U.S. Geological Survey Data Series 577

Equipment and Processing


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Side Scan Sonar

Side scan sonar is a type of technology used to interpret seabed features, material, and textures from acoustic backscatter response intensity. In this application the instrument (towfish) is towed by a cable aft of the vessel. Once activated, a fan-shaped acoustic pulse is repeatedly emitted downward to the seafloor, perpendicular to a vessel's navigation track, collecting a series of swaths stitched together to create a sonogram or seafloor image. The towfish can be operated with a range of frequencies; lower frequencies are recorded at a lower resolution but increasing swath range, and higher frequencies record with high resolution at the cost of swath range. High-intensity returns are indications of hard or dense surface material, such as rock or hard-packed sand, whereas low-intensity returns may infer silt or organic material. Intensity images are used in conjunction with physical "grab" samples or cores for the purposed of ground truthing the side scan mosaic.



Side Scan Sonar Acquisition System Components


Klein System 3900 Dual-Frequency Side Scan Sonar

CodaOctopus Octopus F190 Precision Attitude and Positioning System

OmniSTAR High Performance (HP) Differential GPS Correction

Klein 3900 Side Scan Sonar

Figure 5. Photograph of the Klein 3900 side scan sonar "towfish" instrument.


Side Scan Sonar Acquisition

During the 10CCT02 cruise, a Klein 3900 dual-frequency side scan sonar system (fig. 5) was towed off the port quarter of the vessel to collect information about seafloor surface material. SSS was acquired and recorded using SonarPro in an Extended Triton Format (XTF). All horizontal positions were offset relative to a central ship navigation point. Towfish motion is measured by the internal sensors of the instrument. The towfish altitude (height from seafloor) is calculated by the data processing engine, and in this case the altitude varied considerably during the cruise due to the nature of shallow-water surveying operations. 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.


Side Scan Sonar Processing

The XTF files were converted into CARIS data format structure called Sonar Information Processing System (SIPS) for the purpose of editing and sonar image mosaic creation. The first step in SSS data processing is to correct the altitude, or first return, from the seafloor. This was achieved by a combination of auto-prediction parameters set in Caris SIPS and manual boundary digitization of the water column and seafloor.

The second process was a beam pattern correction, which was accomplished by sampling a series of beams over a homogeneous surface. The purpose of beam pattern correction is to identify and offset the inherent instrument intensity variance as the across-track range increases. Near nadir, the acoustic return will be significantly more intense and decreases as across-track range increases. These phenomena result in a false high-intensity value strip along the centerline of the SSS swath.

Several other SSS editing tools are Angle Varying Gain (AVG) and Time Varying Gain (TVG) corrections, which were used to further adjust the resulting intensity range artifacts, offering a more consistent along- and across-track image. The despeckle editing tool was also employed to identify and mute isolated extreme high- or low-intensity valued pixels relative to adjacent pixels.

After all the individual side scan lines have been examined and edited, the CARIS SIPS engine allows for the creation of Georeferenced Backscatter Rasters (GeoBars). For these particular datasets, a pixel resolution of 1 m was chosen. From the series of GeoBars, a side scan mosaic image was then generated as a GeoBar composite providing a continuous image of a single-scaled intensity value range for geographic comparison.

In shallow-water surveying (depth less than 30 m), a variety of technical phenomena such as multipathing and ping ghosting prove to be problematic in processing SSS data. Multipathing is the bouncing of the acoustics along the seabed and ocean surface before reaching the extent. This phenomenon is amplified by wave chop, rough weather, and shallow depths. Ghosting is another acoustic anomaly produced by a ping rate that is not slow enough for the transducers to decipher one ping from another. This echoing effect is especially the case in very shallow depths (Hayes and Barclay, 2003).

The side scan mosaic in GeoTIFF format can be viewed on the Images page. Also, a downloadable version of this image can be located in the Layouts folder of the GIS Project (216 MB).


