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Data Series 847

Coastal Bathymetry and Backscatter Data Collected in 2012 from the Chandeleur Islands, Louisiana

Index
Abstract
Introduction
Survey Overview and acquisition
Data Processing
Error Analysis
Survey Products
Data Downloads
Logs
Acronyms
References Cited
 

Survey Overview and Data Acquisition

Geophysical survey data were collected during concurrent cruises from July 20 to August 3, 2012, covering an area of approximately 80 square kilometers (km2). Single-beam data (12BIM04) were collected aboard the 22-foot (ft) RV Twin Vee (fig. 2), from which 495 line-km of bathymetry data were collected. Tracklines (248 lines) were primarily oriented perpendicular to shore and spaced approximately 100 meters (m) apart (fig. 3). Additional single-beam data were also acquired along six long-term change lines spaced approximately 1 km apart extending 6 km offshore on both the gulf and sound sides of the islands. These lines were designed to reoccupy previously collected single-beam bathymetric survey lines, allowing for comparison of bathymetric profiles from multiple surveys independent of larger-scale mapping efforts.

Interferometric swath and seismic data (12BIM03) were collected from the 26-ft RV Survey Cat (fig. 4). A total of 458 line-km of data were collected utilizing an integrated suite of geophysical instruments capable of mapping the nearshore environment around the island, including an interferometer that provided swath bathymetry with backscatter (this report) and a high-resolution chirp subbottom profiler that imaged the shallow subbottom (Forde and others, 2013). Swath bathymetry, backscatter, and chirp tracklines (124 lines) were generally shoreline parallel, and line spacing varied from 50 m to 250 m, depending on water depth (fig. 5). Tie lines, or tracklines perpendicular or oblique to shoreline orientation, were also surveyed as a check on bathymetric soundings and to provide dip or apparent dip lines for the chirp profiles. The North American Vertical Datum of 1988 (NAVD88) orthometric elevation range for all of the bathymetric data was -0.42 m to -15.42 m, derived from the National Geodetic Survey's (NGS) geoid model of 2009 (GEOID09).


Single-Beam Bathymetry | Swath Bathymetry

Single-Beam Bathymetry

Navigation

The Global Positioning System (GPS) base station was assembled at a previously installed USGS benchmark identified (TMRK), located on a wide sand flat on the sound side of the island. A second base station was erected on a temporarily installed benchmark (BERM), approximately 6.5 km north of TMRK on an emergent island remnant (fig. 3) in order to reduce the Differential Global Positioning System (DGPS) baseline length between the base station and rover GPS in the northern portion of the survey area. The Ashtech Z-Xtreme GPS receivers (table 1) recorded the 12-channel full-carrier-phase positioning signals (L1/L2) from satellites via a Thales choke-ring antenna. A similar GPS instrument combination was duplicated on the survey vessel (rover). The base receivers and the rover receiver recorded their positions concurrently at 1-second (s) intervals throughout the survey. HYPACK version 10, a marine surveying, positioning, and navigation software package, managed the planned-transect information and provided real-time navigation and instrumentation-status information to the boat operator.

Soundings and Motion

Aboard the RV Twin Vee, depth soundings were acquired at 50-millisecond (ms) intervals using a Knudsen 320BP echosounder system with a 200 kilohertz (kHz) transducer (fig. 2) (table 2). Boat motion was recorded at 50-ms intervals using a Teledyne TSS Dynamic Motion Sensor (TSS DMS-05) (table 3). Data from the GPS receiver, motion sensor, and fathometer were recorded in real-time and merged into a single raw data file (.raw) in HYPACK, with each device string referenced by a device identification code and time stamped to UTC.

Sound Velocity

Sound velocity measurements were collected using an Applied Microsystems Smart Sound Velocity Profiler (SVP). The instrument was periodically cast overboard to observe changes in water column speed of sound (SOS). The recorded daily mean of sound velocity for the duration of the survey was 1532.26 meters/second (m/s).

research vessel Twin Vee, mobilized with the single-beam bathymetry equipment configuration.

Figure 2. The research vessel Twin Vee, mobilized with the single-beam bathymetry equipment configuration. [Click to enlarge].

    Single-beam bathymetry tracklines for the 2012 Chandeleur Islands geophysical survey.

Figure 3. Single-beam bathymetry tracklines for the 2012 Chandeleur Islands geophysical survey. Island area extent is derived from USGS lidar data collected in February 2012. [Click to enlarge]

Table 1. Specifications for the Ashtech Z-Xtreme GPS receivers used during the single-beam bathymetry survey, Ashtech_ZXtreme (194-KB PDF).

Ashtech Z-Xtreme Specifications
Static horizontal 5 millimeters + 1 part per million
Static vertical 10 millimeters + 1 part per million
Kinematic horizontal 1 centimeters + 1 part per million
Kinematic vertical 2 centimeters + 1 part per million
Table 2. Specifications for the Knudsen 320 Echosounder used during the single-beam bathymetry survey (Knudsen_320BP) (1,638-KB PDF).


Knudsen 320 Specifications
Pulse length Up to 4 milliseconds
Water depth range 5 – 5000 meters
Transmission power Up to 1 kilowatt per channel
Resolution 1.0 centimeter
Vertical (depth) accuracy 1.0 centimeter, +/- 0.1 percent of depth value (200 hertz)
Table 3.  Specifications for the Teledyne TSS-DMS-05 motion sensor used during the single-beam bathymetry survey (TSS-DMS-05) (202-KB PDF).

