical) can be gathered during ground processing. The radar polarimetric data yields more detailed information about the surface geometric structure, vegetation cover, and subsurface discontinuities than does image brightness alone (e.g., Elachi et al., 1990; Evans et al., 1986, 1988, 1994; Durden et al., 1989; Van Zyl et al., 1987; Van Zyl, 1989; and Zebker and Norikane, 1987).
The Research Institute, King Fahd University of Petroleum and Minerals, participated in this experiment to assess the utility of SIR-C/X-SAR data for geologic mapping, mapping of sand dune morphology, and the surface and subsurface paleo-drainage systems in the Arabian Peninsula. At NASA's request five trihedral corner reflectors were installed at a site (26 06 00 N and 50 06 00E) near Dhahran for data takes 107.3 and 91.3. These corner reflectors were used in the calibration of the recorded radar signals. SIR-C data obtained from the Space Shuttle Endeavour over the Arabian Peninsula were recorded on two separate missions: 1) SRL-1 data were imaged on 15 strips in April 1994 and 2) SRL-2 data were imaged on 12 additional strips in October 1994 (Figure 1).
SIR-C radar images have been compared to existing geologic maps and Landsat images, at a common 1:500,000 scale, in order to identify new information available. For clarity of illustration, the radar strip ("data takes") numbering shown on Figure 1 has been abbreviated by omitting digits to the right of the decimal. For the complete number designation, refer to Figure 2 or the text.
Together, the 27 strips traverse the equivalent of some 50,000 km length and cover about 3 million square kilometers of the land area of the Arabian Peninsula. They provide a unique, synoptic view of the geology of this largely remote and barren desert terrain. Generally speaking, fourteen of the strips were flown along an approximate S33°E (descending) azimuth and thirteen were flown on a N33°E (ascending) azimuth (Figure 2). Eight are left-looking and 19 are right-looking. Incidence angles of the strips vary from 22° to 58°. The width of these strips ranges from 28 km to 91 km and length from 900 km to 3350 km. Cross-track resolutions range from 23 m to 166 m and along-track resolutions from 50 m to 140 m. Recorded pixel size is 12.5 m at full-resolution but, for practical purposes, the images of this compilation were composed of 50 m pixels.
For reference purposes, Figure 4 is a geologic map which illustrates the distribution of cropping out formations in Saudi Arabia. Figure 5 is a stratigraphic column which describes the lithologies of the geologic formations identified on the images and places discussion and annotation of the 10 images into a geologic time framework. The stratigraphic abbreviations, explained for each image, are those used on USGS/DGMR geological maps. Many of the strips extend into countries adjoining Saudi Arabia, and example images shown are entirely within the Arabian tectonic plate (Figure 6). Locations of the 10 images are shown on Figure 7.
The facilities at the Remote Sensing Center (RSC), The Research Institute, King Fahad University of Petroleum and Minerals, Dhahran, Saudi Arabia, were extensively used in this experiment (Figure 8). The RSC contains state of the art Image Processing System (IPS) and Geographic Information System (GIS), supported by a complete photographic laboratory, as well as the expertise for processing, analyzing, and printing remote sensing data acquired from space platforms and development and design of geographic information databases. The Center cooperates with leading international agencies such as the National Aeronautics and Space Administration (NASA), European Space Agency (ESA), and National Space Development Agency of Japan (NASDA), and participates in the development and applications of remote sensing technology.
The Image Processing System at the Center is organized around a Silicon Graphics Indigo 2 High Impact workstation. The system configuration is geared to handle large volumes of data contained in the remotely sensed images taken from satellites. The other components of the system include a 10 GB tape storage, CD drive, 11 GB of on line disk storage, A4 and A0 size color printers, an A0 size digitizer, two display terminals, and an off-line Optronics C-4500 film scanner and recorder. The primary software available in the system are ERDAS Imagine, ER Mapper, and MicroImage. These packages are capable of handling a variety of image processing tasks which can be generally organized as utility, geocoding, multispectral analysis, radiometric and spatial enhancements, 3-d viewing, and image display.
