With the increasing development of underground spaces adjacent to urban levees, contact seepage frequently occurs at the interface between the soil and underground structures. However, traditional geophysical detection methods are often rendered ineffective in such environments due to spatial restrictions and detection blind spots. To address these challenges, this paper proposes a vertical-array lateral scanning detection method. This approach utilizes electrical resistivity tomography (ERT) with flat-base electrodes and ground-penetrating radar (GPR) to acquire data directly from vertical wall surfaces. The feasibility of this method is validated through numerical simulations and field data. The results indicate that the proposed method effectively overcomes the high-resistance shielding effect of hardened walls and clearly reveals the electrical structure of the soil behind the wall. Specifically, the contact seepage zone manifests as a layered low-resistivity feature immediately adjacent to the wall, while the penetrating leakage channel presents as a continuous low-resistivity anomaly extending from the contact interface deep into the levee body. These findings confirm the applicability of this technology for the qualitative identification and effective detection of hazards in complex, space-restricted urban environments.