oa Numerical Experiments on Seafloor-deployed DAS with Helical Fibers for CO2 Sequestration Monitoring with Time-lapse Elastic FWI

Distributed acoustic sensing (DAS) with optical fiber cables has actual and potential applications, including energy applications, because it can densely measure the vibrations along the optical fiber compared with conventional receivers. For example, we already utilize DAS measurement for subsurface seismic monitoring by installing an optical fiber cable in a borehole. DAS with optical fibers deployed at the seafloor has also the potential for CO2 sequestration monitoring. However, seafloor or surface-deployed DAS for seismic acquisition still has a technical challenge for the directional sensitivity of fibers. DAS with a straight fiber cable does not effectively detect the vertical component of seismic waves, such as reflected P-waves. A method to address this challenge is helically wound fiber (HWF), which theoretically measures the summation of the vertical and horizontal components of vibration. However, there is no report on deploying HWFs for DAS measurement at the seafloor. Therefore, this study investigates the validity of seafloor-deployed DAS with HWFs for CO2 sequestration monitoring using time-lapse elastic full waveform inversion (FWI) for 4D seismic data. We show numerical examples to evaluate the relation between the accuracy of the time-lapse change estimated by FWI and wrapping angles of HWFs in seafloor-deployed DAS.