## 5 Conclusions  
In this paper we demonstrated a method for estimating the sound speed at the seafloor, \(c_{0}\), and the sub-bottom velocity gradient, \(K\), at relatively deep (\(\sim\)1000 m) water depths, based on standard commercial 2D seismic-profile data. The method proposed was especially useful for the characterization of low-sound-speed seafloor compositions (silts and clays), suggesting a cost-effective and convenient alternative to the conventional reflection- and refraction-based methods. A geoacoustic travel-time-modeling and inversion procedure is presented here, which allows for the unique, simultaneous estimation of the seafloor and sub-seafloor parameters and the explicit quantification of the uncertainties, subject to the basic modeling assumptions of a constant sound-speed gradient below the seafloor. For the base of the southeastern Mediterranean Sea continental margin, the seafloor sound speed \(c_{0}\) that was estimated at bottom depths of 1030-1250 m, was distributed between 1495 and 1590 m s\({}^{-1}\) with an average of 1537 \(\pm\) 13 m s\({}^{-1}\). \(c_{0}\) that was estimated at bottom depths of 1320-1360 m was distributed between 1595 and 1625 m s\({}^{-1}\) with an average of 1613 \(\pm\) 12 m s\({}^{-1}\). The average sound speed over the whole studied area was 1556 \(\pm\) 13 m s\({}^{-1}\). The estimated \(c_{0}\) values were consistent with the clay-to-silt content of the sediments in the study area. The results constrained the existence of a sub-seafloor velocity gradient, \(K\), which was distributed between 0.75 and 0.85 m s\({}^{-1}\) with an average of 0.80 \(\pm\) 0.035 s\({}^{-1}\). Such a gradient is consistent with sedimentary compaction at the passive-margin setting of the study area. The consistent spatial distribution of the estimated values and parallel depth-migration-velocity analysis demonstrated the validity of our estimates. The proposed method allowed us to estimate the sound speed at the water-sediment interface where its contrast with the sound speed of the bottom water is low (at the order of 1%). Figure 14: The sound speed \(c_{b}\) (left axis, blue solid line) and the bottom density \(\rho_{b}\) (right axis, red dashed line) vs. the porosity \(P\), calculated with the empirical Akal formula [13] for 1200 m bottom depth and the sound speed of water near the bottom of 1527 m s\({}^{-1}\) (black dotted line). Sound speed profile in the water is after [54]. **Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/jmse9121423/s1](https://www.mdpi.com/article/10.3390/jmse9121423/s1)]([https://www.mdpi.com/article/10.3390/jmse9121423/s1](https://www.mdpi.com/article/10.3390/jmse9121423/s1)), Figure S1: The resulting depth migrated section of TGS line ISEM-2073; Figure S2: The distribu-tion of the interval velocities vs the depts at the middle of each interval, estimated through mi-gration analysis of the seismic line ISEM-2073. **Author Contributions:** Conceptualization, E.U., Y.M. ; data curation, O.G., G.L. ; formal analysis, E.U., O.G., G.L., Y.M. ; investigation, E.U. ; methodology, E.U., B.K., Y.M. ; project Administration, S.C., T.K., Y.M. ; resources, S.C., T.K., Y.M. ; software, E.U. ; supervision, B.K., S.C., T.K., Y.M. ; validation, Y.M. ; writing - original draft, E.U. ; writing - review & editing, O.G., G.L., B.K., S.C., T.K., Y.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This research received no external funding. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The seismic data utilized in this study are archived in the State of Israel Ministry of Energy national data archives, with restricted availability by permission from the State's Petroleum Commissioner Office. **Acknowledgments:** We thank the Oil Commissioner's Office, State of Israel Ministry of Energy, for permitting and providing the seismic data used in this research; the Applied Marine Exploration Lab.