Comparison of straight and helical fibers for time-lapse elastic FWI of synthetic seafloor DAS data

Abstract In recent years, optical fiber sensing has been widely employed in various fields, including geophysics. Distributed acoustic sensing (DAS) with optical fibers can measure vibrations along optical fibers more densely than geophones or hydrophones. Applications of borehole-deployed DAS as well as surface- and seafloor-deployed DAS for seismic measurements are increasing. Velocity model building plays a significant role in such seismic applications of DAS as imaging and time-lapse monitoring. Seafloor or surface DAS measurements with a straight fiber (SF) are limited to measuring one (horizontal) component of vibrations, which poses limitations for velocity estimation. This shortcoming of SF can be addressed by using helically wound fiber (HWF) (a fiber wrapped around a core in a helical or spiral pattern) to potentially detect both the vertical and horizontal vibration components. However, a comparison of velocity estimation from seismic data measured by SF and HWF has not been discussed. This study analyzes the performance of elastic full-waveform inversion (FWI) (an iterative method for parameter estimation using multicomponent wavefields) for synthetic seafloor-deployed DAS data recorded by SF and HWF. We conduct numerical experiments using elastic FWI and time-lapse elastic FWI on a synthetic subsurface model data set to evaluate the potential of seafloor-deployed DAS measurements with SF and HWF.