朝鲜第六次核爆后InSAR地表形变测量与分析

Measurement and analysis of surface deformation after the sixth nuclear explosion in the Democratic People’s Republic of Korea using InSAR

利用InSAR小基线集(SBAS)方法得到了朝鲜第六次核爆后其中心17 km×22 km范围内一些部位不同时刻(2017年9月10日~2018年6月1日每12 d间隔)的累积地表形变量.将这些测量点依空间相邻关系聚集成14个集合,按照集合内各点平均相干性进行加权平均得到各集合的累积形变量.观测结果显示:(1) SBAS-InSAR能有效观测第六次核试验的热辐射后效阶段形变过程,爆炸中心附近在爆炸后10余天仍存在地表抬升现象,随后开始下沉,不同地方下沉速率和下沉量不同;(2)在冬春季可观测到可能主要因围岩内裂隙水冰冻带来的地表下沉减缓甚至抬升的现象, 2018年5月24日因朝鲜对部分核设施进行爆破使地表形变出现抬升.研究结果表明:(1)第六次核试验的热辐射后效阶段主要表现为围岩受高温高压作用变酥变软,变质后的围岩在重力作用下被压实并开始下沉,沉降的时间过程可以用Weibull模型进行拟合分析;(2)考虑受核爆影响的变质岩层厚度等因素建模分析最大沉降量,得到了爆炸中心垂直向影响距离约为1800~2300 m,变质后的岩石形变系数约为7×10^-5~8×10^-5,统计拟合优度为0.8,P值接近于0.

We use the small baseline subset(SBAS) method and Sentinel-1B synthetic aperture radar data to obtain the cumulative surface deformation at highly coherent points and different times(12-d intervals from September 10, 2017 to June 1, 2018)following the sixth nuclear explosion conducted by the Democratic People’s Republic of Korea(DPRK). The study is conducted for a 17 km×22 km area centered on the explosion. Measurement points are aggregated into 14 sets according to their spatial neighborhood. According to the average coherence of each point in the set, the point deformation is weighted and averaged to obtain the cumulative deformation of each set. The location of each set is also weighted and averaged according to the average coherence of each point in standing for all points in the set. The analysis and discussion in this paper are based on these 14 sets for 14 different regions. Results show that the deformation process in the thermal radiation aftereffect stage of the sixth nuclear test can be effectively observed using interferometric synthetic aperture radar. There was still surface uplift near the epicenter for ~10 d after the explosion, after which the surface began to sink. The sinking rate and total sinking amount varied by location. Meanwhile, the phenomenon of subsidence slowing or even the surface uplifting possibly due to the freeze–thaw cycle of water in underground rock in winter was observed. After May 24, 2018,deformation began to rise because the government of the DPRK bombed the entrance of the nuclear facilities. We indirectly demonstrate the reasonableness and consistency of the observation results via the coherence of high-resolution optical images, meteorological data, and interferograms for the area of the nuclear explosion. The spatial and temporal distributions of surface deformation and their causes are then modeled. The results of modeling analysis are as follows.(1)In the thermal radiation aftereffect stage of the DPRK’s sixth nuclear explosion, the surrounding rock softened under high temperature and high pressure, and the surrounding metamorphic rock then compressed under the action of its own gravity and began to sink. This time-varying process is fitted by a deformation prediction model based on the Weibull function, and four parameters-namely the acceleration factor of deformation, comprehensive influence factor of deformation, initial surface deformation, and prediction value of maximum deformation-are obtained. The average correlation coefficient of the fitting curve is about 0.97. Model fitting results show that the sinking deformation tended to stop around May 20.(2) A function model was proposed to analyze the genetic mechanism of surface deformation, where the thickness of the layer of metamorphic rock is taken as the independent variable and the maximum deformation is taken as the dependent variable.The thickness of the layer of metamorphic rock was calculated for explosion burial depths of 450 and 770 m. The vertical impact distance of the explosion from the epicenter was about 1800–2300 m while the deformation coefficient of the metamorphic rock was about 7×10^-5–8×10^-5. The statistical fitting degree R2 is about 0.8 and the P value is close to zero. In conclusion, the scope of the impact of the explosion metamorphism is the intrinsic condition of deformation, and gravity is the driving force of surface subsidence in the explosion area. Differences in the deformation rate and amount in different geomorphological locations are due to different thicknesses of the compressible layer relating to the rock thickness and metamorphism. Such differences have directionality, possibly because the attenuation of high-temperature and highpressure propagation in the direction of tunnel works is less than that in the direction of surrounding rock.