Sound velocity profiles are acquired in order to measure the speed of sound in the water column during survey operations. This information is used to correct any refraction artifacts that may be present within the swath bathymetric data due to unaccounted for changes in the speed of sound throughout the water column. As such, the swath bathymetric acquisition is described here, as well as acquisition procedures for the sound velocity profiles.
Swath-bathymetric and acoustic-backscatter data were acquired with a SEA, Ltd., Submetrix 2000 Series interferometric sonar operating at a 234-kHz frequency. The SEA Submetrix 2000 Series transducers were mounted at the bow of the USGS R/V Rafael. Approximately 37 km of swath bathymetric data were collected.
SEA RTS2000 acquisition software (version year 2005) was used to digitally log the bathymetric data at a maximum 50 meter range (100 meter swath width) and 2048 samples per ping in the SEA SXR format. In shallow water areas, the swath width did not achieve the full 50-meter range, but varied from roughly 5x water depth to the maximum 50-meter range (i.e. swath width varied from approximately 15 meter to 100 meters depending on water depth). Data collection parameters are saved as a RTS2000 session file in SEA SXS format.
An Octopus F180R Attitude and Positioning system (see: http://www.codaoctopus.com/motion/f180/index.asp
) recorded ship motion (heave, pitch, roll, and yaw). These data were transmitted via network connection to the RTS2000 data acquisition software. The Octopus F180R Inertial Measurement Unit (IMU) was mounted directly above the SEA Submetrix 2000 Series transducers, to minimize lever arm offsets that can lead to positioning errors. The F180R uses two L1 antennas for position and heading accuracy. The antennas are mounted on a rigid horizontal pole, 3 meters above the F180R IMU, with a horizontal separation of 1 meter and are offset from the IMU in a forward/aft configuration. The forward offset of the primary antenna from the IMU is 0.5 meters, with no port/starboard offset.
Eight sound-velocity profiles were acquired during survey operations at roughly 1 to 3 hour intervals using an Applied Microsystems SV Plus Velocimeter (Applied Microsystems, 2008).
Vertical accuracy of the raw data based on system specifications may approximate 1% of water depth, 0.01 to 0.15 meters within the survey area. However, overall vertical accuracies on the order of 0.5 meters are assumed based on the following considerations: WAAS navigation vertical accuracies; the Coda Octopus F180 Attitude and Positioning system, used to correct for vessel roll, pitch, heave, and yaw, has a theoretical vertical accuracy of a few mm; refraction artifacts were minimized by acquiring a range of sound velocity profiles with a hand-casted Applied MicroSystems SV Plus sound velocimeter during the survey. Changes in ship draft due to water and fuel usage were not considered.