Time-lapse seismic analysis over basaltic rocks at Skoll High (Vøring Basin) to support CO₂ storage monitoring strategies

Basaltic rocks host abundant pore space and reactive mineral phases that make them attractive targets for CO₂ storage via mineral trapping, yet their seismic response presents significant challenges for monitoring due to strong scattering and wavefield complexity. Within the framework of the PERBAS project (Permanent sequestration of gigatons of CO2 in continental margin basalt deposits), we assess the potential for high-resolution time-lapse (4D) seismic methods to detect subsurface changes in a basalt-dominated offshore setting and to inform efficient monitoring strategies applicable to future CO₂ storage operations.

We analyze two P-Cable high-resolution 3D seismic volumes acquired over Skoll High in the Vøring Basin (mid-Norwegian margin), a 2022 baseline and a 2024 repeat survey that reoccupied 17 of 26 original sail lines. Both datasets used consistent acquisition parameters and were processed with a uniform workflow incorporating geometry assignment, noise attenuation, amplitude correction, and high-resolution binning (6.25 × 6.25 m). A key objective was to maximize seismic repeatability as a prerequisite for robust 4D analysis in basaltic terrain.

Time-lapse calibration included cross-correlation for phase and time shifts, shaping filters, cross-correlation statics, and amplitude cross-normalization. Different calibration windows were defined above the top basalt and including the top basalt sequence in order to analyze  the impact of non-repeatable signal variability within the volcanic complex. The normalized root-mean-square (NRMS) metric was used to quantify repeatability across the repeat survey area.

Our initial results indicate that high spatial repeatability is attainable with P-Cable 3D seismic data in a basalt setting when acquisition and processing are carefully controlled, though areas of complex basalt morphology and structural heterogeneity exhibit higher NRMS values. Difference volumes highlight regions of anomalous repeatability that correlate with geological features. These findings underscore both the promise and limitations of time-lapse seismic monitoring in basaltic reservoirs and contribute to establishing realistic detection thresholds and optimized survey designs for CO₂ storage monitoring.

This study expands the understanding of seismic repeatability in volcanic margin environments and provides groundwork for advancing cost-effective monitoring strategies in basalt-hosted storage projects.