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 channel are assumed to be known and to coincide with 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. Additionally, the cable may feature some degree of curvature even though it was deployed with the intention of a straight line. Consequently, the real channel spacing differs from the theoretical one and may vary alongside the cable axis. On land usually a tap test is carried out before the start of the 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. For many marine fiber optic cables, the position is known to some extent but may be subject to inaccuracies. In this paper, we propose a traveltime-based inversion workflow to determine a more accurate fiber 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.

Plate tectonics characterize transform faults as conservative plate boundaries where the lithosphere is neither created nor destroyed. In the Atlantic, both transform faults and their inactive traces, fracture zones, are interpreted to be structurally heterogeneous, representing thin, intensely fractured, and hydrothermally altered basaltic crust overlying serpentinized mantle. This view, however, has recently been challenged. Instead, transform zone crust might be magmatically augmented at ridge-transform intersections before becoming a fracture zone. Here, we present constraints on the structure of oceanic crust from seismic refraction and wide-angle data obtained along and across the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean. Most notably, both crust along the fracture zone and away from it shows an almost uniform thickness of 5-6 km, closely resembling normal oceanic crust. Further, a well-defined upper mantle refraction branch supports a normal mantle velocity of 8 km/s along the fracture zone valley. Therefore, the St. Paul fracture zone reflects magmatically accreted crust instead of the anomalous hydrated lithosphere. Little variation in crustal thickness and velocity structure along a 200 km long section across the fracture zone suggests that distance to a transform fault had negligible impact on crustal accretion. Alternatively, it could also indicate that a second phase of magmatic accretion at the proximal ridge-transform intersection overprinted features of starved magma supply occurring along the St. Paul transform fault.