MGL1109backsgeo.sd: Multibeam backscatter data collected by the U.S. Geological Survey in the Gulf of Alaska in 2011 during cruise MGL1109, 100-meter gridded data in Fledermaus digital terrain format, geographic coordinate system

Multibeam bathymetry and backscatter data were collected using a 12 kHz Kongsberg Simrad EM-122 multibeam echosounder and SIS (v3.8.2) acquisition software. The system consists of two hull-mounted transducer arrays, a transmit unit (150 deg. x 1 deg.) located along the ship centerline, and a receive unit (1 deg. x 30 deg.) that is abaft and oriented athwart-ships. The transducer pod is tilted 4 degrees towards the bow to reduce noise from cavitation and bubbles. The system forms a swath of 288 beams that produce an across-track profile of 432 soundings. The SIS software processes range/angle data from each ping during acquisition, merging position, inertial-motion, and sound-velocity data to derive a solution for vessel motion and raybending, and calculate sounding depths and positions. Sound velocity information entered into SIS was obtained from the Applied Micro Systems Micro SV mounted on the transducer pod and 52 XBT profiles. XBT profiles were extended to 12 km depth with data generated by the unix program 'mblevitus', a module of the multibeam processing package MB-System (Caress and Chayes, 2006). Average water column salinity of 34.4 ppt was determined by extrapolating data from XCDT casts. Multibeam data were collected nearly continuously throughout the cruise, effectively mapping the seafloor along the course of the ship's track. Beam angle was set at 70 on each side throughout the program and swath widths were generally between 3 and 4 times water depth. SIS logged bathymetry and backscatter solutions to the Simrad '.all' format for archival purposes.

EM122 backscatter was processed and mosaicked using the FMGeocoder module in IVS Fledermaus. The Automatic Processing option (Mosaic only) was utilized with the EM122 Beam Time Series (from *.all raw files) selected as the backscatter source. A single tile mosaic, with 100 m per pixel resolution, was created for the entire survey area. The final mosaic was exported to these file formats: Fledermaus SD, ESRI ASCII Raster, and XYB format (Easting, Northing, dB value).
The following project setup and processing parameters were specified: Project Coordinate System: Horiz. CS: WGS84,UTM,Zone 6N Custom Survey Area: XMIN,YMIN,XMAX,YMAX = 169100, 5385700, 1181500, 6408000 EM122 Sensor Config: Tx1:Xoff,Yoff,Zoff,Roff,Poff,Yoff = 20.92,0.57,8.46,0.16,-0.08,359.99 Tx2:Xoff,Yoff,Zoff,Roff,Poff,Yoff = 16.07,0.05,8.53,0.16,-0.08,0 IMU:Xoff,Yoff,Zoff,Roff,Poff,Yoff,lat = 0,0,0,-0.51,-0.23,0.43,0 Line Processing Parameters: Backscatter: Apply Beam Pattern Correction Beam Angle Cutoffs: start,cut = 0, 90 Backscatter Range: Calibrated,Min,Max = -70,10 AVG: Apply AVG - Flat Algorithm - Window Size 30 Statistics: Log space (dB) Navigation: Linear Interp. Format: Kongsberg(.all) Navigation Src.: Use Default Dual Swath Comp.: First O Sonar Defaults: Default Vals - Custom Override All - Sonar Type: Simrad EM122 MBES PrimFrq,SecFrq,MAN,MNB,ATBW,CTBW,SSS = 12000,0,120,484,2,4,1500 PusleL,PSR,SSR,Spread,TxPwr,RecGain,Abs = 5000,1001,0,0,1,1,30 Mosaicking Parameters: Line Blending = 50 percent Style, FiltType = Blend,dB Mean

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