Monitoring subsidence characteristics of Baishazhou karst area in Wuhan with Sentinel-1 images from 2015 to 2019

The karst region of southwest China includes , Guizhou, Yunnan, Guangxi, Sichuan, Hunan, Hubei, Guangdong and Chongqing. The region has a warm and humid climate with heavy rain, making it a zone of high incidence of geological disasters. China experienced 6,181 geological disasters in 2019, up 108.4% from 2,966 in 2018. These disasters caused the direct economic loss of 1.7 billion yuan, accounting for 61.4% of the total in China. Two hundred and twenty-four people are dead and missing, with 98 people in the karst mountainous areas of Chongqing, Sichuan, Guizhou, and Yunnan.

Numerous Chinese cities, including Guiyang, Guilin, Wuhan, Shenzhen, Guangzhou, etc., are situated in karst regions. According to statistics, more than 30 large and medium-sized cities and 420 counties and cities in China are located in high-risk locations for ground collapse. One of the primary causes of geological disasters like ground collapse and ground deformation in cities is the development of subsurface karst, abundant karst groundwater, and a specific thickness of soil layer. In Wuhan, there is a sizable region of covered karst with extremely complicated geological conditions that are primarily distributed in an NWW-SEE direction. The Yangtze River is crossed by several karst belts. Consequently, karst environmental geological problems are widespread in this region, and karst geological disasters have resulted in considerable financial losses. The survey data shows that there have been more than 40 karst collapses since 1994. Human engineering activities have increased in intensity with the ongoing urbanization. Geological disasters have been happening more frequently year after year.

The technologies such geological radar detection, the automatic monitoring of sensor for water pressure dynamic change in karst pipeline system, the survey monitoring with the precise level, GPS monitoring, Interferometric Synthetic Aperture Radar (InSAR), etc. are used to monitor surface deformation in karst areas. Among them, InSAR has been widely used as a method to monitor wide-range deformation with high precision in recent years. InSAR shows its advantages of all-weather, all-time, extensive spatial coverage, weather durability, and high precision, compared to other conventional monitoring techniques. In this study, the researchers used InSAR technology to conduct time series analysis on the data of Synthetic Aperture Radar (SAR) such as TerraSAR-X, COSMOSkyMed, Sentinel-1, Radarsat-2, etc. This data was obtained in Wuhan at different time periods, which can indicate that the subsidence mainly occurred in soft soil areas and karst areas with intensified human activities.

Situated in Wuhan at 30° 28′ 0′′–30° 32′ 24′′ north latitude and 114° 12′ 30′′–114° 18′ 45′′ east longitude, the study area covers the embankment of the Yangtze River and the Zhoutou sub-district in Hanyang district, Baishazhou sub-district in Wuchang district, and Zhangjiawan sub-district and Qingling sub-district in Hongshan district, in which the alluvial lacustrine plain and the denudation accumulation hillock are dominant landforms and the terrace is developed in the river valley. Holocene loose sediments from the Quaternary primarily cover the surface. Clayey soil makes up the higher portion of the lithology, and silty fine sand makes up the lower portion. The risk of a karst collapse is high because the loose deposits are buried beneath soluble carbonate rocks. The historical karst collapse spots in the study area, mainly situated in the alluvial lacustrine plain. Due to intensified human activities, the incidence of karst collapse has also been increasing year by year, which can be indicated by 23 karst collapses between 1931 and 2019.

In this study, the elevated orbit Sentinel-1 SAR data set covering the high incidence area of karst collapse in Baishazhou, Wuhan, was analyzed with the time series of InSAR. Additionally, the land subsidence in the study area was identified and analyzed from April 2015 to September 2019. In the study area, the surface is mainly covered by loose deposits of the Quaternary Holocene, with cohesive soil in the upper part and fine sand in the lower part. The underlying soluble carbonate rocks under loose deposits may pose a high risk of karst collapse. Land subsidence resulted from the weight of the Quaternary soil covering the karst area has significantly increased by the concentrated summer rain. At high incidence zones of karst collapse in Baishazhou sub-district and Zhangjiawan sub-district, seasonal deformation signals connected to rainfall are discovered based on time series analysis, and the deformation rate is roughly 15 mm·a⁻¹. Attributed to human construction activities, ground subsidence with a maximum deformation rate of 30 mm·a⁻¹ was also discovered in the depot of Metro Line 6 and Qingling sub-district. This study establishes the value of InSAR technology in identifying geological risks in karst regions. A significant amount of SAR data can be offered for geological disasters in karst areas thanks to the development of Sentinel-1 and the upcoming NISAR mission of the United States. It is anticipated that, in the future, land subsidence monitoring products with large range, high precision and hightime-resolution will be available for the prevention and control of geological disasters.