Electrical Resistivity Tomography (ERT) has proven to be a highly sensitive geophysical method for characterizing the dynamics of seawater intrusion. This study uses tank experiments to simulate seawater intrusion, utilizing electrical resistivity tomography to monitor real-time changes in groundwater resistivity during the intrusion process. The objective is to quantitatively reveal the development and evolution mechanisms of seawater intrusion wedges in sandy aquifers, thereby establishing a real-time resistivity monitoring method for groundwater distribution and migration characteristics. This study improves resistivity imaging data processing methods, enhancing both efficiency and accuracy. The refined cross-hole ERT technique is applicable not only to meter-scale indoor experiments; its optimized forward and inverse algorithms can also be directly transferred to regional-scale field monitoring. Experimental results show that the average resistivity in the study area continuously decreases from 57 Ω·m in the initial freshwater state to 1.1 Ω·m at the intrusion stabilization point. Areas with resistivity values below 20 Ω·m corresponded exactly to the brine intrusion zone. The evolution of the freshwater-saltwater interface unfolded in three stages: Initially, the density difference (0.025 g/cm³) dominated, with the saltwater intrusion depth at the aquifer base reaching 0.45 m, significantly exceeding the 0.04 m penetration at the upper section. During the intermediate stage, the interface morphology differentiated into an “upper triangular, lower arc-shaped” configuration. The bottom intrusion distance increased to 1.65 m, and the thickness of the brackish-freshwater mixing zone expanded from 0.1 m to 0.3 m. In the final stage, the interface stabilized and began intruding toward the surface, establishing a new hydrodynamic equilibrium. In addition, the migration rate of saline water at the aquifer base gradually decreased from 6.25 × 10⁻⁴ cm/s initially to 1.16 × 10⁻⁵ cm/s at steady state. These results reflect the dynamic coupling process between seepage and dispersion and demonstrate that this method enables effective real-time monitoring of seawater intrusion development and conditions, as well as early warning capabilities.

With the accelerated advancement of smart city construction, the development of urban underground space in coastal areas faces significant safety challenges due to randomly distributed granite boulders. Conventional geophysical methods often struggle to identify these heterogeneous bodies efficiently. This study proposes a comprehensive detection approach fusing the Cone-based Transient Electromagnetic Method (CTEM) and Microtremor Array Surveying. Taking the eastern coast of Jiaozhou Bay, Qingdao, as the research area, we integrated borehole data to validate the geophysical interpretation. The results demonstrate that the joint inversion accurately delineates four stratigraphic interfaces with depths consistent with borehole logs: the artificial fill (bottom at ~8–12 m), Quaternary sediments (~28–33 m), strongly weathered granite (~59–74 m), and moderately weathered granite. Specifically, boulders within the Quaternary and strongly weathered layers are distinctly identified by dual high-value anomalies: high apparent resistivity (>220 Ωm, approximately 1.5–2 times that of the surrounding rock) and high shear-wave velocity (>660 m/s). Furthermore, the method effectively differentiates boulders from water-rich fractured zones, which exhibit contrasting low resistivity (<50 Ωm) and low shear-wave velocity (<420 m/s). These quantitative findings confirm that the fused CTEM and microtremor technique provides precise spatial localization and reliable identification of boulders in complex coastal geological environments.