The Geographic Information System is built around a Sun Sparc 2 workstation, with 4.5 GB on line disk storage, 5 GB 8mm tape, 150 MB cartridge tape, 1.5 inch tape drive, CD ROM and A0 size penplotter. ARC/INFO is the GIS software package which is capable of all the tasks required for database design, query, and analysis.
The Center maintains a satellite image library for the entire Arabian Peninsula. These images are available in both digital and print form. Provisions for acquiring the most recent data from Landsat and SPOT satellites exist through arrangements with EOSAT and SPOT Image Corporation. Future data needs of the center are envisaged as being fulfilled through the satellite data ground receiving station at King Abdul Aziz City of Science and Technology, Riyadh.
The Center provides basic remote sensing and GIS facilities for applications in a wide variety of scientific fields. In accordance with the plans and present development phase of the Kingdom, the Center's resources are focused towards natural resource surveys and environment monitoring. The Center offers remote sensing, image processing, and GIS services, as well as consultancy and applied research in agricultural surveys, coastal studies, engineering applications, geomorphology, geology, land-use analysis and urban planning, mineral exploration, rangeland resource assessment, sand studies, water resources assessment, and monitoring of oil spills.
With little or no concealing vegetation, the geologic features of the Arabian deserts are truly magnificent in scope and beauty. It is hoped that geologists familiar with the surface geology of Saudi Arabia will gain new appreciation and insights into the significance of the landforms and the geologic history depicted on the 27 radar strips and shown here by 10 representative images of this compilation (Figures 9-17). To those unfamiliar with the geology, a brief discussion is provided below as an introduction to the geology and tectonics of Saudi Arabia.
4.1 REGIONAL GEOLOGY OF SAUDI ARABIA
Geology of the Saudi Arabian tectonic plate is relatively uncomplicated, but has extremely important economic ramifications. Basement rocks exposed in the Arabian shield (Figure 4) were formed by accreting microplates-terranes and volcanic arcs in Proterozoic time (Stoeser and Camp, 1985). The total Phanerozoic age sedimentary section (Figure 5) resting upon the Proterozoic age basement is relatively thin near the Arabian shield, thickens to the east to more than 8 km (Brown, 1966), and has associated facies changes toward the northern Arabian Gulf. This record reflects persistent ongoing subsidence toward the north and east. Downwarping toward the Arabian Gulf continues into modern times, as evidenced by the Gulf itself. Uplift and arching of Cenozoic age strata over the Central Arabian arch show that it has had important components of movement during Cenozoic time.
Strata of Lower Paleozoic age (Cambro-Ordovician, Silurian, and Devonian) are mainly sandstone aquifers which rest unconformably upon the Proterozoic age igneous and metamorphic basement (Figure 5). Toward the end of the Carboniferous period, the platform was broken into a series of north trending horsts and graben during the Hercynian orogeny. After erosion, truncated edges of older Paleozoic age strata were covered by the Permian Khuff transgression (McGilvary and Husseini, 1992).
Apart from the gentle downwarp toward the NE described above, the Arabian plate has remained a rather stable platform throughout the Mesozoic and Cenozoic periods, resulting in remarkably widespread and uniform deposition. Many subtle unconformities within the stratigraphic section (Figure 5) reflect times of sea level changes, broad, gentle regional uplift, and erosion. The most obvious and significant unconformities observed on geological maps, and radar and Landsat images are at the stratigraphic base of the Paleozoic age section, that is, the base of the Permian Khuff-Unayzah Formations, base of the upper Cretaceous Aruma limestone, base of the Paleogene section, and base of the Miocene section. The Tethyan sea was present to the north and east of the subject area during much of Phanerozoic time but was closed in late Miocene time culminating in the Zagros Mountain uplift.
