Online Exclusive: Sonar Goes to the Next Level
Online-only Articles - Online-only 2009
A newly developed mapping sonar demonstrated in Australia gives hydrographers a new tool for meeting increased survey demands.
By Emmanuel Sgherri and Nick Goodwin
The future workloads of hydrographic departments and the survey industry are increasing exponentially with new missions and standards. Adding to the equation are mapping of exclusive economic zones and the continental shelf, maritime boundaries delimitation, habitat mapping, sub-sea surveillance, and detection of illegal loads on the seabed. The dominant question: how to manage this increased survey demand within existing budget constraints.
As children of the world now learn geography in a dynamic mode through a Google-type of visualization with direct access to the dataset, the ocean seabed will soon be visualized in the same way. Google Oceans is in its early days, but no doubt tomorrow millions of users, private and public, including scientists, professionals, environmentalists, fishermen, and students will visit the oceans from their laptops.
This change has already started, and the market’s new needs and demands can be summarized as:
- higher resolution data,
- ortho-rectified imagery, with high positioning accuracy required for proper integration into GIS,
- easy access and dissemination, and
- massive reduction of cost of information, with, “How many pixels do I get for my dollars or per unit of survey time?” becoming the key question.
With these market trends and user requirements in mind, IXSEA has launched a massive R&D program to build a new generation sonar: a mapping sonar. Although the data quality of most side scan sonars today is excellent, a number of key points need to be addressed:
- Positioning accuracy: where is my fish?
- Can I control/compensate the attitude and heading?
- The blind zone at Nadir: this costs me millions in vessel time. Can it be eliminated?
- Can my system generate a better quality dataset in real time for a lower cost?
Based on IXSEA’s technologies in inertial navigation, acoustic positioning, SAS (synthetic aperture sonar) processing, and transducer manufacturing, along with a knowledge of PC-based real-time mapping solutions, a multi-disciplinary team has developed the concept of the world’s first true mapping sonar, known as SHADOWS.
First Demonstration Comes Down Under
A SHADOWS demonstration was conducted in late April 2009 on board the Total Response, a 20-meter survey vessel. The vessel is based in Fremantle, Australia on the country’s west coast, and the trial area was in Cockburn Sound, a few miles southwest of Fremantle Harbour between the mainland and Garden Island.
Mean water depth of the survey area is 20m. The area lies between the mainland and the Gaden Island naval base, and a number of subsea cables and pipelines cross the seabed. Additionally, the area serves as anchorage for coastal freighters and small tankers, and anchor scour and other features mark the sea bottom. The seabed is flat and mainly composed of sand with some small rocky features.
IXSEA had provided a hydraulic winch with a double-armored fiber-optic tow cable. The existing winch on board the vessel first had to be removed for the IXSEA winch to be installed. Meanwhile, the SHADOWS fish was craned aboard the vessel, and the servers and displays were installed inside the vessel operations’ room and tested.
The mobilization was essentially complete by the end of the first day. The next morning, a systems check was conducted with the SHADOWS fish in the water. Then deployment and recovery drills were practiced in-harbor. The vessel was put to sea at midday for full operational testing.
To track the SHADOWS’ tow fish and allow the onboard inertial navigation system (INS) inside the fish to stay fully aligned for creating accurate real-time geo-referenced images, an ultra short base line (USBL) acoustic tracking system was installed on the vessel. IXSEA supplied its global acoustic positioning system (GAPS) USBL, which also incorporates an inboard INS. This allowed rapid mobilization of the USBL system without the need to have a separate calibration done between the USBL and a separate attitude and heading reference unit.
The GAPS transceiver was mounted under the hull of the vessel on a convenient mounting bracket in place for multi-beam sonar installations. A diver installed the transceiver, and then the system was tested with a transponder deployed alongside the vessel.
SHADOWS was deployed off the survey vessel Total Response out of Fremantle, Australia.
A survey grid of 1.2 x 1.2 kilometers was defined for the demonstration. Five contiguous survey lines were planned to cover the area, allowing for a 100m overlap. This overlap was not necessary, as the system provides geo-referenced images, but for the purpose of the demonstration, the overlap was used to provide multiple images of bottom features for comparison purposes. With the 20m water depth, the range (per channel) was limited to 200m, providing a total swath width of 400m. Line spacing was set at 300m.
Lost Item Found
A ship’s anchor and chain were known to have been lost at a location west of Fremantle Harbour en route to the survey area. The trials boat operator, Total AMS, was interested in salvaging the anchor and requested that IXSEA try to image the anchor to confirm its position and size. Water depth at this location was around 20m, and the sonar range was approx 200m (400m swath).
The tow fish was deployed, and it detected the anchor and chain in the first pass. A second pass was made to gather additional images. From the images and using the contact analysis tools, it was possible to show that approx 4m of anchor was visible above the seabed and by derivation show the anchor to be circa 10-12 tons. Additionally, the length of anchor chain was measured to be nearly 200m. The chain was expected to be of 7/8-inch gauge. Anchor marks can be followed through each side through the gap filter picture.
As its first task, SHADOWS located a lost anchor and chain.
Two prism-shaped (four triangular sides) sonar targets were deployed. One had equal side lengths measuring 1m, while the other’s measured .5m. A prism-shaped target was selected because it provides realistic target insonification properties. Unlike a “radar reflector” shape, the prism is equally likely to reflect sound away from the receiver as it is towards it.
The targets were configured to be deployed together, joined by a 10m polypropylene rope. A small metal anchor was connected by polypropylene rope and chain to the smaller target to stop the targets dragging. The larger target was connected to the surface by polypropylene rope where it was marked with two small buoys to aid recovery. A USBL transponder was fixed midway up the line, making the positioning of the target easier and permitting the comparison with the position obtained in the sonar record.
Two prism-shaped targets were used for the test.
With perfect matching between two sonar lines, this example shows the capability of SHADOWS to produce geo-referenced pictures in real time with accurate positioning.
No processing is needed to produce the map (Geotiff mosaic) and adjust the position of profiles to each other. The map is built in real time with a 15-square-cm resolution and automatically generated from the sonar position (no need to enter parameters for the cable lay back and angle of deportation of the fish). The accurate matching between two lines is done automatically by the sonar. Understanding and interpreting what’s on the seabed is facilitated (detection and localization of scattered objects and man-made objects).
Linear features such as pipelines and cables can be clearly visualized in the map over several sonar lines. Export of DXF/Autocad layers can be made directly onto the Geotiff sonar map for object change detection. Tiff higher resolution is required on some areas; post processing can be applied to the selected data to obtain up to 5-square-cm resolution.
The mapping sonar’s performances open a new perspective not only in terms of return on investment but also vessel-time optimization, higher profit margins, and fuel-cost reduction. In a simulation done of the cost of a 10,000-square-km debris survey using a standard digital side scan sonar versus the same performed with mapping sonar, savings on the total costs resulted in the proportion of 1 to 4.
The new mapping sonar offers industry a cost-effective solution by combining high-resolution SAS sonar technology, high accuracy navigation, and real-time mapping to meet today’s new survey challenges and increased survey demand within existing budget constraints.
Emmanuel Sgherri is business development manager for seafloor mapping instruments developed by IXSEA and the SHADOWS mapping sonar. He has been responsible for the development of IXSEA sales in various parts of the world.
Nick Goodwin is IXSEA’s regional business manager in Australasia and formerly director of IXSEA’s U.K. subsidiary. He holds a first class degree in environmental science from the University of Southampton and has more than 18 years of experience in underwater navigation and positioning and its application to survey operations.
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