Multi-Parameter SIR-C/X-SAR for Geoscience Study in

China

Prof. Guo Huadong

Institute of Remote Sensing

Applications Chinese Academy of

Sciences

P.O. Box 9718, Beijing 100101

China

OBJECTIVES

To establish the backscatter models for the typical targets at the land surface and study the penetration phenomena.

To develop the techniques for multi-frequency and multipolarization SAR image processing and specific geoscience information extraction.

To use SAR imagery to detect the geological structure and lithology both in arid and subtropic regions, and study the archaeology and other geoscientific fields.

To develop the interferometric and polarimetric SAR data analysis methods and evaluate their roles in the geoscience study.

PROGRESS

Spaceborne-Airborne-Ground-based Radar Remote

Sensing Experiment

During the first SIR-C/X-SAR mission (SRL-1), the Chinese airborne SAR (CASSAR) campaign was performed in Beijing test site simultaneously when SRL-1 overflew the test site. Meanwhile, the backscattering measurement was carried out on the ground using the truck-mounted scatterometer, and ground-truth data were collected in real time. This campaign is called the spaceborne airborne ground-based radar remote sensing experiment (Figure1).

The CASSAR operated in X-band and HH, VV polarizations with the resolution of 10m°¡10m. The flight altitude was 7200m. The azimuth and range directions were designed to coincide with the SRL1. The incidence angle is 73.93°Ñ°™83.12°Ñ. The truck-mounted scatterometer operated in X and L-band with 4 polarizations. The antenna was fixed in the boom about 12m high. It could illuminate any azimuth directions and elevation angles from 0°Ñto 84°Ñwith 6°Ñinterval. The Lunenburg sphere was used for external calibration before the measurement. The measured distributed targets including bare soil, winter wheat, rice staple, water body, etc. Meanwhile, vegetation and soil parameters were measured in the real time, such as biomass, soil moisture and surface roughness.

The backscattering data acquired in the platforms with different altitudes could be used for calibration and better understanding of interaction mechanisms of electromagnetic radiation with natural surfaces. Then the soil moisture and water content of winter wheat were analyzed using CASSAR data. The calibration results and ground truth data analysis of this campaign are put forward.

Penetration Experiment

The Inner Mongolia test site, which is located in the east of Badanjilin desert, was selected for the penetration phenomena experiment, where the vegetation coverage is quite sparse, and the land surface is mainly covered by the Gobi and sand. The weather is very dry. Before the SIR-C/X-SAR mission, twelve corner reflectors were buried in the sand, in which four are under the natural sand, six in the man-carried dry sand and the last two on the land surface.

The area has two test sites. No.1 site is located in western Alashan Trigonum, which is low-lying land composed of rocks covered by the sand of 1m thickness, with some rocks coming off the surface occasionally. Four corner reflectors are buried here, whose buried depth is 1m. Its purposes are (1) to observe the penetration phenomena of microwave in natural condition and the media moisture effects on microwave attenuation; (2) to expose the an process of microwave in natural sand. No. 2 site was choosed in a Gobi area, where the sand layer of 20cm thickness covered on the alluvium surface. Six corner reflectors were buried in a digged place with 0.93 2.73m depth covered by man-carried sand. Its purpose is to study the attenuation process of microwave in the dry sand and measure the penetrating height and condition. Then the samples were collected for measuring the moisture and complex dielectric constants. The penetration depths through the sand sheet of multifrequency radar signals were calculated and explained.

Targets Discrimination and Detection

a.At the Kunlun site of western China, a joint field investigation was carried out with JPL. The volcanoes northeast of Aksayqin Lake, in the Kunlun Mountains of western China, have been studied by combining field observations and SIR-C/X-SAR images. From the study, the volcanic morphology has been identified, which can be separated from the surrounding bedrock and alluvium (figure 2). The effects of different band and polarization combinations in recognizing volcanic features, lava flows, alluvial fans, and bedrock. A number of rock samples collected from the lava flows were analyzed with purpose of measuring dielectric constant isotopic dating, and determining chemical compositions of the samples.

