Acoustic backscatter from 2013 interferometric swath bathymetry systems survey of Columbia River Mouth, Oregon and Washington

The USGS R/V Parke Snavely was equipped with an Applanix Position and Orientation System for Marine Vessels (POS/MV) inertial motion unit (IMU) for the duration of the survey. The POS/MV received Differentially Corrected Global Positioning System (DGPS) aided positional navigation from dual Trimble model 4000 DGPS receivers, and commercial C-Nav satellite differential stations. The DGPS data were combined with the inertial vessel motion measurements (pitch, roll, and heading) directly within the POS/MV hardware so that high-precision position and attitude corrections were fed in real time to the sonar acquisition equipment. Sound velocity measurements were collected continuously with an Applied Micro Systems Micro sound velocimeter (SV) deployed on the transducer frame for real-time sound velocity adjustments at the transducer-water interface. Additionally, water column sound velocity profiles (SVP) were collected approximately every 2 hours throughout each survey day with an Applied Micro Systems, SvPlus 3472. DGPS data and measurements of vessel motion were combined in the POS/MV hardware to produce a high-precision vessel attitude packet. This packet was transmitted to the Swath Processor acquisition software in real time and combined with instantaneous sound velocity measurements at the transducer head before each ping. The returned samples were projected to the seafloor using a ray-tracing algorithm working with the previously measured sound-velocity profiles in SEA Swath Processor. A series of statistical filters were applied to the raw samples that isolated the seafloor returns from other spurious targets in the water column. Finally, the processed data were stored line-by-line in both raw (.sxr) and processed (.sxp) trackline files. The raw 16-bit backscatter data recorded simultaneously with bathymetry by the SWATHplus system was georeferenced and gain-normalized by the program SXPEGN written by the USGS to enhance the backscatter imagery. The program normalizes for time-varying signal loss and beam directivity differences. The resulting normalized amplitude values are re-scaled to 16 bit, and gridded into a GeoJPEG using GRID Processor Software supplied by SEA. The image was imported into ESRI GIS software and projected to North American Datum of 1983. The image was then converted to a GeoTIFF file using ESRI GIS software.

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