In November 2015, scientists from the U.S. Geological Survey (USGS) conducted an experiment using in-house equipment to image artificially created gas plumes offshore of Santa Cruz, California. 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 (https://walrus.wr.usgs.gov/research/projects/eq_tsu_land_cathaz.html).
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 the 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 the Snavely.
Above:Pete Dal Ferro deploying the bubbler system from an inflatable vessel. The compressed air was stored in the large white cylinder, and the yellow air hose was connected to a garden soaker hose wrapped around a weight. After the weight was lowered about 35 meters into the water, the bubbler was turned on and a stream of bubbles rose in the water column, simulating a seafloor seep. USGS photograph by Gerry Hatcher. [larger version]
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.
Above: Video showing sonar data from the water column and a view of the bubble plume at the sea surface. Left: Real-time imaging of the bubble plume by the Reson 7111 multibeam sonar reveals the seafloor surface (labeled) and the bubble plume rising through the water (red circle). Right: Bubbles reaching the sea-surface. Video:MP4 format | Ogg format | WebM format
Above: 3D animation in which viewer circles around a stationary bubble plume imaged in the water column. The bottom of the bubble plume is at a water depth of about 35 meters (green in animation) and the top at a depth of about 8 meters (red in animation). This technique cannot image bubbles near the surface because water this shallow is beyond the outer beams of the multibeam sonar. The background is a digital elevation model of Santa Cruz, which shows the shape of the land but no structures or vegetation. Video:MP4 format | Ogg format | WebM format