Survey Overview and Data Acquisition

The geophysical data was collected during one USGS cruise (13CCT04) from August 13 to August 23, 2013, covering an area of approximately 205 square kilometers (km²). A total of 775 line-km of data were collected utilizing an integrated suite of geophysical instruments capable of mapping nearshore environments. Data were collected from the 97-ft RV Tommy Munro using an interferometric swath bathymetry (IFB) system that provided swath bathymetry with backscatter, a sidescan sonar (SSS) system that imaged the seafloor's surface (this report), and a high-resolution chirp subbottom profiler that imaged the shallow subbottom (Forde and others, in prep).

Swath bathymetry, backscatter, and chirp tracklines (129 lines) were generally shoreline parallel, and line spacing varied from 50 m to 250 m, depending on water depth (fig. 2). 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 Mean Low or Lower Water Level Datum (MLLW) range for all the bathymetric data was 3.83 m to 18.805 m, derived from applying a National Oceanic and Atmospheric Administration (NOAA) Tide Zone Model for this area.

SWATH BATHYMETRY

SIDESCAN SONAR

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 1), 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 RV Tommy Munro using a 468 kHz Systems Engineering and Assessment Ltd. (SEA), SWATHplus-H (high frequency) interferometric sonar system (table 2). The transducers were pole mounted on the starboard side of the vessel with the Coda Octopus F190R IMU and the transducers as one component and the Coda Octopus GPS antenna mounted atop the pole. For transit, the pole mount rotated out of the water parallel to the gunwal and held onto place with a cradle (fig. 3b). During survey operations, the pole mount is deployed and Coda Octopus GPS antennas are attached to the top providing in-line vertical placement of the instruments (fig. 3d). The 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. 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 3). 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 4). 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).

Figure 2.  Swath bathymetry and sidescan sonar tracklines for the 2013 Coastal Change and Transport geophysical survey off shore of Petit Island, Gulf Islands National Seashore, Mississippi. [Click to enlarge].

Figure 3. A, the research vessel Tommy Munro mobilized with the interferometric swath bathymetry equipment. B, the swath transducers, sound velocity sensor, and IMU wetpod attached to the base of the rigid pole mount. C, the Klein 3900 sidescan sonar "towfish", and D, the pole mount deployed with GPS navigation bracket affixed. [Click to enlarge]

Sidescan sonar

Data Acquisition

During the 13CCT04 cruise, a Klein 3900 dual-frequency sidescan sonar system (fig. 3c, table 5) was towed off the port side of the RV Tommy Munro approximately 5 meters astern. Sidescan data were acquired and recorded using Klein SonarPro software (Klein Associates, Inc.) with corrections for cable out and layback offsets. Horizontal offset values between the sidescan sonar towfish and the DGPS antennas were entered into Sonar Pro. The towfish motion was measured dynamically by internal sensors of the instrument, and towfish altitude (height from seafloor) was calculated by SonarPro. Data files were recorded in an Extended Triton Format (XTF). Ideally a towfish is flown at a relatively considerable distance from the vessel and the other acoustic instruments to avoid interference. Sources of acoustic interference are vessel vibrations and other instruments such as subbottom profilers and interferometric swath systems that utilize similar frequency ranges. However, in shallow-water surveying the optimal distance is often difficult to achieve due to the negative buoyancy of the towfish and risk of unpredicted isolated shoals.

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