The problem of seeing things underwater is one that has vexed divers and others who work in the water since time immemorial. Ordinary vision works OK, in the daytime or with artificial light, in shallow water, in good conditions – but fails spectacularly in poor light or when water conditions are murky or worse. The modern electronic marvel that let us “see” for thousands or even millions of miles through air and space – radar – is completely useless underwater; water is effectively a brick wall to the short electromagnetic wavelengths that radar employs. Yet it is extremely important that underwater craft be able to sense their surroundings; if your craft is unable to see what is around it, you will pay the price for your lack of vision.
Around the beginning of the 20th century, researchers realized that they could use sound waves to “see” underwater.
Unlike radar, sound waves propagate just fine underwater – in fact, they propagate underwater better than they propagate through the air. The first sonar-like devices were used to listen for icebergs; sonar technology advanced enormously during the First World War as a tool to listen for enemy submarines. Sonar still has extensive military uses, but today civilian sonar is the primary area of development.
Although in this post we will talk about “seeing” it should be noted that sonar does not actually produce a direct visual image. Many sonar control suites will translate the sonar data into a visual picture, and high-end sonars actually can produced simulated images that look very much like you are “seeing” what is out there – but the actual data is sound pulses being reflected back. Computers do much of the interpreting of that data stream, making it more accessible to people without advanced training in sonar interpretation.
Most remote-operated vehicles have the ability to carry sonar sets as optional equipment.
ROVs use sonar for a variety of purposes, from mapping the bottom of bodies of water to searching for items in the water.
There are three basic types of sonar available for deployment on an ROV:
Scanning sonar operates in a way that will be familiar to anyone who has ever seen a radar dish turning at an airport. The sonar emitter physically rotates, sending out a pulse of sound as it does so, and simultaneously listens for the echoes returning from any objects in the water. The rotational speed of a scanning sonar balances out the time between pulses and the time it takes for a returning pulse to come back to the sonar.
Scanning sonars do not provide a high degree of resolution. They are able to detect large objects (for example, pilings or ships) but lack the discrimination necessary to spot smaller objects, particularly when those objects are on the bottom or next to another object. Because there is only one beam, scanning sonars have no ability to see behind objects; if one object is behind another object in a line to the scanning sonar, the scanning sonar can see only the closer of the two. The strength of the scanning sonar is that it does cover an area of 360 degrees, although only from 10 to 20 degrees above and below the plane of the sonar.
Multibeam sonars (often referred to as “hydrophones”) are significantly more capable. A multibeam array has multiple sonar emitters, which are all digitally rather than manually moved, allowing for a much higher rate of scanning. A combination of hardware and software allows the multibeam sonar to sweep a wider area of the water – about 120 degrees in all directions. Multibeam sonars do not cover a 360 degree arc, however. They make up for this by having a higher degree of resolution than scanning sonars – at a hundred meters, a multibeam sonar can spot a diver in the water. At 10 meters it can distinguish arms and legs. Multibeam sonars are extremely good at looking in one direction at a time.
Side-Scanning sonars have two arrays, each of which has a sonar beam that traverses a 90-degree arc, one horizontally and one vertically. This means that they can see in almost every direction, except for straight up and down. Side-scanning sonars are used to collect data as an ROV moves through the water; unlike scanning and multibeam sonars, they do not provide information in real-time. Instead, they provide a “historical” view of what was there when the ROV was moving past a certain point. The advantage of side-scanning sonars is that they “see” almost everything, providing great resolution and covering an enormous area. Side-scanning sonars are suitable for wide-area searches for stationary objects (such as bodies or objects on the bottom of the body of water), surveying, and environmental monitoring.
The right type of sonar for your ROV will depend on the missions for that ROV. Each sonar type has advantages and disadvantages for various kinds of work. Aquabotix ROVs can be configured with any of these sonar types. As with other accessories, choosing the right tools for the job will go a long way towards ensuring that you get the best possible results.