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In late August 2018, scientists and technical staff from the USGS Coastal and Marine Hazards and Resources Program completed the acquisition of over 2000 km of multichannel seismic (MCS) data as part of the Mid-Atlantic Resource Imaging Experiment (MATRIX) conducted aboard the R/V Hugh R. Sharp. The seismic program was led by the USGS Gas Hydrates Project and was sponsored by the USGS, the U.S. Department of Energy (DOE) and the Bureau of Ocean Energy Management (BOEM). Shipboard chief scientists for MATRIX were Carolyn Ruppel and Nathan Miller, with Wayne Baldwin leading aspects of cruise’s second leg.
The USGS’s plan for MATRIX originated in 2015, soon after the discovery of hundreds of seafloor methane seeps in the sector of the U.S. Atlantic margin stretching from Cape Hatteras to Georges Bank. Scientists from the USGS Gas Hydrates Project and other USGS groups have participated in or led numerous cruises (e.g., IMMeRSS, DISCOVRE, and other collaborative programs) to acquire geophysical, geochemical, and biological data on these methane seeps. Some of this research focuses on the connection between the seafloor seepage and the underlying gas hydrate dynamics, while other collaborative investigations target the fate of methane in water column and transfer of methane across the sea-air interface.
BOEM’s release of large amounts of legacy industry seismic data for the Atlantic margin through an USGS data portal also catalyzed the development of MATRIX. Characteristic seismic reflectors (bottom simulating reflectors or BSRs) associated with the occurrence of gas hydrate can be identified in some of these industry data. However, the data generally do not extend to deeper water areas where BOEM has identified potential gas hydrate accumulations (e.g., “the whale”) as part of its U.S. marine gas hydrate assessment. In that assessment, the U.S. Atlantic margin is predicted to host approximately as much gas-in-place in methane hydrate as does the northern Gulf of Mexico, a world-class petroleum basin that has been the focus of U.S. gas hydrate research for more than 15 years.
The primary goal of MATRIX was to acquire MCS imagery to constrain gas hydrate distributions between Hudson Canyon and Cape Hatteras on the U.S. Mid-Atlantic margin. In collaboration with BOEM and DOE, the USGS chose the placement of MATRIX lines to fill the gap in modern seismic between a 2014 Extended Continental Shelf (ECS) line acquired by the USGS near Hudson Canyon to the north and the 2014 Eastern North American Margin (ENAM) survey collected with National Science Foundation sponsorship to the south near Cape Hatteras. Both the ECS and ENAM surveys were conducted on the R/V Marcus G. Langseth, a specialized seismic vessel owned by the U.S. National Science Foundation (NSF) and operated by Lamont-Doherty Earth Observatory (LDEO).
Focusing on areas offshore New Jersey, Delaware, Maryland, Virginia, and North Carolina at water depths between the shelf-break (~100 m) and 3700 m, MATRIX surveyed through three potential deepwater hydrate accumulations delineated in this area by BOEM. In addition, MATRIX constrained gas hydrate and shallow free gas distributions in critical upper continental slope areas beneath many of the identified methane seeps.
Submarine slide deposits, shallow faults, sedimentary and erosional features, and regional stratigraphic markers were also imaged during the MATRIX survey. MATRIX also acquired data through the locations of a few deep stratigraphic test wells and exploration wells drilled near the northern part of the Baltimore Canyon Trough in the 1970s and 1980s. USGS researchers and other scientists are expected to use the MATRIX results for analysis of geohazards, sedimentary features, and the post-Paleogene evolution of the U.S. Atlantic margin for decades into the future.
The MATRIX MCS data are of outstanding quality. Vertical resolution is estimated to be ~15 m, and subseafloor penetration ranges from ~1 km on the upper slope to ~3 km in deep water. This is significantly deeper than the expected depths of BSRs typically interpreted as marking the base of the gas hydrate stability zone. In some locations, MATRIX data acquired at more than 3000 m water depth even detect basement beneath thick sedimentary cover.
The MATRIX surveys were conducted using up to 4 airguns (up to 420 in3 air volume) as a seismic source and a streamer as long as 1.2 km (112 to 160 channels) to receive the data. MATRIX was the first USGS-managed program since the 1990s to deploy such a long streamer and to use larger airguns to acquire a regional seismic dataset.
To better constrain water velocities using seismic energy received up to 15 km from the ship, MATRIX successfully deployed ~60 expendable sonobuoys at water depths greater than 1000 m. The USGS split-beam fisheries sonar also continuously collected water column imagery to identify methane plumes emanating from seafloor seeps.
Cooperation with several other institutions allowed access to the full suite of equipment needed to complete a program as ambitious as MATRIX. Airguns were provided by LDEO, Scripps Institute of Oceanography (SIO), and the USGS and operated and maintained by USGS gunners from the Pacific Coastal and Marine Science Center (PCMSC). The streamer combined sections from the USGS and SIO and was configured and managed by the staff from the USGS Woods Hole Coastal and Marine Science Center. LDEO provided engineering plans for the sonobuoy launchers, which were constructed at PCMSC.
MATRIX acquired high-quality seismic data using a 4-airgun array powered by portable compressors operating on a vessel smaller than typically used for regional MCS surveys. The success of MATRIX attests to the versatility of smaller, general-purpose UNOLS research ships such as the R/V Hugh R. Sharp and their potential to more routinely support MCS operations requiring airgun sources.
The USGS completed the MATRIX cruise under an Incidental Harassment Authorization (IHA) granted by National Marine Fisheries Service to ensure compliance with the Marine Mammal Protection Act. Under the terms of the IHA and of the Biological Opinion, which was issued in conjunction with an Endangered Species Act consultation, three professional protected species observers sailed aboard the MATRIX cruise. The observers maintained careful records on seismic operations; marine mammals, turtles, and seabirds seen near the ship; and mitigation measures (e.g., shutdowns of the airgun array) implemented to protect marine species.
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