the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Jan 2026
Gas Migration and Slope Instability in the Danube Fan: Insights from integrated OBS-MCS Seismic Analysis
Abstract. Gas hydrates and deltaic deep-sea fans are main features in continental margin systems, influencing slope stability, fluid migration, and carbon cycling. In the northwestern Black Sea, the Danube Fan remains poorly constrained with respect to subsurface structure, sediment strength, and hydrate dynamics. Here, we present high-resolution multichannel seismic (MCS) and ocean-bottom seismometer (OBS) data to characterise sedimentary structure and fluid-related features. Two integrated OBS–MCS profiles reveal underconsolidated, clay-rich levee deposits interspersed with mass-transport units, chaotic facies, and gas-related anomalies. Derived P- and S-wave velocity models indicate low shear strength and high Vp/Vs-ratios in shallow units, consistent with soft, water-saturated sediments. Deeper layers display compaction-driven velocity increases but remain mechanically weak, rendering the slope prone to failure. Our findings suggest that vertical gas migration is widespread, expressed by seismic chimneys, polarity reversals, and velocity pull-downs, with free gas confined below bottom simulating reflectors and in stratigraphic traps. Hydrates likely occur as sparse, patchy pore-filling accumulations, and the lack of S-wave velocity anomalies suggests they do not act as cementing phases, implying little direct influence on sediment strength or slope stability. The hydrate system appears hydrate-poor, possibly reflecting post-glacial re-equilibration. Overall, lithology, gas migration pathways, and high sedimentation rates emerge as primary controls on hydrate formation and slope instability in the Danube Fan.
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Status: open (until 08 Apr 2026)
- RC1: 'Comment on egusphere-2025-5275', Leon Thomsen, 09 Feb 2026 reply
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RC2: 'Comment on egusphere-2025-5275', Anonymous Referee #2, 28 Mar 2026
reply
Review report
This manuscript presents an integrated analysis of high-resolution MCS and OBS data to investigate sedimentary architecture, gas migration, hydrate occurrence, and slope instability in the Danube Fan, northwestern Black Sea. The study is valuable because it combines P- and S-wave information with seismic stratigraphic interpretation and existing drilling constraints to address an important geohazard problem. The paper is also potentially significant in that it characterizes a hydrate-poor but gas-active endmember, which is less commonly documented than hydrate-rich continental margins. Overall, the dataset is interesting and the manuscript has clear potential for publication.
However, in its present form, several important aspects of the interpretation are not yet sufficiently demonstrated. In particular, the paper needs a clearer explanation of the S-wave phase identification procedure, stronger and more quantitative linkage between the seismic/velocity observations and the geological interpretations, a more explicit integration of the MeBo200 drilling results, and a more cautious justification of the “velocity pull-down / seismic chimney” interpretation given the resolution of the velocity models. I therefore recommend major revision.
Major comments
- The procedure for identifying converted S-wave phases needs to be explained much more clearly.
The manuscript correctly states that assigning converted S-wave phases to the same reflectors is “the most critical” step of the analysis, but the present description is too brief for the reader to understand how this was actually done and how robust the identification is. The authors should explain, in a more step-by-step manner, how the P-wave phases were transferred to the radial component, how conversion points were treated, how ambiguities were resolved, and what criteria were used to reject alternative phase assignments. Since much of the Vp/Vs interpretation depends on this step, this issue is central to the paper. - Several key interpreted features are difficult to see in the figures, and the figures need improvement.
The manuscript discusses features such as the seismic chimney, localized high amplitudes, polarity reversals, and related gas indicators, but in the current figures some of these features are not easy to identify visually. This weakens the persuasiveness of the interpretation. The authors should improve figure clarity, for example by enlarging key panels, adding zoom-ins and arrows. If the “SC” and related anomalies are central to the argument, the figures should allow the reader to recognize them much more easily. - The difference in model expression between Vp and Vs should be explained.
From the presented figures, the Vp models appear to increase relatively smoothly with depth, whereas the Vs models appear more discontinuous or layer-stepped. The manuscript should explain why the two models are represented differently. Is this caused by differences in data coverage, resolution, picking confidence, parameterization, or the modelling strategy for S-wave conversion phases? Without such an explanation, readers may wonder whether the apparent Vs discontinuities are geologically meaningful or simply artefacts of the inversion/model-building procedure. - The comparison with MeBo200 drilling results should be strengthened substantially.
