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Sub-Bottom Profling From Small AUVs Profler Mounted on Man-Portable AUV Reveals New Details of Buried Channel in Narragansett Bay By Dr. Thomas Hiller ��� Arnar Steingrimsson ��� Robert Melvin The Gavia Surveyor AUV equipped with the SBP module on the back deck of the RV Shanna Rose, ready for transit to the survey site. A long with side scan and bathymetry, sub-bottom profling is a key technology for subsea engineering and geophysical investigations, and is a core requirement for many oil and gas, and offshore renewable energy surveys. In the last decade, huge progress in small AUV technology has led to rapid uptake in the offshore survey industry. Advances in power, propulsion, positioning and control have resulted in low-logistics, man-portable vehicles that can carry compact instrumentation payloads and collect survey-quality sonar data, with mission duration and depth capabilities suitable for general survey work. The data quality from the latest generation of payload sensors satisfes engineering inspection and geophysics requirements, where previously a boatmount or ROV-mounted sonar was needed. In addition, the small AUV has the unique ability to access restricted areas and acquire high-resolution data from deeper water. One key capability missing from a low-logistics AUV mix was the ability to picture the sub-bottom geophysics around a survey site, a critical requirement in the oil and gas, and offshore renewable energy sectors. Teledyne Benthos (North Falmouth, Massachusetts) developed an AUV-mountable version of the towed Chirp III sub-bottom profler (SBP), with the frst systems deployed on Teledyne Gavia ehf (K��pavogur, Iceland) AUVs in 2011. Since its inception, the Gavia SBP module has been successfully used on at least two commercial surveys in Europe and South America in 2012 to provide geological details. Sub-Bottom Profler Design In April 2012, a high-frequency chirp SBP mounted on a man-portable AUV was used to survey the geologically remarkable Narragansett Bay to the north of Rhode Island Sound with a team from Teledyne Gavia, Teledyne Benthos, UTEC Survey Inc. (Houston, Texas) and the University of Rhode Island. The data captured from the AUV at a consistent altitude provided details of the refectors in the channel diffcult to obtain using a towed or hull-mounted system. Narragansett Bay is one of the largest estuaries in the United States. Its shoreline developed during the last glaciation from a delta plain as water from the melting Laurentide ice sheet deposited sediment into a glacial lake. Over time, sea levels receded and the glacial lake drained, exposing the bay bottom. As temperatures warmed toward the end of the glaciation, the ice sheet melted and receded north. Sea-level rise fooded the river system, creating the present coastline. Greenwich Bay, part of Narragansett Bay, is now a shallow estuary about 10 kilometers south of Providence, Rhode Island, with a seabed rich in fuvial-deposited stratifed sands and gravels due to the outwash of multiple tributaries into the bay. As one of the most surveyed geological areas, it was chosen by the team for surveying with the Gavia SBP module. A major design challenge was the vehicle���s small diameter. This put strict limits on the size of SBP transducers that could be used, and hence the power and frequency range available. The AUV version of the Chirp III operates in the 14-to-21-kilohertz frequency band with adjustable power and pulse length. A four-element transducer array narrows the beam pattern and provides up to 10 meters of penetration. Despite the size restrictions, an AUV-mounted SBP has several advantages over boat-mounted or towed systems. The AUV can maintain a constant altitude and speed in a very-low-noise vessel environment. Data processing and interpretation is also easier as there are no swell, pitch, roll, cable snatch or wake artifacts. Compared to a ship-mounted www.sea-technology.com FEBRUARY 2013 / st 45