It is currently challenging to accurately detect overburden cavities ahead of shield tunneling in subway tunnels. Given the shallow depth of subway tunnels, this study investigates the principle of super-ahead detection of overburden cavities in shield tunnels using small coil transient electromagnetic detection technology, as well as the transient electromagnetic anomaly characteristics. Numerical simulation studies show that when the transient electromagnetic coil is located directly above the cavity, an area resembling a "butterfly" is generated in the electric field response. The closer the distance to the cavity, the greater the electric field response influenced by the cavity. Due to the rapid attenuation of the transient electric field in the cavity, differences exist in the apparent resistivity responses, allowing for the identification of underground cavities. Through engineering practice, the transient electromagnetic anomaly characteristics of dry cavities mainly manifest as isolated, discontinuous, and easily closed-loop electromagnetic curves, with high resistivity anomalies in apparent resistivity data. Wet cavities exhibit transient electromagnetic anomaly characteristics with some continuity in the electromagnetic curves, often showing convex or concave features, and low resistivity anomalies in apparent resistivity data. Ground transient electromagnetic methods can effectively identify shallow underground cavities.