Data Acquisition

Navigation

The Geographic Positioning System (GPS) base stations were established by USGS personnel for the purpose of this survey. Benchmark PAF1 was located in the eastern part of the survey area on a marsh island west of Fish Bayou, benchmark PAF2 was located in the middle of the survey area on the eastern shore of Oyster Bayou, and benchmark PAF3 was located in the western portion of the survey on the middle shore of Point Au Fer Island (fig. 2). The base stations were equipped with Ashtech Proflex Differential GPS (DGPS) 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).

GPS instrumentation was duplicated on all survey vessels (rovers) with the exception of the R/V Shark (15BIM07) and the R/V Chum (15BIM08) on which Ashtech Global Navigation Satellite Systems (GNSS) were used instead of choke-ring antennas. The base receivers and rover receivers recorded positions concurrently at all times throughout the survey. The rovers R/V Shark and R/V Chum recorded every 0.1 s throughout the survey, while the R/V Jabba Jaw and the R/V Sallenger rovers recorded every 0.2 s.

Soundings

Depth soundings were recorded at 50-millisecond (ms) intervals on all survey platforms using ODOM Echotrac CV 100 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 15BIM05 and 15BIM06; and version 13.0.09.17 for 15BIM07 and 15BIM08), 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 recorded on the R/V Sallenger and R/V Jabba Jaw only, using a Teledyne TSS Dynamic Motion Sensor “DMS-25” and Teledyne TSS Dynamic Motion Sensor “DMS-05,” respectively, every 50 ms (table 4). A motion sensor device was not used on the R/V Shark or the R/V Chum. Motion errors on these vessels were minimized by using a rapid GPS recording interval of 0.1 s, a minimal vertical distance of 1.225 m and 1.296 m, respectively, from antenna to sounding device, and a narrow 4 degree transducer beam.

Sound Velocity

Sound velocity profile (SVP) measurements were collected using two SonTek Castaway conductivity, temperature, and depth (CTD) instruments (table 5) and one Valeport 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 35 sound velocity casts were taken throughout the survey in water depths ranging from 1.13 to 7.00 m. The average sound velocity recorded was 1,532.57 meters per second (m/s), and values ranged from 1,514.20 to 1,542.056 m/s. The 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.

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