Regional dip on top of the basement is very low, generally less than 1° toward the east. Regionally, surface outcrop dips are eastward at values less than that, with strike of the beds curving around the axis of the Central Arabian arch (Figure 4). The cropping out resistant cuestas are mainly carbonate formations of Mesozoic and Cenozoic age.
The sedimentary basin toward the east is host to an ideal combination of favorable source rocks, burial, and thermal history, migration, reservoir rocks, seal, and early trap formation that has resulted in the entrapment of more than 650 billion barrels of recoverable oil reserves in Saudi Arabia, Kuwait, Iraq, Iran, and U.A.E., or some 65% of total world reserves. These are mainly in Jurassic and Cretaceous age carbonate and sandstone reservoirs. Ghawar oil field in Saudi Arabia alone contains 100 billion barrels or 10% of world oil reserves (Saudi Aramco, 1990).
Besides its huge oil reserves, Saudi Arabia brings to mind desert environments, blowing sands, and giant sand dunes. The dune forms tell a fascinating story which is elaborated by Glennie et al. (1994). Figure 3 shows the principal sand seas of Saudi Arabia and prevailing wind directions which form a giant clockwise gyre about the Arabian shield, consistent with existing NE Trade Winds latitudes. In the present hyper-arid climate of Saudi Arabia, these sand seas effectively block the many wadis and paleodrainages issuing from the Arabian shield and breaching the encircling cuestas of the Central Arabian arch (Figure 4).
The tectonic setting of the Arabian plate is shown on Figure 6. Major tectonic elements such as the Arabian shield, Central Arabian arch, basalt extrusives (harrats), the Bahrain anticline, Qatar arch, Ghawar anticline, and Central Arabian graben system are clearly expressed on the SIR-C radar images, some of which also reveal structures and geologic features not previously recognized, for example, the Wadi Sabah fault.
The igneous and metamorphic rocks which comprise the Arabian shield are generally more clearly shown on Landsat images than on SIR-C radar. However, in any tectonic interpretation, new information is added by the radar which should be taken into account. According to Stoeser and Camp (1985) the shield was formed of accreting volcanic arcs and terranes in Proterozoic time.
The Hercynian orogeny is a very significant interruption of the otherwise gentle Phanerozoic tectonic history of the interior of the Arabian platform. A series of north-trending horst blocks, formed in late Carboniferous and early Permian time, have been subtly rejuvenated through Mesozoic and Cenozoic time to form the giant traps, such as Ghawar, which contain the huge oil reserves of Saudi Arabia.
Salt tectonics and domal uplift movements of Infracambrian age halite account for many of the rest of the productive antiform structures, such as Bahrain, Dammam and extending into Burgan and other large, anticlinal structures productive from Cretaceous age sandstones in Kuwait. Compressional anticlines of the Zagros fold belt account for Iran's large gas and oil reserves in Tertiary age Asmari limestone.
Perhaps, the most dramatic and informative insights into the geologic history of the Arabian plate are found around its periphery. Earthquake epicenters, in recorded history, sharply outline the Arabian tectonic plate (Al-Furaih and Al-Aswad, 1994). Separation of the Arabian plate from Africa is interpreted as follows. After initial doming at the Red Sea-Gulf of Aden-African Rift triple junction, in Oligocene time, opening of the Gulf of Aden on the south and the northward propagating opening of the Red Sea-Gulf of Suez to the west of the Arabian tectonic plate has been accompanied by anti-clockwise rotation and northward translation of the plate away from Africa. These movements have resulted in the development of a compressional regime to the north and east of the plate in the Taurus-Bitlis-Zagros fold and thrust belts in Miocene to Quaternary time and an extensional regime on the west (Red Sea) and south (Gulf of Aden).
In late Miocene time, formation of the Zagros mountains fold belt, and sinistral movements along the Aqaba-Dead Sea transform, were initiated. These movements continue to be present as evidenced by earthquake data.
