sensors Article Monitoring the Degradation of Island Permafrost Using Time-Series InSAR Technique: A Case Study of Heihe, China Sai Wang 1, Bing Xu 1,*, Wei Shan 2 , Jiancun Shi 1, Zhiwei Li 1 and Guangcai Feng 1 1 School of Geosciences and Info-Physics, Central South University, Changsha 410083, China; [email protected] (S.W.); [email protected] (J.S.); [email protected] (Z.L.); [email protected] (G.F.) 2 Engineering Consulting and Design Institute, Northeast Forestry University, Harbin 150040, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0731-88830573 Received: 9 February 2019; Accepted: 15 March 2019; Published: 19 March 2019 Abstract: In the context of global warming, the air temperature of the Heihe basin in Northeast China has increased significantly, resulting in the degradation of the island permafrost. In this paper, we used an elaborated time-series Interferometric Synthetic Aperture Radar (InSAR) strategy to monitor the ground deformation in the Heihe area (Heilongjiang Province, China) and then analyzed the permafrost deformation characteristics from June 2007 to December 2010. The results showed that the region presented island permafrost surface deformation, and the deformation rate along the line of sight mainly varied from –70 to 70 mm/a. Based on the analysis of remote sensing and topological measurements, we found that the deformation area generally occurred at lower altitudes and on shady slopes, which is consistent with the distribution characteristics of permafrost islands. Additionally, the deformation of permafrost is highly correlated with the increase of annual minimum temperature, with an average correlation value of –0.80. The accelerated degradation of permafrost in the study area led to the settlement, threatening the infrastructure safety. Our results reveal accelerated degradation characteristics for the island permafrost under the background of rising air temperature, and provide a reference for future relevant research. Keywords: island permafrost; deformation; time-series InSAR; global warming; geohazards 1. Introduction Seasonal permafrost and permafrost are broadly distributed in China, covering 54% and 25% of the land area, respectively. The distribution area of all permafrost is about 1.59 × 106 km2, ranking third in the world. The permafrost distributed in the northern part of Northeast China represents the second largest area of permafrost in China [1]. In recent years, global warming has caused a rise in air temperatures and increasing precipitation, which have accelerated the degradation of permafrost. Due to the widespread degeneration of permafrost, forests are dramatically transforming into swamps, further breaking the ecosystem equilibrium. This situation should receive attention [2]. As a sensitive indicator of climate change, the change in permafrost has attracted special attention around the world. Northeast China is the southern margin of the Eurasian permafrost region. The permafrost of these low-elevation islands is more sensitive to changes in climate and environmental, and has shown obvious signs of degradation in recent years. The surface deformation caused by island permafrost degradation is localized, which is different from the surface deformation (usually the overall uplift or subsidence) caused by permafrost degradation. Additionally, the degradation of island permafrost has a great impact on the local environment, leading to phenomena such as landslides [3], solifluction, and partial expressway collapse and destruction. With the increase of construction Sensors 2019, 19, 1364; doi:10.3390/s19061364 www.mdpi.com/journal/sensors Sensors 2019, 19, x FOR PEER REVIEW 2 of 14 Sensorsor subsidence)2019, 19, 1364 caused by permafrost degradation. Additionally, the degradation of island permafrost2 of 14 has a great impact on the local environment, leading to phenomena such as landslides [3], solifluction, and partial expressway collapse and destruction. With the increase of construction projects, projects, understanding the distribution of surface deformation in the island permafrost area is the key understanding the distribution of surface deformation in the island permafrost area is the key to to disaster warning and prevention. However, compared with the permafrost in the Qinghai–Tibet disaster warning and prevention. However, compared with the permafrost in the Qinghai–Tibet plateau, the permafrost monitoring research in this region is insufficiently studied. More attention plateau, the permafrost monitoring research in this region is insufficiently studied. More attention needs to be paid to the permafrost degradation in the future. needs to be paid to the permafrost degradation in the future. Due to its wide spatial coverage, Interferometric