Integrated ERT and Microtremor (SPAC) Survey for Shallow Karst Detection in a Noisy Corridor: Drilling Verification and Risk Zoning

Concealed shallow karst along long-distance pipeline corridors can trigger subsidence, uneven settlement, and leakage, creating environmental and infrastructure hazards. In the Huyuanxi area (Fuyang District, Hangzhou, Zhejiang, China), strong electromagnetic interference and limited site access motivated an integrated electrical resistivity tomography (ERT) plus ambient-noise microtremor (SPAC) workflow for shallow-karst screening. Three ERT lines (900 m each) were deployed along the pipeline axis and at ±15 m offsets with 10 m spacing using a WDJD-4 system (100 V constant-voltage; Wenner array, 30 layers), followed by resistivity inversion; Res2Dinv v3.65 was adopted as the inversion software. The L2 norm was selected for the objective function, and the error model was set to the default error floor plus 5%. The regularization parameter was set as λ = 0.01, and adaptive gridding was used for the mesh with a minimum cell size of 0.5 m × 0.5 m. The number of iterations was set to 15, with a final root mean square (RMS) misfit of 3.2%. The depth of investigation (DOI) was calculated via the built-in algorithm of the software, yielding a maximum value of 30 m. Low-resistivity anomalies were used to focus eight perpendicular microtremor profiles (3 m spacing) acquired with SmartSolo IGU-16HR 1C and 10 geophones (5 Hz; 1 ms sampling interval) in a nested SPAC array (0.5/1/2 m radii); processing removed segments with SNR < 3 and inverted 2-D Vs structure by damped least-squares. Resistivity sections show 50–8600 Ω·m near surface, including a <100 Ω·m fracture-zone anomaly (28–30 m wide; 8–18 m depth) and a cavity-zone anomaly (55–70 m wide; 10–20 m depth). Joint interpretation places karst development mainly at 10–25 m depth near the bedrock–cover interface (~16 m). At HYXK3, microtremor versus shear-wave logging yielded a void-layer bottom depth of 21.28 m versus 20.12 m (5.76% error) and Vs of 457 versus 446 m/s (2.40% error). Example profiles show microtremor-derived depths (18.3/14.5/18.7 m) consistent with ERT (16.8/15.1/19.5 m; 4.1–8.1% errors). Drilling verification accuracy was approximately 81.7%, precision approximately 90%, recall approximately 74.0%, and the F1-score 81.1% supporting practical corridor risk screening under complex field constraints.