Determination of Marine Fiber-Optic Cable Position and Acquisition Time Shift using Direct Wave Traveltime Inversion

Summary In Distributed Acoustic Sensing (DAS), a fiber-optic cable is used as a distributed seismic sensor, with channels representing successive short sections of the fiber, spaced at defined intervals along the 1D fiber axis. Typically, the positions of these channels are assumed to be a line projection along the cable's position. In reality, a fiber-optic cable contains many fibers that are not perfectly straight and are thus longer than the cable itself. Consequently, the real channel positions may not correspond to a simple interpolation along the cable axis. Moreover, the precise cable coordinates are often sensitive information and may not be provided to the end user who uses the cable for sensing applications. On land, a tap test is usually carried out before the start of a DAS acquisition to determine the exact channel locations. DAS with marine horizontal cables has recently been used for various offshore applications, including seismic imaging. To avoid errors in the seismic image, a precise receiver location is required. In this paper, we propose a traveltime-based inversion workflow to determine a more accurate channel position on the seafloor. Moreover, we show that we can resolve an unknown time shift between the acquisition and the recording system, in addition to the fiber position.