Compressional and shear wave velocity structure of shallow sediments in the north-western Black Sea revealed by OBS data

The Danube deep-sea fan complex in the north-western Black Sea, with its ancient channel-levee systems, hosts multiple bottom-simulating reflections (BSRs) that have been observedin high-resolution reflection seismic data. The multiple BSRs indicate the presence of gas hydrates and free gas. To image the distribution of free gas and gas hydrates on the western flank of the S2 canyon, simultaneously, ocean bottom seismometer (OBS) data and 2-D high-resolution multichannel reflection seismic (HRMRS) data were acquired during the R/V Maria S. Merian (MSM-34) expedition. Along two parallel HRMRS-OBS profiles we recorded the wave field for a wide range of incidence angles. The velocity-structure models for both, P-and S-wave traveltimes, cover a depth down to 1.2 km, providing seismic velocity information below the BSR. For identification of the P-wave phases from OBS to OBS, we aligned the OBS data at zero offset to the 2-D HRMS data at each OBS location. The P-wave velocities show a gradual increase with depth from 1510 m/s directly beneath the seafloor up to 1900 m/s at 1.2 km depth. As the S-waves travel at slower speed than P-waves, S-waves reflection could be traced only in a smallsource-receiver offset with a maximum of ~0.9 km. We assume the reflection horizon to be the point of P-to-S conversion. Seismic S-wave velocities increase from 140 m/s beneath the seafloor up to 860 m/s at 1.2 km depth. These observations allow the determination of the P-to-S-ratio that decreases from 10.6 beneath the seafloor down to 2.2 at 1.2 km depth. The seismic velocities and P-to-S-ratio exclude the presence of gas hydrates above the BSR, but endorse the accumulation of a low concentration of free gas below. The distribution of the gas is predominately controlled by lithology