Artificial-Gas-Seep Test Produces 3D Images of Bubble Plumes in the Ocean
In November 2015, scientists from the U.S. Geological Survey (USGS) Pacific Coastal and Marine Science Center conducted an experiment using in-house equipment to image artificially created gas plumes offshore of Santa Cruz, California.
In November 2015, scientists from the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California conducted an experiment using in-house equipment to image artificially created gas plumes offshore of Santa Cruz. The experiment is part of our preparation for a 2016 survey of California’s Santa Barbara Basin, where we plan to map the seafloor, image sediment layers beneath the seafloor, and detect and map seafloor seeps. One of the goals of the upcoming Santa Barbara Basin study is to better understand the relationship between sub-seafloor fluid flow, faults, and submarine landslides.
Submarine landslides are natural hazards that can damage man-made structures on the seafloor—such as cables, pipelines, and oil platforms—and can trigger tsunamis. They are known to occur in places where the stability of sedimentary deposits along a submarine slope is weakened by the buildup of fluid pressures below the seafloor, also known as “pore-fluid overpressure.” One of the most important and detectable indicators of pore-fluid overpressure is the discharge of water or gas from the seabed in the form of fluid seeps. The ability to detect and map gas-bubble plumes in the water column enables researchers to identify active seafloor seeps and examine their relationship to the underlying geology, the pathways in which fluids move through the sediment, and the potential landslide hazards.
In order to simulate an active seafloor seep for testing purposes, we outfitted a small inflatable vessel with a storage cylinder containing compressed air. The compressed air was piped about 35 meters down an air hose to a weight suspended in the water column. At the weight, the compressed air was expelled through a double loop of garden soaker hose. Once turned on, this inexpensive setup produced a vigorous “curtain” of small-diameter bubbles that expanded as they rose through the water column. In the November experiment, we launched a small boat from the deck of 34-foot research vessel (R/V) Parke Snavely, activated the bubbler, and left the boat-plus-bubbler to drift while we made mapping passes with a multibeam sonar mounted on Snavely.
Multibeam sonars emit sound waves and receive their echoes in the shape of a fan beneath the vessel, using the time it takes for sound pulses to travel to and from the seafloor, or an object in the water column, to calculate distances to the seafloor or the object. The fan shape enables multibeam sonars to map swaths of seafloor, the width of a given swath typically being two to seven times the water depth, depending on such factors as sea state and bottom type. In our seep-imaging test, we used a Reson 7111 multibeam sonar, which can map in waters ranging from approximately 5 to 1,000 meters deep. The sonar sends out sound pulses, or “pings,” as fast as 20 times per second in very shallow water but is limited to slower rates in deeper water, where sound takes longer to travel down to the seafloor and back. In contrast to single-beam echo sounders, which collect one data point per ping, the multibeam sonar collects hundreds of data points per ping, enabling the quick assembly of three-dimensional images.
Under normal operations, we typically use the Reson 7111 just to conduct seafloor mapping. For mapping seeps, however, we configured the system to collect data across the entire interval between the sea surface and the seafloor. Recording this type of information provides scientists the ability to detect, visualize, and interpret active gas bubbles in the water column and to identify the location of seafloor fluid seeps. The artificially created plume of bubbles was clearly visible in the data we collected in November. This successful plume experiment demonstrates our ability to map water-column features in 3D using in-house equipment and personnel at the USGS. One challenge we encountered during the test was recording the enormous volume of data generated from imaging the water column. We are addressing this problem by modifying the computer hardware in the recording system so that it can accommodate the large data-transfer rates. We hope this type of data collection will become routine during USGS multibeam sonar surveys and thus broaden the scope of scientific problems that can be tackled using standard seafloor-mapping equipment and data.
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