Synthetic Aperture Radar (InSAR), as an all-weather Due to its wide spatial coverage, Interferometric Synthetic Aperture Radar (InSAR), as an all- monitoring method with competitive accuracy, has been widely used to obtain ground deformation weather monitoring method with competitive accuracy, has been widely used to obtain ground measurements. The traditional differential InSAR technique is inevitably affected by spatiotemporal deformation measurements. The traditional differential InSAR technique is inevitably affected by decorrelation,spatiotemporal atmospheric decorrelation, delay, atmospheric and random delay, noise, and which random limits noise, the measurementwhich limits the accuracy measurement to some extentaccuracy [4]. Toto some overcome extent these [4]. To limitations, overcome time-series these limitations, InSAR techniquestime-series suchInSAR as techniques persistent scatterersuch as interferometrypersistent scatterer (PS-InSAR) interferometry [5] and (PS-InSAR) small-baseline [5] and subset small-baseline interferometry subset (SBAS-InSAR) interferometry have (SBAS- been proposedInSAR) have [6]. Thesebeen proposed time-series [6]. InSAR These techniquestime-series canInSAR monitor techniques small deformationcan monitor small over adeformation long period, areover suitable a long for period, surface are deformation suitable for inversionsurface deformation in the study inversion area, and in have the beenstudy successfully area, and have applied been in manysuccessfully cases. Theapplied InSAR in techniquemany cases. has The been InSAR widely te usedchnique for monitoringhas been widely surface used deformation for monitoring caused bysurface earthquakes deformation [7], underground caused by earthquakes mining [8], [7], landslides underground [9], oil mining and gas [8], extraction landslides [10 [9],], groundwater oil and gas extractionextraction [ 11[10],], and groundwater thawing and extraction freezing [11], of permafrostand thawing [12 and,13 ].freezing of permafrost [12,13]. InIn this this study, study, we focuswe focus on the on degradation the degradation monitoring monitoring of island of permafrost island permafrost around the around expressway the fromexpressway Bei’an to from Heihe Bei’an in Heilongjiang to Heihe in Province,Heilongjiang China. Province, First, a setChina. of 20 First, scenes a set L-band of 20 ALOS/PALSARscenes L-band stripmapALOS/PALSAR images stripmap acquired images from Juneacquired 2006 from to December June 2006 to 2010 December were processed 2010 were with processed an elaborated with an time-serieselaborated InSARtime-series strategy InSAR to obtainstrategy deformation to obtain deformation of the study of area. the Subsequently,study area. Subsequently, the deformation the ofdeformation island permafrost of island was permafrost analyzed was in combination analyzed in withcombination elevation, with slope elevation, direction, slope and direction, temperature. and Moreover,temperature. we Moreover, analyzedthe we correlationanalyzed the between correlation thedeformation between the ofdeformation island permafrost of island and permafrost elevated temperature.and elevated Finally,temperature. we further Finally, analyzed we further the anal potentialyzed the geological potential hazards geological associated hazards withassociated island permafrostwith island degradation. permafrost degradation. 2.2. Study Study AreaArea TheThe study study area area is locatedis located in the in Sunwu–Aihuithe Sunwu–Aihui area, Heihearea, Heihe City, in City, Northern in Northern Heilongjiang Heilongjiang Province, ChinaProvince, (see China Figure 1(see). The Figure plains 1). ofThe the plains study of area the havestudy low area mountainous have low mountainous terrain, with terrain, an elevation with an of betweenelevation 140 of andbetween 425 m.140 The and elevation 425 m. The of theelevation study areaof the gradually study area decreases gradually from decreases the northwest from the to thenorthwest southeast. to the southeast. FigureFigure 1. 1.The The studystudy areaarea andand itsits Digital Elevation Model (D (DEM).EM). The The red red rectangle rectangle on on the the left left outlines outlines thethe spatial spatial coverage coverage ofof thethe studystudy area,area, andand thethe rightright inset shows the the DEM DEM of of the the study study area, area, with with the the bold black dashed line denoting the expressway from Bei’an to Heihe and the white rectangle denoting the coverage of the ALOS/PALSAR data used in this study. Sensors 2019, 19, 1364 3 of 14 Sensors 2019, 19, x FOR PEER REVIEW 3 of 14 bold black dashed line denoting the expressway from Bei’an to Heihe and the white rectangle The central part of the image is intermountain