Mitigating surface noise and attenuation in continuous monitoring of geologically stored CO₂ using a borehole...

We investigate strategies to mitigate the impact of environmental noise and surface influences in long-term monitoring of subsurface reservoirs using the borehole-deployable Portable Active Seismic Source (B-PASS). Unlike traditional surface seismic sources, the B-PASS system minimizes the impact of surface environmental factors such as temperature fluctuations and groundwater variations. This capability makes it particularly effective for long-term applications, especially CO₂ storage monitoring. Field tests of B-PASS were conducted in a gas field underlain by a shallow, highly attenuating beach sand. This study tested two source configurations: vertical motion and horizontal motion. By stacking the repeatable signals from B-PASS, we achieved effective noise reduction and enhanced the quality of the seismic signal. The source remained deployed in the borehole for 10 days; stability was quantitatively assessed over 7 days comprising approximately 8,000 sweeps., with stacking providing an hourly temporal resolution. The results demonstrated significant signal propagation to the farthest receiver, located approximately 450 m away, even through a highly attenuating layer. Deeper source deployment proved advantageous, enhancing the signal-to-noise (S/N) ratio for P-waves by up to five times compared to a surface source. The field experiments further demonstrated that environmental noise significantly influenced signal propagation, and midnight was identified as the optimal operational period due to less ambient noise. Therefore, by considering the daily temporal variation of the S/N ratio of the field before the long-term monitoring, we can identify the optimal operation time of the monitoring. To determine the optimal number of signal stacks, we developed a method to evaluate how the S/N ratio varies with stacking number and offset (source-receiver distance). Based on these results, the B-PASS demonstrates its potential as a reliable solution for continuous monitoring, even in geologically challenging environments with surface attenuation and noise, by optimizing the daily monitoring schedule and the number of signal stacks.