A prototype vertical line array (VLA) has been constructed for imaging the shallow subbottom in hydrate stability zone of the northern Gulf of Mexico. It is 200m long and consists of 16 hydrophone channels, each channel being digitized to 16 bits at a rate of 10,000 samples per second. Recording is done by a battery-powered module near the base of the VLA.
An acoustic doppler current profiler looks upward from the base of the VLA to aid in determining the geometric configuration of the VLA in the water column. The circuitry provides an option for incorporating tilt meters and compasses along the VLA to improve that determination.
The VLA has been deployed in 830m of water and tested by making runs on it firing a surface-towed 80in3 watergun. A source signature was recorded for each shot by a single-channel hydrophone towed 500m below the source. Examples of the test data are presented.
Results indicate that the tests were generally successful but that some modifications to the VLA design are desirable. Immediate plans are to construct two other VLAs to the new design and modify the prototype to match. The three VLAs will then be deployed to comprise a net for triangulating upon and tracking sounds in the water column. These sounds will be used to monitor possible changes to the geologic structures in the shallow subbottom.
In the longer-term term, it is planned to deploy a net of five VLAs to provide redundancy and error correction for the triangulation/tracking. That net will provide the heart of a multi-sensor remote station which will monitor an area of sea floor for a period of 3-to-5 five years. Data will be transmitted to shore via an oil platform or a moored bouy.