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Geophysical imaging relies on specialized tools to detect anomalies in the water column or to map features on or beneath the seafloor. Equipment may be towed, mounted on the side of a ship, or attached to the ship’s hull. Many geophysical techniques rely on transmitting an acoustic signal of a particular frequency and analyzing the information in the returning signal to infer the properties of the seafloor or water column feature from which the “bounce” is received.
During the IMMeRSS Mid-Atlantic cruise, scientists will use acoustic instrumentation to detect methane plumes emitted from seafloor seep sites and to map the characteristics of the seafloor near seep sites. The first critical technology is the Reson 7160 multibeam swath sonar system, which operates at a nominal frequency of 44 kHz. This multibeam system uses up to 512 beams arranged in a fan to map seafloor bathymetry and texture (backscatter). The backscatter data can provide clues about the locations of rough seafloor that may correlate with outcrops of methane-derived authigenic carbonates.
The multibeam sonar system also records the acoustic returns from water column features like the bubbles that make up methane plumes emanating from seafloor seeps. The original discovery of hundreds of seafloor seeps on the northern U.S. Atlantic margin was based on the analysis of multibeam sonar data from the hull-mounted 30 kHz system on NOAA’s Okeanos Explorer. NOAA’s Office of Ocean Exploration and Research was primarily using the multibeam sonar to develop high-resolution bathymetric maps of the margin when the water column data revealed the existence of the methane plumes. For the IMMeRSS cruise, the multibeam sonar will be deployed on a special sidemount on the port side of the R/V Sharp.
USGS researchers will also use a Simrad EK60 split-beam echosounder to detect active water column methane plumes, which could be prime targets for ROV dives. The EK60 was originally devised for fisheries studies, but in recent years has gained popularity among marine acousticians and geophysicists to infer bubble size and constrain methane fluxes from the seafloor. The EK60 can accommodate multiple transducers, but the USGS will use only a 38 kHz transducer, which will be mounted in the retractable keel on the R/V Sharp. The EK60 has a narrow cone of ensonification, which means that the ship must be almost directly over a seafloor seep to detect the resulting methane plume. Thus, EK60 data contrasts with multibeam sonar water column data, which identifies methane plumes in a swath whose width is several times the water depth. On the other hand, the EK60 plume data provide critical quantitative information that the multibeam sonar data do not. The multibeam and EK60 sonars are therefore complementary tools for locating and characterizing active methane seeps.
Geophysical imaging methods are also commonly used to detect features beneath the seafloor. For example, seismic methods use a strong, impulsive source to image layers of sediment, pathways for gas migration, and faults. The USGS has previously conducted seismic imaging cruises in the U.S Mid-Atlantic margin seep areas, and will not be acquiring more seismic data on the IMMeRSS cruise.