Three-Dimensional Amplitude versus Offset Analysis for Gas Hydrate Identification at Woolsey Mound: Gulf of Mexico

Round 1

Reviewer 1 Report

This work focuses on the Woolsey Mound, which is a hydrate complex in the Gulf of Mexico, exhibiting gas hydrate outcrops. The study aims to determine the hydrate stability zone base without extensive bottom BSRs. By analysing seismic data, shallow bright spots were targeted, and hypothesised to indicate the hydrate stability zone base, showing Class 4 AVO anomalies. Moreover, Time-lapse analysis is used to correlate changes in AVO curve strength to decreasing hydrate concentrations. This work can be potentially interesting for the readers of Geohazards journal and can be published after the following moderate comments were addressed by the authors:

1.     The Introduction section reads well. I suggest that the authors enrich this part by including a discussion regarding the geohazards associated with natural gas hydrate dissociation in cold environments. The authors may also refer to some relevant articles such as:

-        Farahani et al., 2021. Insights into the climate-driven evolution of gas hydrate-bearing permafrost sediments: Implications for prediction of environmental impacts and security of energy in cold regions. RSC Advances, 11(24), pp.14334-14346.

-        Farahani et al., 2021. Development of a coupled geophysical–geothermal scheme for quantification of hydrates in gas hydrate-bearing permafrost sediments. Physical Chemistry Chemical Physics, 23(42), pp.24249-24264.

2.     Is Figure 1 generated by the authors? If not, please include the reference. I have the same comment about Figure 6.

3.     What is NMO? I suggest that the authors define the abbreviations as the readers may not have different knowledge and research backgrounds. For example, some readers might not know that TGS offers seismic services. More information can be added to clarify this to the readers.

4.     Figure 7: Can the authors add more details to the caption regarding the bright spots?

5.     Can the authors add more details about the other classes of AVO response?

6.     Figures 8, 9, 11, and 12: The details are very difficult to read. Please replace them with high-quality Figures. I have this comment about the other relevant Figures.

7.     The correlation between the seismic response of hydrate-bearing sediments and hydrate saturation substantially depends on the pore-scale distribution of hydrate crystals in pore space. How does AVO analysis can account for this aspect?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

This manuscript (MS) explains how the AVO method was used to identify the base of the hydrate stability field by focusing on the shallow, high amplitude negative polarity BSRs at Woolsey Mound in the northern Gulf of Mexico by comparing two 3-D pre-stack datasets acquired over a time interval of 4 years (2010 and 2014). It wishes to apply this new technology to judge other gas hydrate areas of the Gulf of Mexico where BSRs are not regionally extensive and characterize the hydrate system's dynamic nature by analyzing the AVO curves' strength. I read several of this MS with great interest. There are some uncertainties or puzzles that need further clarification before it can be accepted for publication.

1.    The authors should thoroughly state whether the data set used for the AVO analysis is sufficiently convincing in this study. As we all know, three discontinuous points can usually show the trend of a line more accurately than merely two points. I personally reckon that in addition to the 2010 and 2014 datasets mentioned in the MS, relevant 3-D seismic data of this site from at least another interval year (preferably 4 years or more) should be joined to draw conclusions.

2.    In section 3.1 and 3.2, I am curious whether the different working environments during the two cruises in 2010 and 2014 will have any impact on the data, such as seasons, seawater conditions, seismic wave speed collection operation methods, etc.

3.    Figs. 4-5, 8-9, 11-12, 14-15, 17-18, and 20-25 are unclear for me and pretty hard to read.

4.    A gas hydrate stability zone phase diagram should be added into Fig.1.

5.    Since this study aims to identify the baseline of hydrate zones, the alternation comparison of the targeted bright spots at different inlines between 2010 and 2014 data should be provided after Fig. 7, 10, 13, and 16.

6.    There have no contour plots in Fig. 7, 10, 13, and 16.

7.    Section 4.1, I still don't understand how to verify whether the bright spots that can produce an AVO response arise from the trapping of free gas beneath the gas hydrate.

8.    Section 4.2, I feel it very strange why the decomposition and formation performance of gas hydrates simultaneously occur at the same depth? If so, the dynamic alternation of local salt formations should be reflected in AVO curves and pointed out between 2010 and 2014.

9.    Similar doubts, in section 4.2, how much has the hydrate concentration changed here?

10.  The entire paper always talks about the gas hydrate system in seafloor sediments, so, how does the occurrence and distribution of local hydrates, and their according change after four years? What is the identification accuracy of this 3-D Amplitude Versus Offset Analysis?

The quality of English is good.

Author Response

Please see attachment

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The author answered all my questions and the current status of the manuscript is acceptable.