Detecting the Depth of a Subsurface Brine Layer in Lop Nur Lake Basin Using Polarimetric L-Band SAR

Hindawi Publishing Corporation Journal of Sensors Volume 2015, Article ID 245790, 11 pages http://dx.doi.org/10.1155/2015/245790 Research Article Detecting the Depth of a Subsurface Brine Layer in Lop Nur Lake Basin Using Polarimetric L-Band SAR Chang-An Liu,1,2 Huaze Gong,1 Yun Shao,1 and Bingyan Li3 1 Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Datun Road, Beijing 100101, China University of Chinese Academy of Science, Beijing 100049, China 3 China University of Geosciences Beijing 100083, China 2 Correspondence should be addressed to Huaze Gong; Received 28 October 2014; Revised 15 January 2015; Accepted 5 February 2015 Academic Editor: Yoshio Yamaguchi Copyright © 2015 Chang-An Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Lop Nur once was a huge lake located in northwestern China. At present, there is no surface water in Lop Nur Lake basin and on SAR images it looks like an “Ear.” The objective of this paper is to retrieve the depth of subsurface brine layer in Lop Nur by copolarized phase difference of surface scattering. Based on field investigation and analysis of sample properties, a two-layer scattering structure was proposed with detailed explanations of scattering mechanisms. The relationship between copolarized phase difference and the brine layer depth in the region of Lop Nur were studied. The copolarized phase difference of surface scattering was extracted by model-based polarimetric decomposition method. A good linear correlation between measured subsurface brine layer depth and copolarized phase difference with R2 reaching 0.82 was found. Furthermore, the subsurface brine layer depth of the entire lake area was analyzed. According to the retrieved maps, some interesting phenomena were found, and several hypotheses about the past water withdrawal process and the environmental evolution had been proposed to theoretically explain these phenomena. Based on the penetration capability of SAR the reconstruction of historical evolution process of Lop Nur will be an interesting topic for future research. 1. Introduction Lop Nur was once an extensive lake located in the Xinjiang Uygur Autonomous Region in the eastern Tarim Basin, in northwestern China. Lop Nur is located in a typical arid region, which is highly sensitive to climate change, and its history of environmental changes is significant for understanding historic global climate change [1–3]. The Lop Nur basin was an important section of the ancient “Silk Road,” famous as the prosperous communication channel between Eastern and Western cultures. However, Lop Nur has completely dried up during the 20th century, leaving earshaped concentric strips (interchanging bright-grey appearance) visible in remote sensing images (Figure 1) [4–6]. Many assumptions have been made about its formation, but they remain to be proven by scientific evidence, and therefore its formation has remained a mystery [7]. It has been called the “Drought Pole” and the “Sea of Death” because of its extremely dry conditions and poor accessibility [6]. The lake area may expand and shrink several times because of the periodic arrival of moist airflow or flooding. It serves as a region of salt and water accumulation in the Tarim Basin and is therefore rich in mineral salts with many salts at the surface and very low moisture content. With the penetration ability of synthetic aperture radar (SAR) signals and the arid environment of Lop Nur, it is believed that SAR can detect subsurface targets and reflect the information of the thickness of the dry soil layer [8]. One of the significant advantages of SAR is penetrating dry soil. In 1981, space shuttle Columbia installed an Lband HH polarization imaging radar system SIR-A and obtained many images. Scientists have made many valuable applications of the results. Most notably, McCauley of the United States Geological Survey found ancient river and human remains in the eastern Sahara desert under sand cover using SIR-A image analysis, which caused a sensation in the remote sensing and meteorological communities [9]. Blom found igneous dikes buried under up to 2 m of alluvium in 2 Journal of Sensors 40∘ 40󳰀 0󳰀󳰀 N C A 40∘ 20󳰀 0󳰀󳰀 N III B I Lop Nur Lake basin II 40∘ 0󳰀 0󳰀󳰀 N 40∘ 0󳰀 0󳰀󳰀 N 40∘ 20󳰀 0󳰀󳰀 N 40∘ 40󳰀 0󳰀󳰀 N 41∘ 0󳰀 0󳰀󳰀 N 41∘ 0󳰀 0󳰀󳰀 N 90∘ 0󳰀 0󳰀󳰀 E 90∘ 20󳰀 0󳰀󳰀 E 90∘ 40󳰀 0󳰀󳰀 E 91∘ 0󳰀 0󳰀󳰀 E 91∘ 20󳰀 0󳰀󳰀 E 91∘ 40󳰀 0󳰀󳰀 E 90∘ 0󳰀 0󳰀󳰀 E 90∘ 20󳰀 0󳰀󳰀 E 90∘ 40󳰀 0󳰀󳰀 E 91∘ 0󳰀 0󳰀󳰀 E 91∘ 20󳰀 0󳰀󳰀 E 91∘ 40󳰀 0󳰀󳰀 E 0 20 (km) 40 60 N Figure 1: Field investigation routes and sampling site locations. A, B, and C are field routes in 2006, and I, II, and III are field routes in 2008. The dashed rectangle is the coverage of full-polarimetric SAR data used in the last part of this paper and the red dots are positions of sampling sites of 2013. There is a large salt pond in the middle of the image as a black square. ALOS-PALSAR image (HH polarization, ScanSAR mode) obtained on January 15, 2011, were used as base map. the Mojave Desert of California detected by the SEASAT Lband (23.5-cm wavelength) SAR [10]. These findings suggest that subsurface features with potential tectonic or geomorphic significance may be revealed in other orbital radar images in semiarid terrains. Schaber et al. further studied the effects of physical factors that affect penetration and attenuation [11, 12] and found that the estimated penetration depth of L-band radar is approximately 1.5 m through sandy sediment in the southern desert of Egypt [13]. In addition, there are many scientists using SIR-B and SIR-C to study subsurface geological structures and radar penetration depth in desert areas [14–17]. In radar remote sensing, copolarized phase difference has a lot of physical meaning and special significance. Lasne et al. found that the copolarized phase difference has a good correlation with the depth of the dry sand layer [18]. This phase signature may be used as a new tool to map subsurface moisture in arid regions. In their study, a phase difference of 23∘ between the horizontal (HH) and vertical (VV) channels was clearly observed. It allows the detection of the wet paleosoil down to a greater depth (5.2 m) than when only considering HH and HV amplitude signals (3.5 m). They performed several experiments and published many articles to validate their theory [19–23]. Lop Nur also has a two-layer soil structure area and therefore, based on the theory of Lasne et al., we will study the copolarized phase difference in the Lop Nur Lake Basin. The theory of Lasne et al. is based on the field created by a wet paleosoil layer, which is covered by a dry sand layer. The structure of their research objective is relatively simple, while the structure and chemical composition of the soil layer in the Lop Nur region (...truncated)