 

Interferometric Swath Bathymetry

A swath-sounding sonar system is used to measure the depth in a line extending outward, or across-track, from the sonar transducer.  As the survey vessel moves along a trackline, the swath transducer sends out an orthogonal sonar signal and scans the seabed to each side of the vessel.  It sweeps out an area of depth measurements, referred to as a swath.  The word "interferometric" refers to the acoustic wave technique used to measure soundings.  Acousitc interferometry uses the phase content of the sonar signal to measure the wave front that is returned from the seafloor or other targets such as a seawall.

Interferometric sonar systems require three or more linear arrays of transducer elements, one to transmit acoustic energy and at least two to receive the returning signal. These arrays, also called staves, are located within the black housing known as the transducer.  Depth measurements from interferometric systems are calculated by measuring two factors, the traveltime of a sound pulse and the angle at which the pulse returns to the transducer.  Generally, when a sound pulse is emitted from the transducer it travels to the seafloor and then travels back to the transducer, unless the signal has attenuated.  The time it takes to do this is the travel time, also referred to as the range.  The second measurement is the angle at which the returning sound pulse is received by the transducer.  The combination of these pairs calculates a depth value (SEA Ltd., 2005).

 

Swath Bathymetry System Acquisition Components

SEA SWATHplus-H 468-kHz Interferometric System

CodaOctopus Octopus F190 Precision Attitude and Positioning System

OmniSTAR High Performance (HP) correction - 20-cm accuracy

 

10cct02 Swath Transducer

Figure 6. Pole-mounted swath transducer lowered into position aboard the R/V Tommy Munro.

 

Field Calibration and Acquisition

The swath bathymetry data were collected using SEA SWATHplus-H Interferometric Sonar System (fig. 6) and SEA Swath Processor version 3.6 (fig. 7). Prior to surveying, the system must be calibrated in concert with the CodaOctopus Precision Attitude and Positioning System. This system supplied real-time differential corrections for position using the OmniSTAR reference station network and GNSS constellation, while real-time vessel motion (heave, pitch, and roll) information from the F190 sensor was applied to the acquired swath data. The calibration procedures involved setting the F190 sensor and antennae parameters and offset values into the program, restarting the program with correct values, and proceeding to calibration maneuvers.  The vessel needs to execute some type of motion (circles or figure eights) for approximately 30 minutes during the calibration process.  The calibration program allows the sensor to orient itself relative to the vessel and obtain a lock on position which will ensure accurate motion and position data in real time during swath acquisition.

SEASwath Processing

Figure 7. Screenshot of SEA Ltd. SWATHplus Acquisition and Processing Interface.

 

Calibration of the SWATHplus sonar head involved entering all setup data parameters into a SEA Swath Processor session file (SXS) and then running a roll calibration test (also known as a patch test) to account for any imperfections in the baseline orientation of the instrument.  The roll calibration consisted of seven lines that were 900 m long and spaced 25 m apart to obtain 100 percent overlap coverage.  The lines were surveyed over a homogeneous surface in a manner in which the port swath will overlap the port swath and the starboard swath will overlap the starboard swath.  These preliminary surveyed lines were then input into SEA’s Grid Processor program version 3.07.  The Grid Processor has a roll calibration program that runs a series of calculations on the port to port swaths and the starboard to starboard swaths that produces a roll angle value that is a result of the transducer orientation aboard the respective vessel.  Once less than three-thousandths of angle adjustment occurs,  final transducer roll angle offset values for the port and starboard, respectively, were added to their default angle (-30 degrees) within the SWATHplus configuration prior to processing bathymetry lines.

During acquisition, the differentially corrected positions supplied from OmniSTAR were recorded by the CodaOctopus Octopus F190 and output in the WGS84 datum.  Boat position, motion data (roll, pitch, and heave) from the F190, and swath depth measurements were streamed in real time to a dedicated laptop computer.  Additionally, a Valeport mini Sound Velocity Sensor (SVS) was attached to the transducer head to supply Speed of Sound (SOS) measurements simultaneously.  The SXS used during setup and calibration was updated during the survey to record all streaming data into the SEA Swath Processor raw bathymetry and data coverage formats, respectively (SXR and SXC).  During the survey, several complementing SOS casts were measured using a Valeport mini Sound Velocity Profiler (SVP) and input manually into the SXS file.  All raw data were backed up digitally onto Blu-ray media and terabyte servers.

