NBP09-01 Cruise Report - British Oceanographic Data Centre

6 Seafloor MappingFrank Nitsche, Kathleen Gavahan6.1 ObjectiveA large portion of the bathymetry of the continental margin around Antarctica is still unmapped. In theAmundsen Sea, scientific questions concerning warm water entering the shelf and collecting in deep innershelf troughs depend on the local and regional bathymetry. Detailed bathymetry data were also neededto determine depth and location of CTD and mooring sites as well as for planning Autosub operations.Improving the existing bathymetry of the Amundsen Sea continental shelf also better defines existing troughsand supports reconstruction of past ice flow on the continental shelf.We have thus been running the swath-mapping system throughout NBP0901 The main mapping teamconsisted of Kathleen Gavahan (Raytheon) and Frank Nitsche (LDEO) and was supported by several others,especially Ken Mankoff, Adrian Jenkins, Robin Robertson, Chris Little, Stan Jacobs, Andy Webb, PierreDutrieux, and Sharon Stammerjohn, who helped edit raw multibeam files.6.2 System Description and OperationThe main equipment used for seafloor mapping on this cruise was the Simrad EM-120 swath-mapping system.It operates at a 12 kHz main frequency and receives up to 191 beams from angles up to 65 o to each sideof the ship, providing a resolution of ∼1.5 o of the seafloor and a coverage of ∼4 times the water depth.To adjust for changes in sound velocity, CTD and XBT data were used to update to the local conditionsthroughout the cruise.The raw data are recorded digitally and displayed in real-time. Once a day the raw data were processed andedited manually for outliers and false bottom returns using the mb-system 5 software package. The finaldata are stored in MBsystem format 57 and will be archived at the Antarctic Multibeam Synthesis Database(http://www.marine-geo.org/antarctic/).In addition to the swath-mapping system we almost continuously ran a sub-bottom profiler to provideindependent depth measurements. Sub-bottom data can also be used to determine sediment sample locations.At the start of the cruise a Knudsen 320B echosounder with 3.5 kHz main operating frequency was used.From January 16th 2009 we used the Ocean Data Equipment Corporation Bathy 2000 echosounder andsub-bottom profiler. The Bathy 2000 transmits a chirp signal of 3.5 to 4 kHz. It determines the bottomand images the uppermost subbottom sediment layers down to a few 10s of meters depending on sedimentcharacter. The data were archived digitally in the Bathy2000 variant of the SEGY-format.Additional bathymetry data of previous NB Palmer , JC Ross, Polarstern, and Oden cruises provided extraguidance and helped to avoid accidental duplication of existing tracks.6.3 System Performance and ProblemsThroughout the cruise, both the EM-120 swath-bathymetry and the Bathy2000 echosounder operated normallyunder most conditions, but had problems with receiving good data in sea ice. The Bathy2000 wasonly disturbed by heavy pack ice but the EM-120 multibeam had problems with heavy pack ice and certaintypes of newly formed ice such as grease ice and nilas. The ice gets under the ship, blocking signals comingto and from the transducers, producing bad data even in easy and calm conditions. We rarely encounteredsuch ice conditions during NBP0901, but a larger issue was the drastic drop in data quality in rough seas.NBP09-01 Cruise Report (p. 43 of 83) Revised February 27, 2009