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Sub-Bottom Profiling From Small AUVs

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.

Along with side scan and bathymetry, sub-bottom profiling 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 satisfies engineering inspection and geophysics requirements, where previously a boat-mount 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 profiler (SBP), with the first 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 Profiler 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 reflectors in the channel difficult 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 flooded 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 fluvial-deposited stratified 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 system, the small footprint from a low-flying AUV, combined with the strong returns, results in positional accuracy for small man-made features, such as buried pipelines.

Configuring the AUV
The modular Gavia Offshore Surveyor AUV consists of a nose-cone, battery, control unit and propulsion unit. In commercial survey work, the Offshore Surveyor AUV is normally configured with an inertial navigation system (INS) assisted by a Doppler velocity log (DVL) and differential-ready GPS receiver. Sensor modules are chosen dependent on the data desired, with the vehicle automatically detecting their presence on start-up. The vessel can be configured with one or two lithium-ion battery modules that can be exchanged in the field in a few minutes once depleted, with the two-battery configuration allowing missions up to 12 hours at 4-knot speed.

For the Greenwich Bay SBP missions, the build was: nosecone with obstacle avoidance sonar and imaging camera, battery module, Kearfott Corp. (Little Falls, New Jersey) T-24 INS with Teledyne RD Instruments (Poway, California) WHN1200 DVL, control module with GPS, Teledyne Benthos ATM-900 acoustic modem, Iridium Communications Inc. (McLean, Virginia) A3LA-X telemetry, side scan sonar, SBP and propulsion unit. In this configuration, the AUV was 2.9 meters long and weighed 88 kilograms in air. To continue this article please click here.

Dr. Thomas Hiller has a Ph.D. in experimental physics in the semiconductor sector. He has managed sonar product lines and deployed the first interferometric sonar mounted on a man-portable AUV in Reykjavik Harbor in 2006. He joined Teledyne Gavia in August 2012.

Arnar Steingrimsson is the director of vehicle sales for the Teledyne Benthos group of companies, which includes Teledyne Gavia, Teledyne Benthos and Teledyne Webb Research. He was previously the director of sales and marketing at Hafmynd ehf, which is now Teledyne Gavia.

Robert Melvin is the vice president of engineering at the Teledyne Benthos group of companies. He was previously the engineering manager at Hydroid Inc. He has bachelor's and master's degrees in electrical engineering.

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