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Open-File Report 2011–1226

Bathymetry and Acoustic Backscatter—Elwha River Delta, Washington

Survey Outline

The 2010 mapping mission (U.S. Geological Survey (USGS) Field Activity ID: S-6-10-PS) collected high-resolution bathymetry and coregistered acoustic backscatter of the delta and shallow mudflats offshore of the Elwha River, Strait of Juan de Fuca, Washington State (fig. 1). The survey was completed in 10 days, from February 22 to March 3, 2010, and consisted of 195 survey lines covering approximately 7.2 km2 in elevations ranging from about +1.0 m to -90 m relative to NAVD88 (fig. 1).

Bathymetric surveys were conducted using a 234.5 kHz SEA (Systems Engineering and Assessment Ltd.) SWATHplus-M phase-differencing sidescan sonar. The sonar was pole-mounted on the 34-foot USGS mapping vessel R/V Parke Snavely (fig. 2), and affixed to a hull brace (fig. 3). Real-time kinematic (RTK) GPS position data were passed through a CodaOctopus F180 inertial measurement unit (IMU) to the sonar hardware and data collection software. Sonar heads, GPS antennae, and the IMU were surveyed in place to a common reference frame with a Geodimeter 640 Total Station. The R/V Parke Snavely was outfitted with three networked workstations and a navigation computer for use by the captain and survey crew for data collection and initial processing. See table 1 for the sonar system specifications.

Table 1. SWATHplus-M Sonar Specification (System Engineering and Assessment Ltd., 2004).
Sonar Frequency234.5 kHz
Maximum Water Depth120 m
Maximum Swath Width300 m (typically 7 to 12 times water depth)
Resolution Across Track (best case)5 cm
Transmit Pulse Length34 ms to 500 ms
Ping Repetition Rate
150-m Swath Width10 pings per second
300-m Swath Width5 pings per second
Vertical Accuracy (Range Dependent)
57 m0.1 m
114 m0.2 m
171 m0.3 m
For more information, contact the PCMSC team.

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