Abstract In the southern Hikurangi subduction margin, widespread gas hydrate accumulations are inferred based on the presence of bottom simulating reflections and recovered gas hydrate samples, mainly associated with thrust ridges. We present a detailed analysis of high- and medium-resolution seismic reflection data across Glendhu and Honeycomb ridges, two elongated four-way closure systems at the toe of the deformation wedge. High-amplitude reflections within the gas hydrate stability zone, coincident with high seismic velocities, suggest the presence of highly concentrated gas hydrate accumulations in the core regions of the anticlinal ridges. A novel method involving combined seismic velocity and reflectivity analysis and rock physics modelling is used to estimate hydrate saturations in localised areas. The effective medium model consistently predicts gas hydrate saturations of ~30% of the pore space at Glendhu Ridge and >60% at Honeycomb Ridge, whereas the empirical three-phases weighted equation likely underestimates the amount of gas hydrate present. We note that our estimates are dependent on the vertical resolution of the seismic data (5–14 m), and that the existence of thin layers hosting gas hydrate at higher concentrations is likely based on observations made elsewhere in similar depositional environments. A comparison between the two ridges provides insights into the evolution of thrust related anticlines at the toe of the accretionary wedge. We propose that the main driving mechanism for concentrated hydrate accumulation in the study area is along-strata gas migration. The vertical extent of these accumulations is a function of the steepness of the strata crossing the base of gas hydrate stability, and of the volume of sediments from which fluid flows into each structure. According to our interpretation, older structures situated further landward ofthe deformation front are more likely to host more extensive concentrated hydrate deposits than younger ridges situated at the deformation front and characterised by more gentle folding. The method introduced in this work is useful to retrieve quantitative estimates of gas hydrate saturations based on multi-channel seismic data.

中文翻译：

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新西兰 Hikurangi 边缘南部浓缩天然气水合物沉积物的地震速度和反射率分析

摘要 在 Hikurangi 俯冲边缘南部，根据底部模拟反射和回收的天然气水合物样品的存在，推断出广泛的天然气水合物聚集，主要与逆冲脊有关。我们详细分析了 Glendhu 和 Honeycomb 山脊的高分辨率和中分辨率地震反射数据，这两个山脊是变形楔形尖端的两个细长四向闭合系统。天然气水合物稳定带内的高振幅反射与高地震速度一致，表明背斜脊的核心区域存在高度集中的天然气水合物聚集。一种将地震速度和反射率分析与岩石物理建模相结合的新方法用于估计局部区域的水合物饱和度。有效介质模型一致预测 Glendhu Ridge 孔隙空间的~30% 和 Honeycomb Ridge > 60% 的天然气水合物饱和度，而经验三相加权方程可能低估了天然气水合物的存在量。我们注意到，我们的估计取决于地震数据的垂直分辨率（5-14 m），并且含有较高浓度天然气水合物的薄层的存在可能是基于在类似沉积环境中的其他地方进行的观察。两个山脊之间的比较提供了对增生楔趾部与逆冲相关背斜演化的深入了解。认为研究区水合物集中聚集的主要驱动机制是沿层气运移。这些堆积物的垂直范围是穿过天然气水合物稳定性基础的地层的陡度以及流体流入每个结构的沉积物体积的函数。根据我们的解释，位于变形前缘更靠陆的较老构造比位于变形前缘的较年轻的山脊更有可能拥有更广泛的集中水合物沉积物，并且具有更温和的折叠特征。这项工作中介绍的方法对于基于多道地震数据反演天然气水合物饱和度的定量估计是有用的。位于变形前缘更靠陆的较老构造比位于变形前缘的较年轻的山脊更可能拥有更广泛的集中水合物沉积物，并且具有更温和的折叠特征。这项工作中介绍的方法对于基于多道地震数据反演天然气水合物饱和度的定量估计是有用的。位于变形前缘更靠陆的较老构造比位于变形前缘的较年轻的山脊更可能拥有更广泛的集中水合物沉积物，并且具有更温和的折叠特征。这项工作中介绍的方法对于基于多道地震数据反演天然气水合物饱和度的定量估计是有用的。