Teledyne TSS-DMS-05 Specifications
Roll and pitch 0.05 degrees
Heave 5 percent of heave amplitude or 5 centimeters



Interferometric Swath Bathymetry and Backscatter

Navigation and Motion

The position data string was integrated in real-time using the Coda-Octopus F190R Precision Attitude and Positioning System, which includes a waterproof inertial measurement unit (IMU). The IMU is located between the transducer heads to minimize lever arm geometry errors between the observed depths and associated vessel motion. Real-time corrected positions were acquired from an OmniSTAR HP (High-Precision differential global navigation satellite system) satellite constellation subscription. In addition to the position string, the F190R records heave, roll, and pitch of the vessel during acquisition (table 4), which are used to calculate the precise position of a measured reference point near the head of the transducers. Equipment offsets were entered into the F190 software prior to instrument calibration and survey commencement. Planned survey tracklines were loaded into HYPACK as a navigation guide for the boat operator.

Soundings and Backscatter

Interferometric swath bathymetry and backscatter data were collected aboard the R/V Survey Cat using a 468 kHz Systems Engineering and Assessment Ltd. (SEA), SWATHplus-H (high frequency) interferometric sonar system (table 5). The transducers were mounted on a sled attached to a rail system fastened between the catamaran hulls, allowing the instrument to align directly below the GPS antennae to minimize geometry calculation errors (fig. 4). OmniSTAR HP position correction data and motion data from the IMU were integrated with interferometric soundings in the SWATHplus software package, with position and calibration offsets pre-defined by a session file (.sxs), allowing for the acquisition of real-time- corrected depths. Prior to deployment, all equipment offsets were surveyed in dry dock with a laser total station. During the survey all swath tracklines were recorded in SEA raw data format (.sxr).

Sound Velocity

A Valeport Mini Sound Velocity Sensor (SVS) was attached to the transducer mount and collected continuous speed of sound (SOS) measurements at the depth of the transducers. These values were directly read and incorporated into the SWATHplus acquisition software giving real-time SOS at the transducer head while underway (table 6). In addition, a separate Valeport miniSVP sound velocity profiler (SVP) was used to collect SOS profiles; water surface to seafloor; at strategic intervals throughout the survey (table 7). Accurate SOS values throughout the water column are essential to accurate sea floor mapping with a swath system, particularly at swath beam range extents, and most notably at depths greater than 3 meters (m), where SOS has commonly demonstrated variability due to thermoclines and changes in salinity. If SOS is inaccurate, water column refraction will significantly decrease data precision and accuracy throughout the beam range of the swath system, particularly at the far range, thus increasing manual post-survey processing time and interpolation uncertainty upon creation of a digital elevation model (DEM).

The research vessel Survey Cat mobilized with roof mounted navigation bracket and interferometric swath bathymetry equipment and IMU wetpod cradle, mounted to a sled and rail system between the catamaran hulls.

Figure 4.  The research vessel Survey Cat mobilized with roof mounted navigation bracket, interferometric swath bathymetry equipment, and IMU wetpod cradle, mounted to a sled and rail system between the catamaran hulls. [Click to enlarge].

    Swath bathymetry, backscatter, and chirp tracklines for the 2012 Chandeleur Islands geophysical survey.

Figure 5. Swath bathymetry, and backscatter tracklines for the 2012 Chandeleur Islands geophysical survey. Island area extent is derived from USGS lidar data collected in February 2012. [Click to enlarge]

Table 4. Specifications for the Coda Octopus F190R inertial measurement unit (IMU) used during the swath bathymetry survey, Coda Octopus Group, Inc. (F80 Series) (1.84-MB PDF).

Coda Octopus F190 Specifications
Positional accuracy 0.10 meters with Marinestar subscription
Velocity 0.014 meters/second
Roll and Pitch 0.025 degrees
True Heading 2 meter baseline – 0.05 degrees
4 meter baseline – 0.025 degrees
Heave (1s) 5 centimeters or 5 percent (on-line)

Table 5. Specifications for the SWATHplus-H sonar system (SEA Ltd.) transducers used to collect interferometric swath bathymetry data during the survey (SWATHplus_Specifications) (175-KB PDF).

SWATHplus-H Sonar Specifications
Sonar Frequency 468 kilohertz
Maximum Water Depth 50 meters
Maximum Swath Width 100 meters
Resolution Across Track (best case) 1.0 centimeters
Transmit Pulse Length 8.5 – 500 microseconds
Ping Repetition Rate (300m Swath Width) 7.5 pings per second
Vertical (depth) Accuracy, (range dependent)
57 meter range        0.1 meters

Table 6. Specifications for the Valeport Mini Sound Velocity Sensor (SVS) used to collect continuous sound velocity, co-located with the bathymetry transducers and IMU (Valeport, Ltd.).

Valeport MiniSVS Specifications
Range 1,375-1,900 meters/second
Resolution 0.001 meters/second
Accuracy (100 millimeter path length)
Last factory calibration error
0.001 meters/second
Total maximum theoretical error +/- 0.017 meters/second
-used to 3 decimal places
Acoustic frequency 1 hertz

Table 7. Specifications for the Valeport Mini Sound Velocity Profiler (SVP) used to collect positional sound velocity profiles through the water column (Valeport, Ltd.).

Valeport MiniSVP Specifications
Range 1,375-1,900 meters/second
Resolution 0.001 meters/second
Accuracy
Sound velocity +/- 0.02 meters/second
Temperature +/- 0.01 °Celsius
Pressure +/- 0.05 percent range
Acoustic frequency 1 hertz

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