Mapping the Northeast Lau Basin
Susan Merle
3-D image of the NE Lau Basin seafloor. Bathymetry data were collected during expeditions to the area is 2008, 2009 and on the present mission. Data are 2 times vertically exaggerated. The main objective of this expedition has been to further our understanding of the extensive volcanic activity in the northeast Lau Basin. We wouldn’t know where to look if we didn’t have a good map of the seafloor. Seafloor mapping was performed between TowCam and CTD operations on this mission. Some areas were mapped to fill gaps in our data coverage, others were mapped to perform surface differencing calculations which tell us where new lava has erupted or landslides have occurred. The bathymetry data are gridded and can be displayed as a 2-D map, a 3-D image, or utilized interactively as a layer in a GIS (Geographic Information System) database.
The mapping system is mounted on the hull of the R/V Kilo Moana. A sound signal called a ping is sent from the ship to the seafloor and bounces back to the ship. The length of time that it takes for that round trip is how the seafloor depth is determined. Over the years seafloor mapping systems have improved immensely. During the 1980s the NOAA Vents Program mapped the Juan de Fuca ridge with the first generation of seafloor mapping systems called SeaBeam classic. That system had 16 soundings, called beams, on each ping. Coverage was 75% of the water depth, eg. if the water was 1000 meters deep the swath on the bottom was 750 meters across. The new EM122 12 kHz multibeam system on the R/V Kilo Moana has a possible 432 beams per ping. The coverage averages 4 times water depth (4000 meters across in 1000 meters of water). The resolution of the data has increased dramatically due to the increase in data density. The EM122 is a new generation of multibeam system, capable of mapping the water column as well as the seafloor. The water column data images biomass (fish, zooplankton, etc.) as well as bubble plumes rising from the seafloor. On our past expedition to the Mariana Arc we successfully imaged gas bubble plumes rising from the eruptive vents at the summit of NW Rota volcano (http://nwrota2010.blogspot.com/2010/03/volcano-mapping.html).
On this expedition we departed from the island of Upolu Samoa, steamed southwest crossing the Tonga Trench to the Lau Basin – mapping all the way. Mapped depths range from 150 meters near the island to 7200 meters over the trench. Approximately 4000 square kilometers of seafloor was mapped in the expedition operations area where water depths range from 1000 to 3800 meters.
Bathymetry data collected on the 2010 expedition are outlined in black.
