Sidescan sonar and single channel airgun profiling on the NOAA ship Discoverer

Cruise D1-96-NC July 16 - August 9, 1996

On July 16, 1996, the NOAA ship Discoverer sailed from Seattle, Washington with 25 scientists (116 kb) from the USGS, NOAA and Williamson and Associates aboard. The cruise (D1-96-NC), with Robert Embley (NOAA, Newport, Oregon) and Kim Klitgord (USGS, Menlo Park, California) as co-chief scientists, successfully completed sidescan sonar studies of the Co-Axial segment of the Juan de Fuca Ridge (NOAA VENTS Program) and the Escanaba Trough on the southern Gorda Ridge. The AMS-60SI (98 kb) sidescan sonar system and 4.5 kHz sub-bottom profiler system leased and operated by Williamson and Associates were used for both surveys. At Escanaba Trough, nearly 350 km of near-bottom sidescan sonar and 4.5 kHz sub-bottom penetration data and surface towed airgun seismic reflection data were acquired on a grid of 18 lines over the Northern Escanaba (NESCA) hydrothermal system within the axial valley of Escanaba Trough.

A tight grid of 1 to 2 km spacing swaths in both the NNW and NE directions provided two complete sidescan mosaics with different image angles for interpreting the surficial geologic character of the 18 km x 25 km region centered on the hydrothermal mounds that are the drill target for the ODP Leg 169 on the ship Joides Resolution with Rob Zierenberg (USGS, Menlo Park, California) as co-chief scientist. The sidescan sonar data, seabeam swath bathymetry and sub-bottom data were all processed (95 kb) on board ship by the scientists. The completed sidescan mosaics and sub-bottom profiles will be used on the ODP cruise.

The 4.5 kHz seismic system routinely imaged over 50 meters below the seafloor and provided a superb stratigraphic record of the interlinked volcanism, deformation and turbidite deposition within Escanaba Trough. USGS scientists used these records to identify features that had been seen on previous cruises by submersible vehicles and an underwater towed cameras.

The 160 cu.in. single channel airgun data provided an excellent image of the entire sediment column and basement structures for the entire area, at times exceeding 1000 m of penetration. The Escanaba Trough survey revealed a narrow 4-km wide zone of neotectonic activity with the most active faulting and volcanic activity.

The recent basaltic flows provided bright sidescan targets enabling scientists to map their areal extent and to identify their likely eruption source a fissured volcanic hill 2 km east of the hydrothermal Central Hill of NESCA. The hydrothermal fields on the Central Hill and Southwest Hill were clearly identifiable. Growth faults in the 4.5 kHz data provide a fine data base for establishing the faulting history within the neotectonic zone to complement the volcanic history.

This was the last cruise for the Discoverer, ending a 30-year career of oceanographic research. The ship was escorted in by the Chief Seattle (70 kb), a fire boat, spraying water as the ship approached the locks into Lake Union. Approximately eighty family members and friends of the crew boarded in Everett, Washington to ride the ship through the locks (74 kb) into Lake Union, where the Discoverer is docked at NOAA's Pacific Marine Center.

An abstract about the data collected on this cruise will be presented at the December '96 American Geophysical Union Meeting in San Francisco, California .