Ground-collapse hazards in the sand-pebble strata of Chengdu are highly concealed, and conventional detection methods are often insufficient for identifying potential cavities and loosened zones. To address this issue, this study investigates an advanced urban geohazard detection approach based on microtremor survey techniques and Rayleigh wave dispersion theory. First, considering the depositional characteristics of alluvial sand-pebble strata in the Chengdu Plain and the mechanisms of ground collapse, the evolution of particle-skeleton degradation and cavity development under groundwater disturbance and engineering activities is analyzed. Second, shear-wave velocity (Vs) profiles are obtained by inverting Rayleigh wave dispersion curves, and a subsurface characterization method using Vs as the controlling parameter is established. Furthermore, a comprehensive identification model for loosened zones and cavities in sand-pebble strata is developed by incorporating multi-source constraints. Finally, typical engineering cases are analyzed and verified by excavation.

The results indicate that: (1) the microtremor method can effectively characterize subsurface shear-wave velocity structures within 0～10 m depth, and low-velocity anomalies correspond well to loosened zones and cavity-developed areas; (2) classification criteria for sand-pebble soil density and ground-collapse risk thresholds applicable to the Chengdu area are proposed; and (3) field validation demonstrates strong consistency between detected low-velocity anomalies and the actual spatial distribution of subsurface cavities, confirming the reliability of the proposed method.