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The surveys were conducted using the research vessel R/V Parke Snavely outfitted with an interferometric sidescan sonar for swath mapping and Real-Time Kinematic navigation equipment for accurate shallow water operations.
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The bathymetric surveys were conducted using a 234.5 kHz SEA (Systems Engineering & Assessment Ltd) SWATHplus-M phase-differencing sidescan sonar. The sonar was pole-mounted on the 34-foot USGS mapping vessel R/V Parke Snavely, and affixed to a hull brace. Real-time kinematic (RTK) GPS position data were passed through a CodaOctopus F180 intertial 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 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.
Geodetic control for the survey was established using a shore based Global Positioning System (GPS) base station broadcasting Real Time Kinematic (RTK) corrections to the survey vessel via UHF radio link. The base station was located at the base of Ediz Hook. The base station was programmed using the WGS84 (G1150) reference frame with an Epoch of 2010.1548. Average Opus Solution coordinates for the station are:
Reference Frame: WGS84 (G1150) Epoch: 2010.1548 Latitude: N 48° 08 06.01943 Longitude: W 123° 28 06.90674 Ellipsoid Height: -11.355m
The average values for the derived OPUS solution for MILL are: Reference Frame: ITRF00 [same as WGS84 (G1150)] Epoch: 2010.1548 Latitude: N 48° 08 06.01936 Longitude: W 123° 28 06.90685 Ellipsoid Height: -11.351m
The differences to be added to the RTK broadcast locations are: Latitude: -0.00007 Longitude: +0.00011 Ellipsoid Height: +0.004m
Using UTM coordinates as a comparison: The base station was programmed using the following coordinates: Reference Frame: NAD83, UTM zone 10 Epoch: 2002.0000 Northing: 5331411.376m Easting: 465138.251m Ellipsoid Height: -11.042m Orthometric Height: 9.155m (based Geoid09)
The R/V Snavely was equipped with a CodaOctopus F180 attitude and positioning system for the duration of the survey. The F180 is running F190 firmware, and receives real-time kinematic (RTK) corrections directly. The RTK GPS data (2 cm error ellipse) are combined with the inertial motion measurements directly within the F190 hardware so that high-precision position and attitude corrections are fed in real-time to the sonar acquisition equipment. The WGS84 (G1150) Epoch 2010.1548 3-dimensional reference frame was used for all data acquisition.
Sound velocity measurements were collected continuously with an Applied Micro Systems Micro SV deployed on the transducer frame for real-time sound velocity adjustments at the transducer-water interface. The Micro SV is accurate to +/- 0.03 m/s. In addition, sound velocity profiles (SVP) were collected with an Applied Micro Systems, SvPlus 3472. This instrument provides time-of-flight sound-velocity measurements by using invar rods with a sound-velocity accuracy of ±0.06 m/s, pressure measured by a semiconductor bridge strain gauge to an accuracy of 0.15 percent (Full Scale) and temperature measured by thermistor to an accuracy of 0.05 degrees Celsius (Applied Microsystems Ltd., 2005).
GPS data and measurements of vessel motion are combined in the F180 hardware to produce a high-precision vessel attitude packet. This packet is transmitted to the Swath Processor acquisition software in real-time and combined with instantaneous sound velocity measurements at the transducer head before each ping. Up to 20 pings per second are transmitted with each ping consisting of 2048 samples per side (port and starboard). The returned samples are projected to the seafloor using a ray-tracing algorithm working with the previously measured sound velocity profiles in SEA Swath Processor (version 3.05.18.04). A series of statistical filters are applied to the raw samples that isolate the seafloor returns from other uninteresting targets in the water column. Finally, the processed data is stored line-by-line in both raw (.sxr) and processed (.sxp) trackline files. Processed (.sxp) files were further processed with sxpegn (build 151) by David Finlayson (USGS) to remove erroneous data from the files and make valid gain-normalized amplitude data for processing backscatter data.
The raw 16-bit backscatter recorded simultaneously with the bathymetry by the SWATHplus was georefrenced and gain-normalized by the program SXPEGN, software purpose written by the USGS to enhance the backscatter of the SWATHplus system. The program normalizes for time-varying signal loss and beam directivity differences. The resulting normalized amplitude values are re-scaled to 16-bit, binned at 1m resolution in SEA Grid Processor and exported as point files. The binned points are gridded using Surfer with an inverse distance weighted algorithm using a 3 m search radius and a 0.25 m smoothing parameter. The data were exported from Surfer in ESRI ASCII Grid format.
Normalized acoustic backscatter shows the relative strength of the acoustic signal returned from the seafloor. The normalization process compensates for acoustic signal strength differences due to time varying gain (TVG), spreading and many static sonar artifacts (those that repeat from ping-to-ping). The values are comparable across the entire dataset regaurdless of water depth or distance and angle from the transducer. Low normalized acoustic backscatter values indicate relatively low acoustic target signal strength; High normalized acoustic backscatter values indicate relatively high acoustic target signal strength.
Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Please recognize the U.S. Geological Survey (USGS) as the source of this information. Physical materials are under controlled on-site access. Some USGS information accessed through this means may be preliminary in nature and presented without the approval of the Director of the USGS. This information is provided with the understanding that it is not guaranteed to be correct or complete and conclusions drawn from such information are the responsibility of the user.
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