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 data were collected using an SEA Ltd SWATHplus 234-kHz Interferometric Bathymetric Sonar (<http://www.sea.co.uk/swathplus.aspx?nav=products
> ) during survey operations. The SWATHplus system was mounted as a sidemount and positioned on the starboard side of the R/V Megan Miller. Line spacing varied from 40 meter in ~ 5 to 15 meter water depth to 70 meters in ~15 - 25 meter water depth. Data files were recorded by SEA software SWATHplus 3.05.19 in the SWATHplus SXR format. Data collection parameters are saved into a SWATHplus session file in SEA's SXS format that can be later used for data replay.
Pitch, roll, heading, and heave information were recorded using a Coda Octopus F180R motion reference unit (MRU) (see: <http://www.codaoctopus.com/motion/f180/index.asp
>) and transmitted via network connection to the SWATHplus data collection software. The MRU sensor was mounted directly above the SWATHplus transducers, thereby minimizing "lever arm" offsets which can lead to positioning and depth errors in the bathymetric data. The F180R series MRU uses two L1 antennas for position and heading accuracy. These were mounted directly above the MRU with an offset of 6.76 meters. The antennas are mounted on a rigid horizontal pole with a separation of 1 meter and are offset from the MRU in a forward/aft configuration. The forward offset of the primary antenna from the MRU is 0.5 meters, with no port/starboard offset.
Sound Velocity Profiles (SVP): Fourteen sound velocity casts were collected at various intervals during the USGS cruise 07011 using an Applied Microsystems SV Plus v2 instrument (see <http://www.appliedmicrosystems.com/products/productDetails.aspx?id=1
> for details). Sound velocity profiles (SVP) were generally collected at the eastern and western ends of the survey area, corresponding to the start and/or end of geophysical line data collection. This procedure was primarily followed to coordinate collection of SVP casts with the beginning or end of trackline. This was acceptable because only small variations in the speed of sound were noted throughout the survey area and negligible refraction artifacts were visible within the swath bathymetric data.