## 1 Introduction
### Integral Use of InSAR, GPR, and ERT Methods  
Despite the evident complementarity of Interferometric Synthetic Aperture Radar (InSAR), Ground-Penetrating Radar (GPR), and Electrical Resistivity Tomography (ERT), their integrated application remains relatively uncommon. However, recent studies have demonstrated their potential in various geophysical and geotechnical investigations, particularly subsidence monitoring, karst characterization, groundwater dynamics, and landslide hazard assessment. A study in Galicia, Spain [21], applied InSAR and GPR to monitor subsidence in a coastal urban area between January 2020 and March 2022. The research compared two InSAR techniques--Persistent Scatterer Interferometry (PSI) and Small Baseline Subsets (SBASs)--which detected average deformation velocities of \(-3.0\) mm/yr and \(-4.1\) mm/yr, respectively. Complementary GPR surveys in January 2022 validated the InSAR findings, providing detailed subsurface information crucial for preventive maintenance planning. This study underscores the effectiveness of combining remote sensing and ground-based methods to enhance urban subsidence detection and mitigation efforts. Another study [22] demonstrated the effectiveness of integrating PSInSAR, GPR, and ERT to assess sinkhole activity in vegetated, covered karst environments. Conducted in the Sandhill Reservation of West Central Florida, the research identified a 60 m-diameter irregular depression, with subsidence rates of 1 mm/yr along its flanks, while the center remained stable. GPR and ERT revealed two key subsurface horizons--H1, associated with the water table, and H2, representing a clay-rich layer--highlighting the complexity of sediment structures influencing surface deformations. Additionally, the study emphasized the crucial role of corner reflectors in improving PSInSAR monitoring in vegetated areas, addressing one of the primary challenges in subsidence detection. This work reinforces the importance of a multi-method approach for accurate sinkhole monitoring and risk assessment in karst terrains. In Quebec City, Canada, an integrated hydrogeological, geophysical, and geodetic study [23] combined tracer tests, GPR, ERT, and InSAR to investigate groundwater flow dynamics and cave structures in an urban karst setting. Tracer tests confirmed a direct flow path from a sinking stream to a spring, while GPR and ERT detected voids near the cave pathway, indicating potential instability. InSAR analysis further revealed structural movements in buildings located over a backfilled sinkhole, emphasizing the necessity of continuous geodetic monitoring for urban safety. This research highlights the value of combining multiple techniques to enhance the detection and characterization of subsurface voids in densely populated karst environments, with implications for urban planning and risk mitigation. A broader review [24] explored the application of multidisciplinary approaches to assess and mitigate loess landslide hazards, focusing on studies from 1980 to 2023, particularly in China. The research identified key factors influencing landslide stability, such as internal structure, water content, shear strength, and the impact of irrigation and rainfall. It emphasized that improper irrigation can exacerbate instability, necessitating improved water management strategies. The study incorporated various geophysical and geotechnical methods, including seismic refraction tomography, ERT, GPR, GNSS, InSAR, compression and shear tests, and numerical modeling techniques such as limit equilibrium analysis and finite element modeling. By integrating these methodologies, the research proposed a more comprehensive framework for landslide monitoring and prediction, addressing challenges in data processing and calling for continued advancements in monitoring tools. These studies emphasize integrating InSAR, GPR, and ERT with other complementary techniques to improve the detection and characterization of subsidence, sinkholes, groundwater flow, and landslide hazards.