 

 

Swath Bathymetry Post-Processing

The bathymetric swath post-processing for this survey was performed in a two-step workflow due to constraints in collecting interferometric data with this system. For example, instrument position, navigation, and sound velocity values were threaded in real time to the acquisition files. The advanced editing portion of processing was performed in CARIS, allowing for more dynamic and detailed data editing functions for preparation of finalizing interpolated surface products.


SEA Swath Processor

Raw swath bathymetry (SXR) was post-processed using SEA Swath Processor version 3.7, an upgrade that was available for this dataset.  The SXS files used during acquisition were also used for post-processing, which included roll calibration, position offsets, and SOS profiles.  The SVP casts were entered manually to help correct the speed of sound and reduce the production of refraction artifacts, most pronounced at the extents of the swaths.  The raw SXR files were replayed through SEA Swath Processor to produce processed swath bathymetry files (SXP), which are then corrected and geometrically processed.
 
CARIS Processing

The SXP files were then imported into CARIS HIPS and SIPS version 7.0.2, using the Conversion Wizard tool, which converts the processed data files into the Hydrographic Information Processing System (HIPS) format.  The use of a tide zone model, at tidal datum Mean Low Lower Water (MLLW), was applied with the CARIS Load Tide tool using multiple tide stations in CARIS.  The tide stations that were applied to this dataset are Pascagoula, MS, station ID 8741533 and Gulfport, MS, station ID 8745557.  The tidal zone definition was created and supplied by the National Ocean Service's Center for Operational Oceanographic Products and Services - Hydrographic Planning Team (COOPS-HPT).

After the merge process, each line was filtered and edited using the Swath Editor tool to remove any stray points and reject noisy outer beams.  When a line displayed depth consistency along track, it was possible to use the across-track distance filters to smooth the results.  Other filters applied included maximum and minimum depth, confidence, beam-to-beam slopes, and missing neighbors.  In some instances there were SOS refraction artifacts seen in portions of the bathymetry lines which manifested themselves by appearing like a smile or frown from the across-track view along the seafloor.  The CARIS Refraction Editor, a component within the Swath Editor, was used to manually apply an SOS offset value in portions of lines where sound velocity casts were not sufficiently correcting the data.  The files were remerged after any changes occurred during the editing process.  When the files were clean of errant values, a 10-m Bathymetry Based on Statistical Error (BASE) surface was created.  The 10-m BASE surface represents a depth value averaged from points within 10-m cells along the swath transect.  This BASE surface was further reviewed for any missed, outstanding, or questionable data areas which were edited accordingly and remerged, and a new base surface was recomputed.  Once the BASE surface was uniform, any further review and editing were conducted in the Subset Editor tool, which allows for a three-dimensional composite view of the bathymetry swaths. When the BASE surface was considered complete, the depth values from the base surface were then exported to ASCII format containing UTM positions in x, y, and final depth in z.

This file was imported into ESRI’s ArcMap version 10 and gridded using the Geostatistical Analyst Tool's Radial Basis Function. This allowed the user to adjust interpolation parameters with regard to real data spatial resolution and orientation when predicting values. This method provided for a more accurate and less biased representation of the dataset as a whole. A cross-validation report can be generated when using the Radial Basis Function. This helps the user understand how well the model will predict data values at unknown locations. In general the cross-validation takes one point and predicts its position using the weighted surrounding data values. This predicted value is compared against the actual value of that data point and general statistics are computed. The validation report for the swath bathymetry 50-m grid is listed below.

 

Samples
230,922
Mean
0.00154
RMS
0.05039

 

The swath surfaces can be viewed on the Images page. Also, a composite x,y,z, and pixel intensity ASCII file can be found in the ASCII_Data folder of the GIS Project (216 MB).

 

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