Numerical Estimation of Position and Flux of Naturally Seeping CO2 in Kagoshima Bay Using AUV Observation Data

Abstract

To mitigate global warming, the large volume of CO2 released into the atmosphere must be reduced. CO2 capture and storage (CCS) is one of the promising means for this purpose. In Japan, CCS operators who inject and store CO2 into sub-seabed geological formations are legally required to monitor CO2 leakage in seawater, even though such probability is extremely low; if unusually high CO2 concentration is detected, specifying the position and flux of the associated CO2 seepage is necessary for social responsibility. In this study, we adopted the adjoint marginal sensitivity method to estimate the position and flux of the CO2 naturally seeping on the hillside of a sea knoll on the east of Wakamiko Caldera in Kagoshima Bay, using the pH data observed by an AUV. The tidal currents were time-forwardly simulated and stored to use negatively in the time-backward calculation of the adjoint marginal sensitivity. The result indicated a seepage position on the east hillside of the knoll and a plausible seepage flux. Another observation of the vertical pH distribution implied that there could be two seepage positions at short distances on the same hillside, and the estimated one was in the middle of them. It is then inferred that this method can be applied for the source detection of various marine pollutants and resources.