Cross-hole Radar Attenuation Inversion Based on Structural Coupling

Cross-hole radar is a high-resolution geophysical technique for subsurface characterization. In practical engineering, velocity tomography alone is often insufficient to distinguish different types of geological defects or identify critical features such as water or clay fillings, whereas attenuation is more sensitive to moisture, conductive materials, and fracture fillings. However, attenuation inversion is generally less stable than velocity tomography in strongly heterogeneous media because the measured amplitudes are affected by geometric spreading, scattering, antenna response, and inversion non-uniqueness. To address these issues, this study proposes a sequential velocity-guided attenuation inversion framework for cross-hole radar. Curved-ray traveltime tomography is first used to obtain a stable velocity structure and propagation paths. The gradient of the converged velocity model is then transformed into spatially varying regularization weights for attenuation inversion, allowing the velocity information to act as a soft structural prior rather than being directly imposed on the attenuation model. First-arrival times are picked using the First-arrival Picking through Mathematical Morphology and Edge detection (FPME) method, followed by direct-wave amplitude extraction within a first-arrival-centered time window. Synthetic experiments show that the proposed structural constraint suppresses background artifacts and improves the spatial delineation of attenuation anomalies, particularly in heterogeneous models. Sensitivity and spatially decoupled tests further indicate that moderate structural coupling improves inversion stability while retaining attenuation-data sensitivity. Field application in a karst area of Hechi, Guangxi, shows clearer attenuation anomaly boundaries and reasonable correspondence with the clay-silt overburden and cave- or clay-filled intervals identified from borehole cores. These results demonstrate that velocity-guided attenuation tomography can provide complementary information to velocity imaging for interpreting complex karst structures.