Short-Time DOA-Based 3-D Imaging of Complex Subsurface Targets Using Array-Based Borehole Radar Observations

Abstract

Characterizing the spatial distribution of subsurface geological structures is essential in engineering geology, geophysics, and related fields. Directional borehole radar (DBR), an array-based extension of conventional borehole radar, enables 3-D imaging within a single borehole for subsurface characterization. However, in complex geological environments, subsurface heterogeneity, multitarget coexistence, and strongly coherent scattering often result in severe temporal overlap of echoes, limiting the separability of target responses and degrading the stability and accuracy of conventional 3-D imaging methods. To address this limitation, we propose a short-time direction-of-arrival (ST-DOA)-based 3-D imaging method for DBR. ST-DOA analysis is applied to migrated multichannel radar data to construct a joint time–azimuth representation, which transforms temporally overlapping echoes into separable features in the time–azimuth domain. This enables effective separation of multitarget responses, followed by echo localization and spatial reconstruction to achieve 3-D imaging. We validate the proposed method using synthetic data, laboratory experiments, simulated data, and field measurements. The results demonstrate stable and reliable imaging performance under strong echo overlap and complex scattering conditions, with improved imaging accuracy compared with existing methods. The proposed method improves the imaging capability of DBR, providing new possibilities for accurate mapping of complex subsurface geological structures.