Abstract Gas hydrates are naturally-occurring solid compounds of gas and water within almost all sediment-rich continental margins. Due to the large amounts of methane stored in submarine gas hydrates, they might serve as future reservoirs for offshore marine gas production. Assessing the reservoir characteristics requires reliable estimates of both the gas and gas hydrate concentration, which can be best addressed using geophysical and geological investigations. Here, we demonstrate the power of joint interpretation of interdisciplinary geophysical techniques and geological laboratory experiments. Regional 2D multichannel seismic data provide the broad overview of a hydrate-bearing area. High-resolution 2D and 3D seismic reflection data provide detailed images of two working areas, the buried S1 channel-levee system at 1500 m water depth (well within the gas hydrate stability zone) and a slope failure location, located at 665 m water depth (top limit of the hydrate formation) next to the S2 channel. Detailed compressional and shear wave (Vs) velocity-depth models were derived from four component ocean-bottom seismic data, the latter from P- to S-conversion upon reflection. Due to their steep reflection angles, shear wave events result in less resolved Vs models. Nevertheless, in case of a change in elasticity of the sediment matrix due to gas hydrate cementation, shear wave events can be used as an indicator. As such, Vs can give insight into the nature of hydrate formation throughout the GHSZ. We present new developments in the application of common reflection surface, normal-incidence-point tomography and full waveform inversion techniques to enhance model resolution for the seismic data sets. 2D and 3D controlled-source electromagnetic measurements provide volume information of the resistivity-depth distribution models. Electrical resistivity of the sediment formation depends on its porosity and the resistivity of the pore fluid. Gas hydrate and free gas generally have much higher electrical resistivities than saline pore fluid, and can be assessed using empirical relationships if the porosity and pore fluid salinity are known. Calibration with logging data, laboratory experiments on hydrate- or ice-bearing sediments, and resulting velocity and resistivity values, guide the joint interpretation into more accurate saturation estimations. Beyond that, a joint inversion framework supporting forward calculation of specialized geophysical methods at distributed locations is under development. In this paper, we summarize these individual components of a multi-parameter study, and their joint application to investigate gas hydrate systems, their equilibrium conditions and preservation of bottom-simulating-reflectors. We analyze data from two working areas at different locations and depth levels along the slope of the Danube Fan, which are both characterized by multiple bottom simulating reflectors indicating the presence of gas hydrate. In the first working area we located two depth windows with indications for moderate 16%–24% gas hydrate formation, but no vertical gas migration. In the second working area we observed fluid migration pathways and active gas seepage, limiting gas hydrate formation to less than 10% at the BSR. Some discrepancies remain between seismic-based and electromagnetic-based models of gas and gas hydrate distribution and saturation estimates, indicating that further in-situ investigations are likely required to better understand the gas hydrate systems at our study areas and to calibrate the inversion processes, which will be required for a joint inversion framework as well.

中文翻译：

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黑海多瑙河深海扇地球物理场实验联合解释

摘要 天然气水合物是几乎所有富含沉积物的大陆边缘天然存在的气体和水的固体化合物。由于海底天然气水合物中储存了大量甲烷，它们可能成为未来海上海洋天然气生产的储层。评估储层特征需要对天然气和天然气水合物浓度进行可靠估计，这可以通过地球物理和地质调查得到最好的解决。在这里，我们展示了跨学科地球物理技术和地质实验室实验的联合解释的力量。区域二维多道地震数据提供了含水合物区域的广泛概览。高分辨率 2D 和 3D 地震反射数据提供两个工作区的详细图像，埋藏在 1500 m 水深的 S1 河道-堤防系统（位于天然气水合物稳定带内）和位于 S2 河道旁边 665 m 水深（水合物形成的上限）的斜坡破坏位置。详细的纵波和横波 (Vs) 速度-深度模型源自四个分量海底地震数据，后者在反射时从 P 到 S 转换。由于其陡峭的反射角，横波事件导致解析度较低的 Vs 模型。然而，在由于气体水合物胶结作用导致沉积物基质弹性发生变化的情况下，可以使用横波事件作为指标。因此，Vs 可以深入了解整个 GHSZ 中水合物形成的性质。我们展示了共同反射面应用的新进展，法向入射点断层扫描和全波形反演技术，以提高地震数据集的模型分辨率。2D 和 3D 受控源电磁测量提供电阻率-深度分布模型的体积信息。沉积物地层的电阻率取决于其孔隙度和孔隙流体的电阻率。天然气水合物和游离气体通常比含盐孔隙流体具有更高的电阻率，如果孔隙度和孔隙流体盐度已知，则可以使用经验关系进行评估。用测井数据校准、含水合物或含冰沉积物的实验室实验以及由此产生的速度和电阻率值，指导联合解释更准确的饱和度估计。除此之外，正在开发一个联合反演框架，支持在分布式地点对专业地球物理方法进行前向计算。在本文中，我们总结了多参数研究的这些单独组成部分，以及它们在研究天然气水合物系统、平衡条件和底部模拟反射器的保存方面的联合应用。我们分析了来自多瑙河扇斜坡不同位置和深度水平的两个工作区的数据，这两个工作区的特征都是多个底部模拟反射器，表明存在天然气水合物。在第一个工作区，我们找到了两个深度窗口，指示中等 16%–24% 的天然气水合物形成，但没有垂直天然气运移。在第二个工作区，我们观察到流体运移路径和活性气体渗漏，将 BSR 的天然气水合物形成限制在 10% 以下。基于地震和基于电磁的天然气和天然气水合物分布和饱和度估计模型之间仍然存在一些差异，这表明可能需要进一步的原位调查以更好地了解我们研究区的天然气水合物系统并校准反演过程，这也是联合反演框架所必需的。