Imaging a near-vertical structure with seismic refraction tomography: an offshore study

Seismic refraction tomography (SRT) can be a useful tool for detecting lateral velocity changes in the subsurface. In this work, we leverage a traditional seismic reflection offshore dataset to image a high velocity structure imbedded in sediments. The studied profile is located in the western edge of the South Orkney Microcontinent (SOM). Seafloor depth in this sector ranges between 268 and 304 m. The multichannel streamer of 120 channels and 1.4 km length allowed to detect refraction arrivals from the subsurface and used them as input for SRT to reconstruct a P-wave velocity model of the near–structure. A layered initial model provided the best result retrieving a sharp image of the velocity anomaly. Interpretation of this anomaly was supported by the comparison between the SRT velocity model and the multichannel seismic reflection image. Shallow subparallel layers of sediments characterized by velocities between 1500 and 2000 m/s overly a zone with older deformed sedimentary layers related to velocities ranging between 2500 and 3000 m/s. Within this second layer, a high-velocity anomaly (more than 4000 m/s) is delineated. Geological context supports interpreting this anomalous structure as metamorphic rocks or gabbroic bodies from PMA (Pacific Margin Anomaly) as part of the basement.