Basalt flow rocks, cinder cones, and vents comprising the harrats of western Saudi Arabia are closely related to extensional forces and spreading which opened the Red Sea. The extension is expressed on the radar strips as northwestward trending faults sub-parallel to the Red Sea and faulting parallel to the Gulf of Aden. The faults are quite young, cutting the basalts which are dated Oligocene to Holocene in age, with many being only 1 to 2 million years old.
Arguably, the most spectacular geology in the world is found in Oman, on the SE corner of the Arabian plate, where ophiolites (oceanic crustal rocks which are derived from depths corresponding to the Mohorovicic discontinuity and upper mantle) have been obducted (shoved, ramped) up onto the Arabian plate. These ophiolites represent a major collision with India and the Lut block of Iran in upper Cretaceous (Aruma) time. Compressional deformation related to the Zagros fold belt continued into Neogene time in Oman. Subtle evidence of this compression on the periphery of the Arabian plate is represented by warping, erosion, and unconformities in the interior of the Arabian plate.
5. SIGNIFICANT RESULTS
The interpretation of radar strips was carried out together with the Landsat multispectral images and published geologic maps of the area. In many ways, multispectral Landsat images are easier to work with and interpret than the radar strips but comparisons demonstrate much new supplementary data, especially in regards to penetration of dry sand by the radar signal which allows imaging of the sub-sand features. The mapping of sand-covered geologic features is discussed in Dabbagh et al. (1997).
The 27 radar strips have been interpreted to map metamorphic and igneous rocks of the Arabian shield, faults, joints, folds, fold belts, salt domes, formation contacts, unconformities, sabkhas, dunes, sand seas, harrats (basalt volcanic cones, craters and flows), alluvial fans, deltas, oolite shoals, wadi drainage patterns, agricultural and cultural development, karstic topography, oil fields, wind and oil slicks, a meteorite impact crater, and other geological and topographic features. Some examples are presented in Al-Hinai et al. (1997). Paleodrainage studies are particularly enhanced by the radar. The paleo-drainage mapping capability of the Shuttle Imaging Radar is evaluated by Dabbagh et al. (1995). Geological features have clear surface expression on radar whereas published geologic maps and Landsat images exhibit no indication or only faint clues to their presence. This report presents only ten representative SIR-C scenes which illustrate typical landforms and cultural features visible on the radar images and previously unrecognized geologic features below thin, dry sand cover. A separate comprehensive report of the findings is to be published in book form by KFUPM Press.
A section of data take 107.30 from SRL-1 (Figure 9) shows Wadi Sahba fault (f) in central Saudi Arabia. As a result of the clear expression of Wadi Sahba fault here, the Durmah-Nissah fault system to the west appears to be extending 250 km eastward along Wadi Sahba to the south end of Ghawar. Ad Dahna dunes and sand sheets on the north side of the Sahba fault appear transparent to the L-band radar. The karst (k) topography and a sharply delineated drainage system show clearly beneath the sand cover. This is an excellent example of sand penetration by the SIR-C L-band radar exposing geologic structure that is not apparent from other geologic data. Longitudinal dunes on the north side of the fault reach 10 m in height and show as dark streaks, whereas the transparent inter-dunal sand is about 2 m thick. South of the fault, and along it, sand cover is greater than 10 m and shows on the image in black to dark gray tones. Stratigraphic units show dip toward the east into the critical syncline to the west of Ghawar. This discovery will be a major addition to the ongoing evaluation of the tectonic setup and its contribution to the formation of hydrocarbon reservoirs in the area.
