Data Acquisition

Navigation

Three Geographic Positioning Systems (GPS) base stations were occupied during this survey. Benchmark MRK3 was located in the northern part of the survey area on the sound side of the islands near the Pelican fishing camp, benchmark SM01 was located in the middle of the survey area in the middle of the island at Monkey Bayou, and benchmark GRIG was located in the southern end of the survey on an abandoned oil drilling platform (fig. 2). The base stations were equipped with Ashtech Proflex GPS receivers (table 2) recording 12-channel, full-carrier-phase positioning signals (L1/L2) from satellites via Thales Choke-ring antennas, recording every 0.1 seconds (s).

This GPS instrumentation was duplicated on all survey vessels (rovers) with the exception of the R/V Chum (15BIM03) on which an Ashtech Global Navigation Satellite System (GNSS) antenna was used instead of a choke-ring antenna. The base and rover receivers recorded positions concurrently at all times throughout the survey. Rover R/V Chum and R/V Jabba Jaw recoded at intervals of 0.1 s and 0.2 s respectively, for the duration of the survey. R/V Sallenger recorded at intervals of 1.0 s for the first 4 days, and 0.2 s thereafter.

Soundings

Depth soundings were recorded at 50-millisecond (ms) intervals on all survey platforms using ODOM Echotrac CV100 fathometers (table 3) and eChart software version 1.4. All components including the GPS antenna, motion sensor, and the echo sounder transducers were mounted in-line vertically to avoid error resulting from offsets, helping to maintain low overall error for the survey. On each vessel, all data from the GPS receiver, motion sensor, and fathometer were recorded in real-time to a single raw data file (.raw) within the hydrographic survey software HYPACK (version 14.0.9.47 for 15BIM01 and 15BIM02; version 13.0.09.17 for 15BIM03), with each device string referenced by an identification code and a Coordinated Universal Time (UTC) time stamp to the nearest millisecond. HYPACK software was also used in conjunction with eChart software to monitor sounding data quality at all times throughout the survey.

Vessel Motion

Boat motion was recoded on the R/V Sallenger and R/V Jabba Jaw only, using Teledyne TSS Dynamic Motion Sensor “DMS-25” and Teledyne TSS Dynamic Motion Sensor “DMS-05” sensors, respectively, every 50 ms (table 4). A motion sensor was not used on the R/V Chum. To minimize motion errors on this vessel, a rapid GPS recording interval of 0.1 s, a minimal vertical distance of 1.29 m from antenna to sounding device, and a narrow 4 degree transducer beam were utilized.

Sound Velocity

Sound velocity profile (SVP) measurements were collected using two SonTek CastAway conductivity, temperature, and depth (CTD) instruments (table 5) as well as a Valport Mini Sound Velocity Profiler (SVP) (table 6). The instruments were periodically cast overboard to observe changes in the speed of sound (SOS). A total of 188 successful sound velocity casts were taken throughout the survey in water depths ranging from 0.13 to 13.96 m. The average sound velocity recorded was 1527.93 meters per second (m/s), and values ranged from 1519.85 to 1539.23 m/s. The recorded profiles were later incorporated into the post-processing software to correct the depth for temporal and spatial changes in the speed of sound (SOS) throughout the water column.

Note: To view PDF documents, you will need the latest version of Adobe Reader or similar software.

Disclaimer: Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.