Addressing Karst-Related Challenges in Seismic Imaging - A Case Study from Onshore Abu Dhabi

Abstract The karsted UER and Simsima formations in Abu Dhabi represent a major challenge for seismic imaging as well as for drilling. The heterogeneity of the eroded limestone results in strong lateral velocity variations that distort the deeper seismic image. The conventional approach using pre-stack time migration (PSTM) and pre-stack depth migration (PSDM) results in a strong reflection at the top of the UER, with poor imaging quality in the section below. This paper outlines the challenges of this onshore dataset and proposes a new workflow using multi-wave inversion (MWI), TLFWI, wave equation deconvolution, Q-tomography and QPSDM, which helps to better detect the karsts bodies and improve the image quality below UER. The dataset was acquired and initially processed in 2021. The data were of high quality for most of the survey but deteriorated in the north-east, due to thickening of Miocene evaporites in the near surface and the presence of karsting, which poses challenges for seismic imaging both onshore and offshore Abu Dhabi. A re-processing project was launched in 2023 with the aim to enhance the image quality in the area affected by the thick evaporites and karsting. An advanced velocity model building flow was used. The combination of multi-wave inversion for the near surface velocity model and 10Hz TLFWI produced a high-resolution velocity model that accurately imaged the complex overburden and the fast velocities associated with the karsts. The application of wave equation deconvolution, alongside standard deconvolution techniques, reduced the presence of high frequency ringing from the shallow evaporite sequence. Q-tomography was able to identify Q anomalies, with interval Q values as low as 20 within the karsts. The final QPSDM image shows improved amplitude fidelity, frequency content, and imaging below the karsts when compared with the legacy data. By the end, we demonstrate that this combination of high-end approaches, i.e. TLFWI and wave equation deconvolution, solves the encountered challenges, which helps to improve geological understanding and drilling plans as well as to open more opportunities for discoveries in the sections hiding below the karsts.