ROV-Based HD Sonar Enables Realtime 3D Perspective


Ocean News & Technology
By Jesse Rodocker, Seabotix, and Joe Burch, Sound Metrics

Living in a three dimensional world (3D) has taught our brains to see images in three dimensions, even though the observations that pass through our eyes are only two dimensional (2D). It's the slightly differing information contained in multiple 2D perspectives that allows us to extrapolate 3D images. The same concept is now being realized in underwater robotic work in zero optical visibility water using high-definition sonar.

Use of robotic systems such as remotely operated vehicles (ROVs) is ever evolving. There are numerous challenges when operating in an underwater environment - unstable conditions, poor visibility, hazardous structures, and unpredictable currents, to name a few. Beyond these environmental conditions, operating underwater can have task-specific requirements. A small, properly equipped maneuverable ROV can deal with most underwater environmental challenges and provide sufficient information for making decisions.

ROV systems are used to conduct surveys, search and locate targets, and collect data and/or physical samples as well as complete other tasks. Virtually all missions require some form of visual image collection. However, capturing optical data can often be difficult due to poor visibility. Water turbidity, poor lighting, and the inability to get within close proximity of the target are but a few of the obstacles.

ROV systems today are fitted with increasingly improved optical cameras, bright lighting systems, and video enhancement technologies. However, when visibility is compromised by turbid water conditions, optics are often of little use. Light wavelengths are small compared to the size of suspended particles. These particles cause the light waves to be blocked, deflected, and scattered, yielding poor or no visibility of the target. However, sound wavelengths are large compared to these suspended particles, which in turn allows most of the energy to pass. Knowing this, underwater operations have long become dependent on sonar technology to not only locate, but also help identify targets in turbid water.

Scanning sonars have provided ROV operators a tool to help locate and navigate to targets. Once located, the targets can frequently be identified by an optical visual inspection. Added benefits of a scanning sonar is its full 360 degree scan sector that provides valuable situational awareness. Use of lower frequencies allows for longer ranges, and a single channel transducer can provide clean, low noise images. Improving the technology to a multibeam imaging sonar provides fast image update rates, which allows real-time imaging from a moving platform and/or moving targets. Rather than an operator having to interpret low-resolution scanning sonar data, they may now be provided with more videolike imagery.

The performance of an imaging sonar is determined by a number of specifications, most notably the operating frequency, acoustic beam width, and dynamic range. Generally speaking, a lower frequency increases the distance at which an object can be detected, whereas higher frequencies and smaller beam widths deliver clearer images and higher definition at closer ranges.

Sound Metrics, the creators of the high-resolution DIDSON (Dual-frequency IDentification Sonar) lens based multi-beam sonar has developed their next generation sonar, ARIS (Adaptive Resolution Imaging Sonar). The ARIS uses the same DIDSON technology to beamform crisp, clear images at the speed of sound through acoustic lenses. ARIS models include the Explorer 1200 (1.2 and 0.7MHz frequencies), the Explorer 1800 (1.8 and 1.2MHz frequencies), and the all new Explorer 3000 (3.0 and 1.8MHz frequencies). Sound Metrics is introducing a whole new category of High Definition Sonar imaging with the Explorer 3000. Its improved image clarity is the result of increasing the physical transducer count by 30% and using a new lens prescription that produces finer resolution from each transducer. All the DIDSON Technology sonars offer a lower frequency used for object detection along with a higher frequency for identification imaging.

SeaBotix Inc. has developed a new class of MiniROV systems with the 2011 launch of the vectored Little Benthic Vehicle (vLBV). The vLBV was the culmination of years worth of field operations, client feedback, and development. Vectored platforms offer a number of advantages over conventional ROVs, including greater stability, increased maneuverability, enhanced capability, and improved sensor payload. By placing four thrusters in a vectored configuration, the ROV can maneuver around objects in more demanding conditions. All these features of the vLBV have been realized in a system weighing only 18kg, which does not require any special deployment/handling equipment.

ARIS Explorer 3000 HD Sonar using DIDSON technology (top). The vLBV in a structured environment showing the ARIS sonar, vLBV and Scanning sonar (bottom).

The ARIS is mounted to a skid that can be tilted
up or down 15 degrees depending on mission

Tritech 360 degree scanning sonar image of plane
wreck demonstrating situational awareness and large
area coverage. Range is 30 meters.

Expanding on the vLBV's mechanical capabilities are built-in high resolution sensors, including pitch, roll, heading, depth, and temperature. This onboard data are accessible at the surface for interfacing with thirdparty software to further enhance captured data. Additionally, the vLBV's use of dual vertical thrusters offers the ability to incorporate active roll compensation, resulting in improved stability. Taking the degree of automation and stability to a higher level is auto altitude.

In the past, vectored platforms have predominately been available only as much larger systems. DIDSON, being a larger sonar, has required a larger ROV platform to be a viable option of a multi-beam imaging sensor. With the introduction of the ARIS in a smaller, lighter package than its predecessor, integration onto smaller ROV systems like the vLBV is now possible.

Combining their strengths, SeaBotix and Sound Metrics have set out to augment ROV-based sonar applications. The vLBV provides the stable, capable platform and the ARIS delivers clear, high-definition sonar imagery.

Now getting back to three dimensional thoughts as they pertain to the observer of ROV-captured data. All the sonars mentioned above produce data in two dimensions; range and cross range. Range is the radial distance away from the ROV, and cross-range is measured left and right of the center axis in front of the ROV. The challenge is to quickly perceive vertical information about a target (the third dimension). This provides differing perspectives of an object, allowing the brain to do what it does naturally - interpret the visual information into 3D imagery. Rolling the sonar 90 degrees slowly about the forward axis of the vehicle is how the observer is provided with differing two dimension perspectives. Rolling the sensor exchanges horizontal left right target information for images showing vertical target information. This gradually transitions from the usual apparent top down perspective through a 90 degree roll into an apparent side perspective of the object, which yields the depth perception. The observerÆs brain has just integrated the varying 2D sonar views into a natural 3D perspective of the imaged object without any conscious effort.

An operator can now stand off a target and with confidence, capture real-time, high-quality imagery in black water conditions. This roll capability enables measurements of objects in all three dimensions from accurate sonar data. This roll capability also provides the operator with the ability to match sonar orientation to the slope of the terrain. By rolling the sonar to become parallel with the terrain, the sonar image quality can be maximized. Rolling the sonar allows for terrain matching of the image to surfaces in any orientation, from horizontal to vertical.

Incorporating a variety of technologies and sensors into a single system presents a variety of opportunities. The vLBV fitted with a scanning sonar, ARIS imaging sonar, acoustic positioning, optical camera, grabber, and lighting is a formidable tool for a diverse range of missions. A scanning sonar provides the longer range, 360 degree coverage to locate and navigate to targets where the ARIS provides high-definition imagery and accurate dimensional information. The optical camera and lighting provides real-time visual data where conditions permit and the grabber offers possible object recovery.

The possible applications are vast in number. Operators can use the system to search for objects such as mines, where the added height (Z-axis) information can be used to more accurately identify and classify the target. There is also broad area search capability coupled to ultra-fine image detail on a stable platform that can operate in demanding environmental conditions.

For more information on the vLBV visit

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