Abstract <p>This study investigates the variations in the electromagnetic properties of asphalt pavement under multimedia conditions, with the aim of elucidating how complex internal pavement structures affect the detection performance of three-dimensional ground-penetrating radar (3D GPR). By integrating 3D GPR with forward modeling using gprMax, the research establishes road models incorporating cracks and voids across a range of working conditions. The methodology involves: constructing detailed, layered random heterogeneous media models with systematic configuration of material parameters and defect characteristics; performing electromagnetic wave propagation simulations under diverse scenarios with gprMax to generate GPR response images; and conducting integrated waveform and time-frequency analyses to comprehensively extract the influence patterns of multiple media, various defects, and interfering structures on radar signals. The results reveal that while cracks and voids exhibit distinct reflection patterns, their GPR responses are substantially affected by the properties of the materials and the surrounding environment. Strongly reflective structures complicate signal interpretation and can obscure defect signatures. Time-frequency analysis highlights energy concentrations and frequency shifts associated with different defects. Field validation via core sampling demonstrates strong concordance between simulated and actual data, thereby confirming the reliability of the modeling approach. This work offers theoretical support and practical guidance for more accurate and efficient GPR-based road defect detection in complex subsurface environments.</p>