## 3 Research Method
### Model Development
#### 3.2.3 Monitoring Scenario 3: Estimation of Spoil Volume Changes  
To ensure the stability of SDAs and prevent potential soil erosion, water contamination, and landslides, dynamic monitoring of the spoils is essential. This study utilizes the differential interferometric synthetic aperture radar (D-InSAR) method to focus on monitoring the elevation and volume changes of spoil, which involves five key steps: imagery coregistration and cropping, interferogram pairing and baseline estimation, interferogram formation and phase unwrapping, surface deformation calculation, and dynamic estimation of spoil volume changes. D-InSAR utilizes two interferometric SAR imagery of the same region acquired at different times. The first interferogram is generated from SAR imagery captured before and after deformation, containing only topographic information. The second interferogram, similarly derived from SAR imagery taken before and after deformation, includes both topographic phase information and additional phase variations resulting from surface deformation. By performing differential processing--specifically, subtracting the first interferogram from the second interferogram--D-InSAR effectively eliminates the topographic phase component. This subtraction operation isolates the phase changes solely attributable to surface deformation, thereby enabling the precise measurement of subtle ground movements. Step 1: Imagery coregistration and cropping  
Coregistering Sentinel-1 SAR imagery from different time periods to ensure spatial consistency is a crucial step for subsequent analysis. The coregistration process is completed by setting the master and slave imagery, subswath, satellite orbit files used, and auxiliary digital elevation model data. Additionally, to enhance processing speed and efficiency, the relevant swath and burst parameters are extracted based on the latitude and longitude data of the study area, and the appropriate imagery range is cropped accordingly. Step 2: Interferogram pairing and baseline estimation  
The master and slave imagery for both ascending and descending orbit experiments are selected. The multilook factors to the correct ratio and perform multilook processing on the interferometric pairs are set. Based on the resolution principles, the time baseline threshold and spatial baseline threshold for both the ascending and descending orbit experiments are set, ultimately forming multiple interferometric pairs. Additionally, baseline estimation to determine whether the baseline of the interferometric pairs is below the critical value and whether good interferometric results can be achieved is performed. Step 3: Interferogram formation and phase unwrapping  
Interferometric processing on the imagery is performed based on the spatio-temporal baseline distribution formed by the interferometric pairs to generate differential interferograms, removing terrain phase effects using external digital elevation model data. Then, filtering and enhancement are applied to the differential interferograms, and the minimum cost flow method for phase unwrapping is used to obtain the true phase values in the study area. By reviewing each generated coherence map, filtered interferogram, and unwrapped phase map, interferometric pairs with poor quality are manually removed. Step 4: Surface deformation calculation  
A model is constructed on pixels with high coherence, using singular value decomposition to invert the deformation rate estimates, and residual terrain influences. Then, phase residuals are calculated and residual separation is performed. This process ultimately provides the surface subsidence and uplift data within the study area. Step 5: Dynamic estimation of spoil volume changes  
Based on the concept of mathematical segmentation, SDA in HSRs can be decomposed into irregular shapes composed of multiple rectangular prisms with square bases.