Faults and joint systems in the Eocene age cherty limestone north of Al Jauf are seen on this subsection of 145.50 SRL-1 strip (Figure 10). Cretaceous Aruma (Ka) and Eocene cherty limestones (Tlc) unconformably overlay folded and faulted Devonian strata (Dj) in which at least two directions of fracturing are evident. Faults and joints (f) in the Eocene cherty limestone are remarkably expressed on this radar image whereas only a few faults are shown on geologic maps. The faults are enhanced by the shadowing effect of the oblique illumination of the side-looking radar. They are mostly NW-trending and sub-parallel to the Red Sea and cut the basalts (Qtb), some of which have ages of less than 2 million years. The left third of the image is the SE end of Harat Shamah. A few volcanic cones and effusive centers are marked with (c). Duricrust (Qdc, Caliche), with its characteristic smooth, gray appearance, is seen just to the north of the Great Nafud sand sea.
Another example of fracturing and folding of basement rocks detected on the radar strips is in the subsection of data take 59.62 SRL-2, Figure 11. The left 80% of this image shows basement rocks of the Arabian shield composed of faulted marble, schists (sc), slate, graywack, conglomerate (sgs), andesite agglomerates (aa), granites (gn) cut by fractures and dikes (d), and granite stocks and plutons (gp). The unconformably overlaying, NE-dipping Permian Khuff carbonates (Pk) are clearly offset by the major fault (F) separating the Ar Rayan terrane and the Al Amar suture of Stoescer and Camp (1985). Projected northward across gravel plains (Qg) and the Nafud As Sirr (NS) sand "river", the fault appears to die out to the north of a tight, north-plunging anticline (a) expressed in well-bedded, Triassic Jilh carbonates, shale and sandstone (Trj) in the upper right corner. This anticline and much detail of fracturing and folding of the basement rocks are not shown on geologic quadrangle maps.
Several large areas of Cenozoic age basalt (Qtb, harrat) volcanic flows-cinders, vents and cones were detected on the radar strips and an example is shown here in a section of data take 145.50 SRL-1 (Figure 12) over western Saudi Arabia. The Harrat Shama volcanic field extends for some 500 km northwestward through Saudi Arabia to Jordan and as far north as Damascus. Some of the many volcanic cones and extrusive centers (c) have been marked on the image. North-trending fracturing (f) of the basalts and the underlaying cherty, brittle Eocene limestone and marl (dark, Tlc) is present. The fractures are subparallel to the Dead sea rift and may have served as conduits along which the basalt could rise to the surface. Some of the fractures have been marked and alignment of cones along fractures can be noted.
Karst features covered by thin sand have also been mapped on several strips. Figure 13 is a subsection of data take 113.70 SRL-1 showing a part of the Umm er Radhuma Formation in central Saudi Arabia. Paleogene age Umm er Radhuma (Tu) cherty limestone and dolomite is characterized by severe karst development (k) throughout the Kingdom. Here, large areas of dissolution (dark) may be seen which are partly related to fracturing of this brittle formation. The Umm er Radhuma Formation is a principal acquifer in eastern Saudi Arabia. It is unconformably overlain by Mio-Pliocene age sandstone and marl (Tsm), outcrops of which appear on the right side of this image which is about 30 km west of Jubat Jabrin. The Ad Dahna eolian "sand river" (Qes) appears only about one-half of the width shown on geologic maps because the edges are thin enough to be transparent to the L-band radar. Dark longitudinal dunes are apparent along he edges of Ad Dahna, being thicker than the intervening sand sheets. From NW (left) to SE (right), The stratigraphic succession is as follows: Cretaceous age Buwwaib (Kbu), Biyad (Kb) and Wasia (Kw) sandstones, Cretaceous Aruma limestone (Ka) and its sandy equivalent (Kas), Paleogene Umm er Radhuma (Tu) and finally Mio-Pliocene age sandstone and marl (Tsm).
The SIR-C L-band radar data have proved to be very useful in mapping paleodrainage systems partly covered by thin sand. Figure 14 shows Wadi Khirr paleodrainage system in NW Saudi Arabia unveiled by the L-band radar. This image was acquired during the data take 97.60 SRL-1. Here longitudinal dunes and sand sheets of Al Labbah, along the NE edge of the Nafud sand sea, smear dark streaks across the light gray background of the Upper Cretaceous Aruma limestone. The limestone is visible beneath the sand whereas it appears completely covered on Landsat images and on geologic maps. The dark streaks are thicker sand development in the linear, longitudinal dunes. Wadis flow northward down the dip slope of Aruma limestone. Wadi Khirr is thought to be a paleodrinage because of its wide, deep development compared to the other smaller wadis. The radar response is black where sand is thick against the east bank of the wadi.