b.Zhao Qing test site in southern China is located in the subtropical region, where the application of remote sensing in geological survey and mapping is limited by the dense vegetation and soil cover and poor data availability due to clouds and rain. So it was selected to demonstrate the advantages of multifrequency and multipolarization SAR technology in detecting the geological structures and lithology underneath a vegetation canopy to evaluate the capacities and limitation of SAR system for geological mapping in a subtropical region. The study presents the preliminary interpretation of geological structures, beds, and lithostratigraphic units from the shuttle imaging radar data in the test site(figure 3).

c.At the Yanchi site of north-central China, the study for the ancient segment of the Great Wall of China was done using the SIR-C/X-SAR image. The Great Wall has been eroded and buried in place by centuries of blowing sand.

INSAR for DEM Generation

This research used SIR-C/X-SAR L-band data in Xinjing for extraction of 3D information. Xinjiang is located in the north of Tibet Plateau, Northwest China. The population is sparse and the tectonic activity is frequent in the test site. It is the best site for the study of INSAR for crust deformation, which is one of the supersites for SIR C/X-SAR. Meanwhile, the vegetation coverage is less there, which is suitable for INSAR research.

The procedure of INSAR for DEM includes INSAR signal data processing, normalization of imaging parameters, precise registration of images, phase correction for flat terrain and phase unwrapping. After the above processing procedure, slant-range DEM could be obtained. The ortho projected DEM could be calculated using imaging parameters.

SIGNIFICANT RESULTS

During the first SIR-C/X-SAR mission, the truck- mounted scatterometer data acquired in the real time were used for calibration of the Chinese airborne CASSAR data and SRL-1 survey images in the Beijing test site. In the image linear range, the regression precision is less than 2 dB (except L-band data). However, due to the limited dynamic linear range of distributed targets, it is better to use active calibrators to extend the dynamic range if calibration. The analysis shows that the correlation is higher between soil moisture and SAR image than between the water content of winter wheat.

The penetration experiment indicates the depth of SIR- C/X SAR penetrating the dry sand in Alashan area is 2.82m, but some objects in the place more than 2.82m depth can still be monitored. The limit detecting depth can be calculated from the correlation between penetration depth and radar response intensity. L- band can detect the bedrock under the sand sheet. The research are (1) the depth of sand sheet is 1m thus the bedrock within 0.65m can be detected. (2) less than 1/e energy, which is backscattered by both the sand layer and rock, can reach the radar antenna. Therefor, when the water content is less than 6% without great depth, the bedrock under the sand can be detected.

The volcanic terrain, including in nine cinder cone and two types of lava flows (pahoehoe and aa lavas), can be identified from SIR-C/X-SAR images and field observations(figure 4). On the basis of analyses of backscatter coefficients, LHV appears best for discriminating the two types lava flows and alluvium and bedrock. This indicates that depolarization is important and that the roughness scales important for discrimination of the surfaces are nearer the scale of L-band (25cm) than the shorter wavelengths. CHV exhibits a similar effect to that of LHH for separating different surface features, which may be the result of some depolarization at the C-band scale (5cm) as well. In general, the shorter wavelengths of Cand X-band are poorer for delineating the surfaces mapped in this study. Geological study has shown that the volcanoes in this area are high in K2O and were erupted between 7.45 and 3.97 Ma. The ages suggest at least two volcanic eruption phases. The compositions of the volcanoes are associated with collisional tectonics.

SIR-C/X-SAR data for geological applications in the Zhao Qing test site highlight the advantages of the multifrequency and multipolarization data sets in detecting geological structures and discriminating lithology. The advantages are summarized as follows: (1) the high resolution, to reveal the geological features and details underneath the vegetation canopy; (2) multiparameters, to produce color composite images containing both the spectral and polarization information, improving the interpretability in geological applications; and (3) wide swath and less variations of distortion: to provide a large imaging area to trace and compare structures and their evolution. The three advantages of the SIR-C/X-SAR data indicate its potentials in the geological mapping, mineral exporation, lithlogy and rock type discrimination, and regional structural study. The major achievements of this study are as followings: (1) modification and updating of regional structures and their relationships; (2) discovery of unknown faults and lineaments which are not reported in the geological literature to date; (3) reveal the stratification and triangular facets of formation which indicate the occurrence of bedded structures underneath the vegetation canopy. This suggests SAR data have great potential in geological applications. Especially in a moist vegetated environment, it has incomparable advantages over visible and infrared remote sensing technology.

