Acoustic Backscatter of the Sacramento River, from the Feather River to Knights Landing, California in February 2011

Acoustic-survey data were collected using a 468.75 kHz SEA (Systems Engineering & Assessment Ltd) SWATHplus-H phase-differencing sidescan sonar. The survey vessel was equipped with a CodaOctopus F180 attitude and positioning system for the duration of the survey. The F180 received Real Time Kinematic (RTK) corrected Global Positioning System (GPS) data that were broadcast from shore to the survey vessel via UHF radio link. The RTK GPS data were combined with the inertial vessel motion measurements (pitch, roll, and heading) directly within the F180 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 deployed on the transducer frame for real-time sound velocity adjustments at the transducer-water interface. In addition, sound velocity profiles (SVPs) were collected with an Applied Micro Systems, SvPlus 3472. GPS data and measurements of vessel motion were combined in the F180 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 SVPs in SEA Swath Processor. A series of statistical filters were applied to the raw samples to isolate 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-H system was georeferenced and gain-normalized by the program SXPEGN, which was 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. For further information on the parameters used for each instrument, see the accompanying methods summary in the report for the Data Release for S-03-11-CA.pdf (https://www.sciencebase.gov/catalog/file/get/588f8d74e4b072a7ac08c35e?name=Report_for_Data_Release_Survey_S-03-11-CA.pdf).

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