The discussion repeatedly refers to previous MeBo200 drilling results, but the connection remains too general. Because the drilling information is an important ground truth for lithology, porosity, gas occurrence, and hydrate interpretation, the manuscript should specify more clearly where the drill sites are relative to the OBS/MCS lines, what depths were sampled, and which lithologic or physical-property observations are directly relevant to the interpreted units. It would greatly improve the paper if a simplified stratigraphic or lithologic column from MeBo200 were added to Fig. 7 or a discussion figure, especially near OBS 3005 where the comparison appears most relevant. - The discussion is currently too qualitative and should be made more quantitative.
At present, the discussion often moves quickly from observed seismic or velocity features to fairly broad geological interpretations. The paper would be much stronger if the authors more explicitly stated what aspect of the interpretation is directly constrained by the new results, what is only suggestive, and what is inferred mainly from analogy with previous work. For example, how strongly do the observed Vp/Vs values constrain sediment consolidation state, free-gas occurrence, hydrate distribution, or mechanical weakness? Can the authors provide thresholds, ranges, or quantitative comparisons with literature and drilling results? At present it is difficult to judge how much of the final interpretation is uniquely supported by the dataset. - The interpretation of a velocity pull-down / seismic chimney requires more careful justification.
The manuscript interprets a feature beneath profile A as a seismic chimney associated with a velocity pull-down, and the discussion further uses this feature as evidence for vertical gas migration. However, given the stated model limitations — only four OBS stations per line, limited offsets, no clear refracted phases across the array, and a modelling approach that is local beneath each instrument — it is not obvious that the velocity model has sufficient lateral resolution to resolve a true pull-down effect in a robust manner. The authors should clarify what evidence specifically supports this interpretation. Is the pull-down seen primarily in the MCS image, in the velocity model, or both? Could it instead reflect imaging artefacts, structural complexity, or local stratigraphic effects? The interpretation may still be reasonable, but it currently appears more certain than the underlying resolution would justify.
Minor comments
- The uncertainty discussion is useful, but it would help to distinguish more explicitly between uncertainty in travel-time fit, model parameter uncertainty, and interpretational uncertainty. Low RMS and χ² values do not automatically imply uniqueness of the geological interpretation.
- The manuscript should more clearly separate observations from interpretations in the Results section. In several places, features are already assigned specific geological meanings before the reader is shown the reasoning in the Discussion.
Citation: https://doi.org/10.5194/egusphere-2025-5275-RC2 - The procedure for identifying converted S-wave phases needs to be explained much more clearly.
Data sets
2D multichannel seismic profiles during Maria S. Merian cruise MSM34, Black Sea J. Bialas and M. Riedel https://doi.org/10.1594/PANGAEA.921576
3D P-cable seismic data during Maria S. Merian cruise MSM34, Black Sea J. Bialas et al. https://doi.org/10.1594/PANGAEA.921631
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- 1
This is a useful study, and should be published, subject addressing of the following technica l issues:
*Line 121. The vector rotation implied here seems to be innocuous, a simple mathematical operation. But it assumes that the rotated data is a vector. While the incoming wave is certainly a vector, the data as recorded may not be, since the various recorded components may have different instrumental responses. This is called the issue of vector infidelity. Each recorded component is certainly unfaithful (an input impulse does result in a recorded impulse), but this does not lead to vector infidelity, if each component is unfaithful in the same way. The ms does not show data or discussion to give the reader confidence on this issue.
Separately, the ms does not indicate which part of the data provide the basis for the rotation. Is it the direct arrival through the water?
*Fig. 3bd. The figure shows the polarity reversing at normal incidence, as the source passes the instrument; this is as it should be. But the data should (according to simple theory) go smoothly to zero during this transit; instead the data show strong horizontally-polarized arrivals at normal incidence. This behavior has been noted previously, and deserves a substantive discussion here.
*Fig.4. The caption here refers to a velocity decrease, which is not shown in Fig. 6. Please discuss.
*Fig. 5. The so-called BSR's do not simulate the bottom very accurately. Please discuss.
Also, minor issues are noted in the attached ms.