Mitigating Surface Influence Challenges in Continuous CO2 Monitoring Using the Borehole Portable Active Seismic Source (B-PASS)

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 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 with shallow formations consisting of highly attenuating beach sand. Using stacked repeatable signals, we achieved effective noise reduction and enhanced seismic data quality. Two source configurations—vertical and horizontal motion—were tested, demonstrating significant signal propagation distances of approximately 450 meters in highly attenuating layers. Deeper source deployment proved advantageous, reducing noise interference in body waves and enhancing the signal-to-noise ratio (S/N ratio) by up to 5 times compared to surface source deployment. A 19-hour stability test involving 2,250 shots demonstrated consistent waveform reliability, with stacking providing an hourly temporal resolution. Environmental noise significantly influenced signal propagation, and midnight was identified as the optimal operational period due to reduced 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 clarify the optimal stacking number, furthermore, we constructed the relationship among the stacked number, offset (source-receiver distance), and S/N ratio. These findings suggest that the B-PASS system offers a reliable solution for continuous seismic monitoring even in attenuated and environmentally influcenced geological layers by considering the optimal monitoring time and staking number.