Imaging the Hydrate Stability Zone Using a Vertical Array: First Results

Abstract

Abstract There is interest in using vertical arrays of hydrophones to monitor near-sea-floor gas hydrates over extended periods of time. The technology is being developed to use nets of such arrays to track ships and use the sounds from them to image shallow geologic features. It is expected that such monitoring will extend knowledge of hydrate formation and dissociation as well as of associated influences on the occurrence of sea-floor instabilities. Imaging features within the hydrate stability zone of the northern Gulf of Mexico has attracted increasing interest dur-ing the past few years. A vertical array of hydrophones, spe-cially designed for high resolution in the shallow sediments, was deployed recently and signals from a surface-towed seis-mic source recorded. A horizontal hydrophone array was towed some distance below the source to record simultane-ously a near-zero-offset seismic reflection profile. Signals from both the vertical and the horizontal array are presented. Subbottom information provided by the two agree. That information also agrees with previously acquired seismic profiles and the thickness of the hydrate stability zone as indi-cated by heat-flow measurements. Introduction A remote, multi-sensor sea-floor station planned for the conti-nental slope of the northern Gulf of Mexico will use vertical line arrays (VLAs) of hydrophones to monitor changes in the shallow sub-sea-bottom over an extended period of time. A prototype VLA has been designed and constructed as part of the development leading to deploying that station. During the summer of 2002, the prototype VLA was de-ployed and recovered successfully in the Mississippi Canyon area. The water was over 800m deep. Runs were made on the VLA site firing a surface-towed seismic source. Simultane-ously, a single-channel hydrophone array was towed beneath the source to provide a high-resolution seismic reflection pro-file. Signals from both the prototype VLA and the single-channel array are compared. It is concluded that their subbottom con-figurations agree. Those configurations also agree with previ-ously acquired seismic profiles and the thickness of the hy-drate stability zone as indicated by heat-flow measurements. Eventually, a net of five VLAs will form the heart of the monitoring station. It is intended that the VLA net will trian-gulate on and track surface vessels. The noise of the vessels will be recorded and used as an acoustic source for imaging geologic structures within the hydrate stability zone. In this way, temporal changes to the sub-sea-floor structure can be monitored more-or-less continuously without a survey ship being dedicated to patrolling the area. Design Details The design of the prototype VLA is illustrated in fig.1. It has a total length of slightly more than 200m. It is supported in the water column by glass-sphere flotation and is fixed to the sea floor by an expendable concrete anchor. The upper portion of the VLA consists of 16 acoustic chan-nels evenly spaced at 12.5m intervals. Each acoustic channel comprises a hydrophone and preamplifier. The lower portion accommodates electronic devices in pressure housings. These include a data logger, a battery pack, an acoustic-doppler cur-rent profiler and acoustic releases to disengage the anchor.