Acoustic backscatter data from USGS Field Activity S-8-08-SC, northern Santa Barbara Channel, southern California

The backscatter data were collected using 234.5 kHz SEA (AP) Ltd. SWATHplus-M phase-differencing sidescan sonar aboard the USGS R/V Parke Snavely. The survey vessel was equipped with a CodaOctopus F190 attitude and position system. This system combines Real Time Kinematic (RTK) Global Positioning System (GPS) data (2 cm error ellipse) with inertial motion measurements so that high-precision position and attitude corrections are fed in real-time to the sonar acquisition equipment. Up to 10 pings per second are transmitted with each ping consisting of 2048 samples per side (port and starboard). The returned samples were projected to the seafloor using a ray-tracing algorithm working with the previously measured sound velocity profiles. A series of statistical filters were applied to the raw samples that discriminate the seafloor returns from other unintended targets in the water column. Finally, the processed data were stored, line-by-line in processed trackline files. The individual soundings stored in the processed trackline files were combined into a 2-m resolution rasters in the Grid Processor software. The 2-m binned amplitude values were exported from Grid Processor as an XYZ point cloud. The WGS84 (G1150) ellipsoidal coordinates of the survey were transformed into NAD83 (CORS96) ellipsoidal coordinates using a 7-parameter Helmert transformation available in the CS2CS program in the Proj.4 library (Version 442 for 32-bit Windows). The seven required parameters for the procedure were calculated using an Epoch date of 8 July 2008 according to procedure outlined in Soler and Snay (2004). Finally, the resulting NAD83 (CORS96) elevation postings were transformed into NAVD 1988 using the NOAA VDatum (Version 2.2.4) with the Geoid03 model. A script divided the 2-m binned point cloud into 2 x 2 km tiles. Each tile was gridded using Surfer (Version 9.2.397) and a linear Krigging algorithm and 0.20 cm nugget (the mean standard deviation of all 2-m cells in the survey). Finally, the 2-km tiles were mosaicked into a contiguous raster and converted to an Esri GRID.

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