Sea Technology

FEB 2013

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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VISIT US AT US HYDRO 2013, MARCH 25-28, 2013, NEW ORLEANS, LA further investigation. Target areas had been chosen from previous towed SBP surveys of the area, and a suitable buried channel in the center of the bay was identifed in the frst few vehicle runs. After each run, the SBP data, stored in the SEG-Y format, was downloaded from the AUV and analyzed in Chesapeake Technology Inc.���s (Mountain View, California) SonarWiz software to check the data quality and choose the parameters for the next mission. The area chosen for the frst set of test lines was a 100meter-wide shallow buried channel running west to east in the middle of Greenwich Bay. The charts show little surface expression of the channel in the survey area. A survey line across the buried channel was repeated on each test mission using different settings for chirp length, power and receiver gain. The settings were chosen to minimize ringing and saturation, and optimize defnition of the sub-sea refectors in the middle of the channel. The water depth in the channel���s center is around 5 meters, with layered sediments visible in the SBP data to about 10 meters below the seabed. A small survey of the buried channel was carried out using the optimized settings. This showed consistent defnition of the refectors in the channel and tie-in of refectors between the cross lines. During data processing, fltering and interpretation were easier than for the shallow, short-tow system as there were no swell, pitch, roll or cable-snatch artifacts. After completing the shallow-area survey, the vessel transited to a deeper channel. A short survey pattern was planned to fnd a suitable test line. An angular unconformity with fat and dipping refectors was identifed in between Greenwich Bay and Patience Island at 15 meters depth. A series of missions were then executed to optimize the SBP settings at different altitudes. The trials showed consistent defnition of refectors up to 14 milliseconds below the seabed. Analysis of the frst seabed return from the SBP data gave an indication of the system���s vertical resolution, with a full width at half maximum corresponding to a resolution of about 20 centimeters. This can be compared with the theoretical bandwidth-limited resolution of about 10 centimeters. Triple Processing Power Conclusions The surveys in Narragansett Bay showed details in the frst 10 meters of sedimentation previously unseen by the team. The University of Rhode Island has indicated an interest in retrieving a core sample from the survey area to correlate with the SBP records. The inherent, low-fying, constant altitude of an AUV minimized the sub-bottom acoustic footprint, allowing a fner data resolution. The results demonstrated the advantages of an AUV integrated SBP solution in shallow-water surveys, particularly the ease of deployment, the stable lownoise platform and detail of the data set. Small AUVs owned by commercial organizations are now carrying out surveys and inspection tasks on a weekly basis. These systems are delivering survey data at high productivity and low logistics costs. It can be expected that the small AUV will become increasingly important in many subsea survey and inspection roles. The Teledyne Benthos SBP integrated with the Gavia AUV expands the range of tasks these vehicles can carry out for offshore survey applications. n HYPACK ACOUSTiC Complete package for side scan, sub-bottom, and ADCP data processing SIDESCAN PROCESSING HYPACK�� SIDESCAN PROCESSING ACOUSTIC allows you to process side scan sonar, snippets, and backscatter data collected in HYPACK�� or data collected by 3rd party systems. SUBBOTTOM PROCESSING HYPACK�� ACOUSTIC allows you to edit and review sub-bottom data collected in HYPACK�� or other packages that produce industry standard SEG-Y files. For more information visit us at www.hypack.com 48 st / FEBRUARY 2013 www.sea-technology.com ADCP PROCESSING HYPACK�� ACOUSTIC allows you to analyze data from vesselmounted ADCP systems and generate flow calculations. It also allows you to perform averaging and to output 2-D or 3-D current.

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