Ray‑tracing-based seismic illumination analysis for subsalt imaging in gas storage exploration: a case study of a salt diapir in central Iran

Accurate seismic imaging beneath complex velocity structures, particularly salt bodies, remains a significant challenge in assessing underground gas storage sites. This study presents a detailed seismic illumination analysis conducted using a robust ray-tracing methodology to assess the feasibility of high-fidelity subsurface imaging of complex salt flanks in a challenging land-based 3D seismic acquisition environment. Both survey-independent and survey-dependent ray-tracing techniques are employed to map the spatial distribution and angular content of seismic energy reaching potential target reflectors located beneath and adjacent to the salt body. The survey-independent analysis provides an intrinsic assessment, revealing regions characterized by poor coverage due to ray-path turning and masking effects inherent to the salt geometry. Subsequent survey-dependent analysis quantifies the achievable illumination quality with the proposed acquisition geometry, highlighting critical gaps in both spatial and azimuthal coverage, particularly near steep salt boundaries where wide-angle reflections are essential. The results demonstrate that salt flanks, which could provide the turning or wide-angle rays, exhibit severe illumination deficits, regardless of the migration algorithm employed. Conversely, areas with sufficient illumination coverage (including both steep and wide-angle rays) show a strong correlation with the potential for high-fidelity subsurface imaging. This illumination-based framework confirms that the fundamental limitation in imaging complex salt bodies is the geometric accessibility of the targets by seismic energy, rather than the limitations of imaging algorithms alone. The findings provide critical constraints for assessing the inherent resolvability of subsalt targets based on wave-propagation physics within given survey constraints, offering a valuable tool for survey design and feasibility studies.