Seasat Radar Images Research Articles

Construction and analysis of simulated Venera and Magellan images of Venus

One-look Seasat radar images covering the Gran Desierto dune complex, Sonora, Mexico; the Appalachian Valley and Ridge Province; and accreted terranes in the central interior of Alaska were digitally processed to simulate both Venera 15, 16 images (1 to 3 km resolution) of Venus and image data expected from the Magellan mission (120 to 300 m resolution). Sinc filters were used to simulate the appropriate range and azimuth resolutions, speckle was introduced as multiplicative noise, and additive Gaussian noise was included to simulate expected signal to thermal noise ratios. The Gran Desierto dunes, the largest complex in North America, are not discernable in the Venera simulation, whereas the higher resolution Magellan simulation shows the dominant dune patterns and specular reflections from dune faces oriented perpendicular to the incident radar. Anticlinal and synclinal structures are evident in both simulations over the Appalachians, mainly because differential weathering and erosion left resistant units as topographic highs that delineate the folds. The Magellan simulation also shows that fluvial processes have dominated erosion and exposure of the folded structures. Mountainous terrains and their degree of erosion are discernable in both simulations over Alaska, although only the Magellan simulation shows that fluvial, glacial, and aeolian processes have all been active in shaping the landscape. Neither simulation provides evidence that diverse lithotectonic terranes in Alaska were juxtaposed (i.e., accreted), since the primary evidence needed is lithological, whereas radar returns are dominated by topography and surface roughness, parameters only weakly indicative of lithology. Venera data show clear evidence for volcanic and tectonic terrains on Venus, i.e., endogenic landforms (V. L. Barsukov et al. 1986, Proc. Lunar Planet. Sci. Conf. XVII, Part 2, J. Geophys. Res. 91, D378–D398). On the other hand, the simulations suggest that determination of the nature and extent of terrain modification by exogenic processes (e.g., atmosphere-surface weathering, erosion, deposition) on Venus will remain uncertain, since the length scales of features diagnostic of such processes may be too small to be discerned from Venera data. Substantial differential weathering, erosion, and deposition may have occurred on Venus, enhancing the appearance of both volcanic and tectonic features. Alternatively, rates of resurfacing by volcanic and tectonic processes may have been much higher than rates of atmosphere-surface interactions, producing a surface dominated by volcanic burial and tectonic disruption. The simulations suggest that Magellan data may provide the critical fine-scale morphological information needed to test between these two alternative resurfacing scenarios.

Comparison of submarine relief features on a radar satellite image and on a Skylab satellite photograph

Abstract We compare satellite imagery obtained by an optical sensor and by synthetic aperture radar for a shallow ocean area showing submarine relief features. A Skylab photograph and a Seasat radar image of the North American cast coast (Nantucket Shoals) taken at different dates, but at the same tidal phase and under comparable weather conditions, are analysed. It is shown that the radar imaging as well as the optical imaging is caused by roughness variations of the water surface due to tidal flow over submarine relief. It is investigated whether optical imaging is affected by backscattering by suspended sediment in the water column, by reflection from the sea floor or by variations of the surface roughness associated with wind and tidal flow over underwater bottom topography. We conclude from the analysis of the densities in the blue, green and red layers of the Skylab colour film that specularly reflected sunlight at the rough ocean surface is the dominant imaging mechanism. In cases where the underwa...

MAPPING OF FORESTED WETLAND: USE OF SEASAT RADAR IMAGES TO COMPLEMENT CONVENTIONAL SOURCES

Distinguishing forested wetland from dry forest using aerial photographs is handicapped because photographs often do not reveal the presence of water below tree canopies. Radar images obtained by the Seasat satellite reveal forested wetland as highly reflective patterns on the coastal plain between Maryland and Florida. Seasat radar images may complement aerial photographs for compiling maps of wetland. A test with experienced photointerpreters revealed that interpretation accuracy was significantly higher when using Seasat radar images than when using only conventional sources.

Mapping of glacial landforms from Seasat radar images

Glacial landforms in the drumlin drift belt of Ireland and the Alaska Range can be identified and mapped from Seasat synthetic-aperture radar (SAR) images. Drumlins cover 60% of the Ireland scene. The width/length ratio of individual drumlins can be measured on the SAR images, allowing regional differences in drumlin shape to be mapped. This cannot be done with corresponding Landsat multispectral scanner (MSS) images because of lower spatial resolution and because of shadowing effects that vary seasonally. The Alaska scene shows the extent and nature of morphological features such as medial and lateral moraines, stagnant ice, and fluted ground moraine in glaciated valleys. Perception of these features on corresponding Landsat MSS images is limited by seasonal differences in solar illumination. Because SAR is not affected by such differences or by cloud cover, it is particularly well suited for monitoring glacial movement. The disadvantage of distorted high-relief features on Seasat SAR images can be reduced in future SAR systems by modifying the radar illumination geometry.

Detection of subsurface features in SEASAT radar images of Means Valley, Mojave Desert, California

Igneous dikes buried beneath as much as 2 m of alluvium in the Mojave Desert of California were detected by the SEASAT L-band (23.5-cm wavelength) synthetic-aperture radar (SAR) in 1978. The roughness and dihedral configuration of the dikes are favorable to generation of strong radar echos. The soil-moisture levels in 1978 were likely below the critical 1% level. The other permissive conditions for radar penetration of a fine-grained and thin alluvial cover are present. Our findings suggest that subsurface features with potential tectonic or geomorphic significance may be revealed in other orbital radar images of semiarid terrains.

Cliff and slope topography of part of the grand canyon, Arizona as characterized on a seasat radar image

Cliff and slope topography of part of the grand canyon, Arizona as characterized on a seasat radar image

Cover photograph Seasat radar image of the Phoenix, Arizona, Region

Cover photograph Seasat radar image of the Phoenix, Arizona, Region

Seasat Radar Image of San Andreas Fault, California

On a Seasat radar image (23.5-cm wavelength) of the Durmid Hills in southern California, the San Andreas fault is expressed as a prominent southeast-trending tonal lineament that is bright on the southwest side and dark on the northeast side. Field investigation established that the bright signature corresponds to outcrops of the Borrego Formation, which weathers to a rough surface. The dark signature corresponds to the Lake Cahuilla sand and silt deposits which are smooth at the wavelength of the Seasat radar. These signatures and field characteristics agree with calculations of the smooth and rough radar criteria. On Landsat and Skylab images of the Durmid Hills, the Borrego and Lake Cahuilla surfaces have similar bright tones and the San Andreas fault is not detectable On a side-looking airborne radar image (0.86-cm wavelength), both the Borrego and Lake Cahuilla surfaces appear rough, which results in bright signatures on both sides of the San Andreas fault. Because of this lack of roughness contrast, the fault cannot be distinguished on the aircraft image acquired at a short radar wavelength. The wavelength of the Seasat radar system is well suited for mapping Durmid Hills geologic features that are obscure on the other remote sensing images evaluated in this report.

Seasat Radar Image of San Andreas Fault, California: ERRATUM

AAPG Bulltein, May 1980, v. 64, no. 5, p. 627, Seasat Radar Image of San Andreas Fault, California, by Floyd F. Sabins et al. Table 2 was incorrect as published. The revised version is shown below. Table 2. Calculation of Roughness Criteria End_of_Article - Last_Page 1730------------