Meteorite impact craters are shown on the radar imagery. Figure 15, a section of data take 33.70 SRL-2, shows a circular feature in Jiddat al-Harasis (Oman) which is believed to be a six km ring Habhab meteorite crater. Other larger but indistinct circular patterns to the NW of Habhab crater suggest the possibility of additional impact features. However, this interpretation is obscured by the karstification process, evidenced by the dark spots (sinkholes) and irregular, blind drainage pattern in Oligocene age chalky limestone marl (To).
Figure 16, a section of data take 65.60 SRL-1, shows variety of sand dune types. Quaternary barchan (Qb), megabarchan (Qmb), longitudinal (Ql, linear), dome (Qd) and barchanoid (Qbd) dunes seen here are along the south edge of the Rub'al Khali. The various dune types are particularly well displayed due to their large size, and highlight-shadow effect of the radar illumination. Note the change in dune types, sharp and distinct in part and transitional in part. Two apparently different wind directions are indicated by the arrows. The barchan direction is compatible with present day wind directions, whereas the longitudinal megabarchan dunes may represent older Pleistocene age dunes. Chalky, cherty, dolomitic limestone of the Eocene Dammam Formation (Td) is the cropping out formation on the north flank of Hadramaut arch.
Another geologic feature which has been mapped in many radar strips is sabkha (salt flat). Figure 17, a section of data take 33.70 SRL-2, show Umm as Samim (US) and interdune areas forming a huge inland sabkha (salt flat) at the eastern edge of the Rub Al Khali. The bright return from Umm as Samim is from the rough crusty salt-gypsum surface. It is the largest and best known of the inland sabkhas. Runoff from the mountains of Oman to the NE and highlands of the Huqf-Dhofar-Hadramaut arches to the east and south is funneled toward the Rub'al Khali and Umm as Samim where evaporation occurs. The dark gravel plain (gp) and alluvial fans (af) are a product of flash floods descending from the mountains. An unmapped wadi (uw, left) is very clearly shown here cutting into Oligo-Miocene (Tom) limestone, chalk, marl and gypsum bedrock which forms the light gray tones SE of Umm as Samim. Longitudinal (ld), megabarchan (mb), dome (dd), and barchan dunes (bd) may be identifed. Close agreement between the megabarchan dunes seen on the image and those on the 1963 USGS geologic map indicates that these giant dunes have not changed in shape over the last 30 years.
The oil fields and associated facilities have been very clearly visible on many strips because of the specular radar reflection from their metallic structures. Figure 18, a section of data take 123.12 SRL-1, shows the northern Arabian Gulf and includes some major oil fields (Salsal, Ribyan, Dawl, Hamur, Lawhah, Maharah, Marjan) located between Kharj Island and Ras Safaniyah. The fields (of) are recognized as groupings of bright corner reflections from the production platforms. Of particular interest is the dark mottling upon the water surface thought to be wind shadows of slicks and oilslicks on the surface of the water as indicated by the dark mottling. Wind and oil do indeed smooth the waters so that the radar signal is reflected away from the transmitting and receiving antenna, thus causing the dark tone on the image. Well-defined dark streaks closely associated with the production platforms are believed to be oil slicks as indicated. The broad poorly defined dark splotches are probably wind slicks.
