Characterization of Gas Hydrate Physical Properties with a Walkaway Vertical Seismic Profile on ODP Leg 163

9504610霍尔布鲁克该项目在JOIDES RESOLUTION计划于1995年10月至12月进行的钻探巡航中，增加了一项研究布莱克外山脊沉积物中天然气水合物的“走走式垂直地震剖面”实验。钻探的一个主要目的是检查富含天然气的深海沉积物的物理性质，据推测，这些沉积物在海底以下较浅的深度以冻结状态(天然气水合物)束缚，并被含有充满未冻结天然气的孔隙水的沉积物覆盖。这种类型的沉积气是从大量的大陆边缘环境中获知的，而美国东海岸外的布莱克外山脊提供了最著名的天然气水合物气田的例子，这些气田没有受到断层或其他构造干扰的严重影响。霍尔布鲁克已经提议进行一项实验，在海底下的钻孔中使用传感器来接收来自第二艘船上的声源的声音信号。记录的旅行时间将准确地描述钻探地点附近区域的详细速度结构，特别是穿过底部模拟反射层，或“BSR”，它被假设为定义从冻结到未冻结气体的过渡深度。关于这类过渡区的速度结构的测量很少，而且提供这种独特的“BSR”信号所需的冻结和非冻结形式的气体量也不是很清楚。这里提出的实验，连同将从钻井中回收的样本，将显著提高我们对深海海洋环境中天然气物理的理解，并将提高我们估计在其他只有地震数据但没有进行钻探的地区存在多少天然气的能力。如果天然气水合物被证明含有大量可获得的天然气，那么它们的经济潜力是值得关注的；天然气水合物也可能在气候中发挥重要作用，因为它们可能为大量的甲烷提供汇，甲烷是一种重要的“温室气体”。了解控制其在海洋中分布的因素是海洋钻探方案的一个高度优先事项，也是最近在北美和南美洲西部边缘附近进行的其他钻探巡航的一个重要目标。***

9504610 Holbrook This project adds a "walkaway vertical seismic profile" experiment to study gas hydrate in sediments of the Blake Outer Ridge to a scheduled drilling cruise of JOIDES Resolution in October-December 1995. A primary objective of the drilling is to examine the physical properties of deep-ocean sediments enriched in natural gas, which is presumed to be bound in a frozen state (gas hydrate) at shallower depths below the sea floor, and underlain by sediments containing pore waters charged with unfrozen gas. This type of sedimentary gas is known from a large number of continental margin environments, and the Blake Outer Ridge, off the U.S. East Coast, provides the best known example of a field of gas hydrates which are not heavily affected by faulting or other tectonic disturbance. Holbrook has proposed to conduct an experiment using sensors in the drill hole beneath the sea floor to receive acoustic signals from a sound source on a second ship. The travel times recorded will accurately characterize the detailed velocity structure of the region near the drill site, especially across the bottom simulating reflector, or "BSR," which is hypothesized to define the depth of transition from frozen to unfrozen gas. Very few measurements exist on the velocity structure at this type of transition zone, and the amount of gas required in both frozen and unfrozen form to provide this unique "BSR" signal is not well understood. The experiment proposed here, in conjunction with the samples to be recovered from drilling, will significantly improve our understanding of the physics of natural gas in the deep marine environment, and should improve our ability to estimate how much gas is present in other areas where only seismic data is available, and no drilling has been done. The economic potential of gas hydrates is of interest if they prove to contain large amounts of accessible natural gas; gas hydrates may also play an important role in climate, as they may provide a sink for significant amounts of methane, an important "greenhouse gas." Understanding the factors which control their distribution in the oceans is a high priority of the Ocean Drilling Program, and has been an important objective on other recent drilling cruises off the western margins of both North and South America. ***