U.S. Geological Survey Data Series 563
Side Scan Sonar |
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Side Scan Sonar Systems 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. |
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Side Scan Sonar Acquisition System Components The side scan sonar components used during Cruise 10CCT01 are as follows:
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During the 10CCT01 cruise a Klein 3900 dual-frequency side scan sonar system (fig. 5) was towed on the port side of the vessel to collect information about surface sediment material. SSS was acquired and recorded using SonarPro software in an Extended Triton Format (XTF). Differential GPS position from OmniSTAR was recorded by the Octopus F190 Precision Attitude and Positioning System and output in the World Geodetic System of 1984 (WGS84). The towfish altitude varied considerably during the cruise due to the nature of shallow-water surveying operations. Ideally, SSS is flown at a relatively considerable distance from the vessel and other instruments to avoid acoustical interference. Typical sources of acoustical interference are vessel vibrations and other instruments such as sub-bottom profilers and interferometric swath systems that utilize similar frequency ranges. However, in shallow-water surveying the optimal distance is difficult to achieve due to the negative buoyancy of the towfish and the effect of unanticipated isolated shoals.
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Side Scan Sonar Processing The XTF files collected were converted into CARIS data format structure called Sonar Information Processing System (SIPS) for the purpose of editing and sidescan mosaic creation. All horizontal positions were offset relative to the position of the Octopus F190 Precision Attitude and Positioning System antennae. The side scan mosaic can be viewed on the Images page. Also, the GeoTIFF image can be found in the Data Products folder. Please reference the read-me file for file names and details. |
Interferometric Swath Bathymetry |
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Interferometric Swath Systems A swath-sounding sonar system is used to measure the depth in a line extending outward from the sonar transducer. As the survey vessel moves along a trackline, the swath transducer sends out sonar signals at a right angles to the trackline and is scanning 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 technique used to measure soundings. The interferometric technique 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. |
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The swath system components used during Cruise 10CCT01 are as follows:
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The swath bathymetry was collected using the SEA Ltd. SWATHplus-H Interferometric system (fig. 6) and SEA Swath Processor software version 3.6 (fig. 7). The system must be calibrated before the survey commences. This involves inserting the CodaOctopus Octopus F190 Precision and Attitude Positioning System's sensor and antennae positions and offset values into the program, restarting the program with these correct offsets, 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.
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Calibration of the SEA SWATHplus sonar head involved entering all setup data parameters into a SEA SWATHplus session file (SXS) and then running a roll calibration test also known as a patch test. 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 are surveyed in a manner in which the port swath will overlap the previous port swath and the starboard swath will overlap the previous starboard swath. These surveyed lines were then input into SEA Ltd.'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 reflects the transducer orientation aboard the respective vessel. These final transducer roll angle values for the port and starboard, respectively, were added to their default angle (-30 degrees) within the SXS configuration file prior to processing of bathymetry lines. During acquisition, the differentially corrected positions supplied from OmniSTAR were recorded by the Octopus F190 and output in the WGS84 datum. Boat position and motion data (roll, pitch, and heave) from the Octopus F190, and swath depth measurements were streamed in real time to a dedicated laptop computer. 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 used during survey to record all streaming data into the SEA Swath Processor Raw File (SXR) format and the SEA Swath Processor Coverage File (SXC) format. During the survey, three SOS casts were measured using a Valeport Mini Sound Velocity Probe (SVP) and recorded manually. All data were backed up digitally onto Blu-ray media and terabyte servers. |
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SEA Swath Processor
The swath and side scan sonar surfaces can be viewed on the Images page. Also, the files can be found in the Data Products folder. Please reference the read-me file for file names and details. |