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).
Name	Value
Sonar Frequency	234.5 kHz
Maximum Water Depth	120 m
Maximum Swath Width	300 m (typically 7 to 12 times water depth)
Resolution Across Track (best case)	5 cm
Transmit Pulse Length	34 ms to 500 ms
Ping Repetition Rate
150-m Swath Width	10 pings per second
300-m Swath Width	5 pings per second
Vertical Accuracy (Range Dependent)
57 m	0.1 m
114 m	0.2 m
171 m	0.3 m

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