Acoustic velocity of near-surface dolomitic limestones and their subsurface seismic imaging implications

Carbonate rocks were recognized as highly heterogeneous, and the process of dolomitization was known to introduce further intricacies to the complex characteristics of near-surface carbonate strata. However, studies that investigated the geophysical aspects of near-surface dolomitic strata and their implications for subsurface seismic imaging were historically uncommon, despite their significance. The role of near-surface dolomites as wavefield complexity in subsurface seismic reflection images was investigated. Two datasets were utilized: a Late Jurassic outcrop featuring thinly bedded dolomite strata amalgamated with argillaceous limestone, and a near-surface borehole where dolomites were interbedded with Paleocene to Miocene clastics. Two-dimensional finite-difference acoustic wave propagation forward modeling and reverse time migration (RTM) were performed using velocity models constructed from geological observations of the outcrop and borehole. The results demonstrated that the collective arrangement of near-surface dolomitic strata with non-dolomitized argillaceous limestones and clastics acted as “screening” layers that generated seismic wave reverberation. This phenomenon prevented primary seismic energy from propagating effectively to deeper reflectors. An increase in the bed count and thickness of the near-surface dolomite layers was found to increase the complexity of the associated multiples in the simulated seismic data. The failure to account for these effects degraded the quality of the RTM image and could lead to a completely inaccurate subsurface interpretation.