The interpretations made in this experiment generally support the currently held views about the geologic and tectonic setting of the area, yet provide several new insights to enhance the current understanding. Cropping out strata of Paleozoic, Mesozoic, and Cenozoic age, dipping eastward away from the Arabian shield, reflect the architecture of the Arabian tectonic plate (Figures 4 and 6). Basal lower Paleozoic, basal Permian, basal Aruma Formation (upper Cretaceous), basal Tertiary, and basal Neogene unconformities are clearly demonstrated. The unconformities between these units become apparent by observing broad, regional, stratigraphic relationships between the Neogene age limestones and marls and underlaying Paleogene age limestones and dolomites of the stable Arabian tectonic plate.
Faults, folds, joints, formation contacts, salt domes and other geological and topographic features are readily apparent on the radar strips. In some instances these structures have surface expression whereas published geologic maps and Landsat provide only faint indications of such structure. Bedding attitudes may be readily estimated. Fundamentals of the formation of the Zagros fold belt are demonstrated on the radar images. Faulting is clearly shown in many areas, most notably the Majmaah, Drumah, Nisah and Sahba segments of the Central Arabian graben system. Fracturing of cherty limestones near Al Jauf provides a new understanding of processes involved in the opening of the Red Sea and extrusion of basalts (harrat) in western Saudi Arabia.
Linear, barchan, domal and sheet sand dune fields of the Nafud, Ad Dahna, Jafurah and Rub' al Khali sand seas have been differentiated ,as well as the megabarchan dunes with intervening inland sabkhas of the Rub' al Khali. Thick sand areas are commonly dark and featureless. The shadow-highlight effect, that one would expect from high dunes and low interdune areas, is obvious on the SIR-C strips in areas of large dune development. Several outstanding examples of L-band radar penetration of thin, dry sand sheets have been shown. Lag gravels on the surface of widespread gravel plains throughout Saudi Arabia attest to wind winnowing of alluvial sediments.
Tertiary and Quaternary age volcanic vents and flows, representing tectonically young volcanic activity in western Saudi Arabia, are easily recognizable. These Tertiary-Quaternary age basalt flows (harrats) generally exhibit a characteristic bright radar return and could be mapped in more detail than in this study by showing the different episodes of extrusion.
Our geologic and geomorphic remote sensing interpretation of the synoptic views of Landsat and spaceborne shuttle imaging radar supports the conclusions of Glennie et al., (1994), that is, that the climate and stream runoff in Pleistocene glacial, pluvial times were vastly different from the hyper-arid climate of today. The many examples of Pleistocene paleodrainage record an important history of humid Quaternary climate in Saudi Arabia which is very different from present day arid conditions. Wide large valleys, huge alluvial fan complexes, extensive gravel plains, and the large Wadi Batin delta, etc., attest to periods of more copious rainfall than present.
Karst topography in Tertiary age strata further demonstrate times of heavy rainfall and dissolution of carbonate rocks throughout the kingdom. Stream drainage patterns are sharply etched into cropping out strata, occasionally following fracture systems. Karst is commonly present and indicated by sinkholes, disappearing streams, and indistinct blind stream patterns on broad limestone outcrops.
In brief, the SIR-C L-band radar data not only confirm previous geological mapping but clearly bring out much information that is totally original, provide insights not previously possible, and thus establish the radar strips as a source of considerable cultural, economic, and geologic value. We have shown only a sampling of many possible examples. Future detailed studies and comparisons could be done with C-band and X-band radar images using data enhancement techniques to bring out features of particular interest. Field checking of selected areas is another priority for future work.
The project team wishes to acknowledge NASA/JPL support in providing data and other related material under the SIR-C/X-SAR science program. The Deputy Ministry for Mineral Resources (DMMR), Jeddah, provided administrative and logistical support. Digital data of all 67 images in this report were processed and printed at the Remote Sensing Center, Research Institute, KFUPM, mainly by Mr. Abdul Muqtadir who read the digital data, scaled and processed these data, and prepared the images, mosaics, and strips. Assistance with interpretation of image strips and preparation of overlays was provided by Wasim Ahmed and Syed Aftab Alam. The extensive drafting work was done by Mr. Saadat M. Rana of the Research Institute Technical Services.
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