Abstract: The development of urban underground spaces imposes increasingly stringent requirements on the fine characterization of shallow geological bodies and on construction risk control. Constrained by roads, underground pipelines, traffic systems, and existing structures, traditional linear, L-shaped, T-shaped, and circular arrays often suffer from limited layout flexibility, insufficient azimuth coverage, and poor continuity of three-dimensional imaging under complex urban surface conditions. To address the challenge of accurately detecting the spatial distribution of stone strip layers during shield tunneling beneath ancient seawall relics, this study adopts a three-line array as the basic observation unit to realize dense, random, or semi-random layouts within restricted sites. Multi-node synchronous acquisition technology and the extended spatial autocorrelation method are applied to extract dispersion curves, invert shallow shear-wave velocity structures, and establish a three-dimensional S-wave velocity model of the study area. Field tests and drilling verification were conducted in the ancient seawall section of Hangzhou Metro Line 18. The results demonstrate that three-dimensional microtremor detection based on three-line arrays with dense measuring points offers flexible deployment, minimal construction interference, and superior lateral imaging capability. This approach effectively identifies weak interlayers, inhomogeneous soils, and concealed high-velocity anomalies in complex urban environments, providing reliable technical support for relic protection, shield construction risk assessment, and the detection of concealed geological hazards in urban underground space development.