Holographic subsurface radar for near‐surface geophysics: State‐of‐the‐art, critical comparison with conventional GPR and open challenges

Near‐surface geophysics and non‐destructive testing have an urgent demand for high‐resolution, easy‐to‐interpret subsurface detection techniques. As the mainstream technology in this field, ground‐penetrating radar (GPR) has been widely commercialized and applied, yet it often suffers from insufficient azimuth resolution for shallow small targets and produces complex B‐scan profiles that lack intuitive on‐site interpretability. Holographic subsurface radar (HSR), as an emerging branch of surface‐penetrating radar technology, has attracted increasing attention for its unique advantages in these scenarios, with a relatively lower hardware cost and higher azimuth resolution. This review focuses on HSR with GPR data used as a practical engineering benchmark. It systematically traces the full development history of HSR from pioneering work to the latest commercial and research‐grade systems, elaborates on the fundamental imaging principle, resolution characteristics and mainstream imaging algorithms and analyses the adaptability of conventional GPR algorithms to HSR systems. Unique clutter characteristics of HSR are clarified, and state‐of‐the‐art clutter suppression technologies are summarized along with their field limitations. A full‐dimensional quantitative comparison between HSR and conventional GPR defines the clear application boundaries, differentiated advantages and core engineering bottlenecks of HSR in near‐surface geophysics. Open questions and future research directions are outlined to provide a systematic, practice‐oriented reference for GPR researchers, equipment manufacturers and frontline engineering practitioners.