In SIR-C/X-SAR images, two generations of the Great Wall of China can be recognized(figure 5). The most recent version of the wall was built by the Ming Dynasty during the 14th century. An older version, built during the Sui Dynasty, runs parallel to the present wall. Using radar to look at archeological structures has been very powerful because the radar is sensitive to vertical structure, such as wall. For these segments of the Great Wall buried in place by the winds blowing, they can also show up quite clearly in radar image due to the radarís ability to penetrate through layers of dry sand. This indicates that the multiple channels of SIR-C/X-SAR system have the capability to detect different kinds of structures.

In the phase image and coherence image acquired from INSAR data (figure 6a), it can be seen that, the phase variance coincides with the terrain height. The coherence is related to S/N of data, coredistration precision, look numbers, etc. The acquired DEM is compared with the 1:100,000 topographic map, which is only topographic data here. DEM shows much information in details(figure 6b-d).

PUBLICATIONS

Guo Huadong, Zhu Liangpu, Shao Yun and Lu Xinqiao,

1996, Detection of Structural and Lithological

Feature Underneath a Vegetation Canopy Using SIR-C/X

SAR Data in Zhao Qing Test Site of Southern China, Journal

of Geophysical Research, 101(E10):23101-23108.

Guo Huadong, Liao Jingjuan, Wang Changlin, Wang

Chao, Thomas G. Farr and Diane L. Evans, 1997,

Use of Multifrequency and Multipolarization Imaging Radar

for Volcano Mapping in the Kunlun Mountains of Western

China, Remote Sensing of Environment, 59(2), p364-374

Guo Huadong, 1997, Spaceborne Multifrenquency, Polarimetric and

Interferometric Radar for Detection of the Targets on Earth Surface

and Subsurface, Journal of Remote Sening

(Chinese), Vol.1, No.1, p32-39

Wang Chao, Guo Huadong, Li Lin, 1995, Spaceborne

Airborne-Ground based Radar Remote Sensing

Experiment, Chinese Science Bulletin, 40 (24).

Wang Chao, 1997, Using SIR-C Interferometric Data for

DEM Acquisition, Journal of Remote Sening

(Chinese),Vol.1, No.1, p46-49

List of the figures

Figure 1. The sketch of spaceborne-airborne-ground based remote sensing experiment at Beijing test site of China. (File name: fig1.jpq)

Figure 2. The color composite image of LHH (red), LHV (green), and CHH (blue) at Kunlun site of western China. It appears that the volcanic morpholoy can be separated from the surrouding bedrock and alluvium. (File name: fig2.tif)

Figure 3. (a)The color composite image of LHH (red), LHV (green), and CHV (blue) . A group of color stripes in cyan, greyish brown represents the beds of sedimentary rocks underneath the vegetation cover. (b) the color composite image of LHH (red), CHV (green), and XVV (blue) at Zhao Qing test site of southern China. It shows the structural and lithological features. (c) Inferred gold-bearing arcuate structure at Gold Pit village. (File name: fig3.jpg)

Figure 4. Geological interpretation map of the SIR-C/X-SAR image at Kunlun site of western China. 1. cinder cone and number; 2. aa lava; 3. pahoehoe lava; 4. Triassic strata; 5. Quaternary alluvium; 6. sample sites. (File name: fig4.tif)

Figure 5. The color composite image of LHH (red), LHV (green), and CHV (blue) at the Yanchi site of north-central China. Two generations (Ming and Sui Dynasty) of the Great Wall can be recognized. (File name: fig5.jpg)

Figure 6. Using SIR-C interferometric data for DEM acquisition at Kunlun site of western China. (a) the phase coherence image; (b) the DEM acquired from L band, VV polarization data; (c) the 3D view of DEM; (d) the 1:100,000 topographic map in the test site. (File name: fig6.jpg)