A Cost-Effective and Efficient Geological Safety Survey Method for Early-Stage Site Screening of Carbon Sequestration: A Case Study of the Lishui Sag

Marine geological carbon sequestration (GCS) is crucial for achieving carbon neutrality, with site geological safety being paramount. To address the high cost of extensive multi-channel seismic (MCS) surveys during the early site screening stage, especially for those underexplored basins, this paper proposes an integrated and cost-effective exploration method that combines gravity, magnetic, and sub-bottom profiling (SBP) surveys. This method enables efficient areal scanning for initial assessment of geological safety during early-stage site screening. The Lishui Sag, which has relatively well-defined structural information, was selected as a test site to validate the effectiveness of this method. The survey results show that the faults identified from the horizontal gradient magnitude (HGM) of gravity and magnetic anomalies exhibit good consistency with the major faults interpreted from MCS profiles, with average horizontal position offsets of approximately 2.3 km and 2.4 km, and average strike deviations of about 5.7° and 7.2°, respectively. While SBP detection effectively reveals shallow geohazards such as shallow gases. By leveraging the complementary strengths of geophysical data, this method can reliably identify key geological risk factors of carbon sequestration (e.g., faults, magmatic intrusions, shallow gases) during the early-stage site screening phase (regional scale) while significantly reducing survey costs. Although this method is not intended to provide a comprehensive geological safety assessment for final CO2 storage site approval, it offers an integrated survey approach that balances reliability and cost-effectiveness for early-stage, regional-scale risk assessment in site screening.