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Instrumenting the seafloorThree new technologies for monitoring seafloor hazards are being developed at Scripps to examine the Gaviota slope. Each has its own specific capabilities and research strategy based on the instrument's spatial coverage and data-collection frequency so that the scientists can observe both sudden and long-term geological changes. The first is a torpedo-shaped autonomous underwater vehicle (AUV) about 3.5 meters (12 feet) long and manufactured by Bluefin Robotics in Boston. Under the direction of geophysicist Gerald D'Spain, the AUV was outfitted with a state-of-the-art navigation system, an onboard pressure sensor, and a multibeam acoustic imaging sonar with a resolution of decimeters (several inches) when flying just above the seafloor. 
The AUV is launched from a ship and then follows a preprogrammed underwater flight plan, making multiple passes over areas of interest to survey and map features. It worked well in initial tests conducted off San Diego, and has since been back to the engineering lab for further development. It will be deployed at the Gaviota slide several times over the next few years in a series of repeated surveys D'Spain describes as "mowing the same patch," in an attempt to map any changes to the seafloor topography over periods of months and years. BP supplied funds for acquisition and integration of the navigation and sonar systems while the Office of Naval Research funded the vehicle and several of the onboard systems. The second system is an array of high-precision acoustic transmitters and receivers that form a geodetic monitoring network similar to satellite-based measurement systems such as GPS used on land to detect surface deformation and earthquakes. The system was first developed in the 1980s by Scripps marine geophysicist Fred Spiess to study the movement of tectonic plates in the deep sea over months and years. It takes advantage of the fact that acoustic signals move through seawater quickly and with a consistency that allow their travel times to be used for precise positioning. The instruments are placed on the seafloor to straddle a feature of interest and by correlating signal transmission and reception times, the network can detect changes in distances of as little as 5 millimeters (0.2 inches). Scripps geophysicist David Chadwell has modified and adapted the acoustic network to work in a tighter configuration and to record on a daily basis for the Santa Barbara study. The system was set out at Gaviota last November from Scripps R/V Roger Revelle and continues to record its data, which will be recovered by ship later this year. If an earthquake occurs in the area, Chadwell plans to retrieve the data sooner. Development of the third technology involves converting a standard electronic surveying tool, called an electronic distance meter, to work in the marine environment, which is proving to be a bit more difficult than anticipated. On land the meters emit light, usually infrared, to a distant reflector and measure the light's return time to calculate distances several kilometers (miles) away with an accuracy of 1 to 2 millimeters (0.04-0.08 inches). Geophysicist Mark Zumberge had the idea that the light could be measured similarly by transmitting a beam into an fiber optic cable stretched between two anchors placed several hundred meters (yards) apart on the seafloor. If the two anchors are displaced with respect to one another by a geological process then the length of the fibers changes. The instrument records the change. For protection, the optical fibers are encased in hermetically sealed stainless steel tubing. Zumberge took a prototype fiber-optic strain sensor, or FOSS, on R/V Roger Revelle during the November cruise and set it across the Gaviota fault. But when researchers laid it out at a depth of 400 meters (1,315 feet), the cable snapped, because Zumberge said, "we simply couldn't keep the tension from becoming too great." It was a setback, but he and colleagues are seeking further funding to continue their portion of the program. With dozens of potential underwater landslide slopes along the West Coast from Seattle to San Diego that could trigger tsunamis in highly populated areas, there are plenty of reasons to continue the research. And for BP, there are very practical potential applications when it comes to offshore facilities. "Even though we don't yet have extensive trial results, at BP we're already reaping the benefits in terms of new ideas and inspirations," BP senior advisor Hugh Banon said. "Collaborations with academic institutions that are at the forefront of ocean sciences are a key element in supporting our business. And Scripps is certainly among the very best." |
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