How about we learn some stuff about AUV’s aka Autonomous Underwater Vehicles. One of the key aspects to their abilities is in the first word, autonomous, which means they are independent of any surface controlling tethers. They are free swimming, preprogrammed automatons that can perform a number of exceptionally important tasks when undertaking underwater search operations. But before we get to what they can do lets discuss a bit more about what they are.
The Waitt Institute AUV, Mary Ann, awaits testing on deck before the Florida Straits expedition embarks. Photo Credit: Michael Dessner
The girls (as we affectionately refer to them; specifically Mary Ann and Ginger) are 13 foot, 2000 pound torpedo looking devices. They are comprised primarily of titanium and syntactic foam (which is a medium of molded glass microspheres that provide buoyancy). Inside the syntactic hull, mounted to the titanium strongback frame, are all the housings for the instrumentation and the sensors which you will see peaking through the yellow syntactic at various intervals. The bottles and cans that hold the electrical junctions and instrumentation are either pressure resistant due to the extreme nature of their construction from thick walled titanium or are oil filled which allows them to compensate for the extreme pressures to which they are subjected. The vehicles are rated to what is commonly accepted as full ocean depth, 6000 meters, although the abysses of the planet are much, much deeper. The pressures that the vehicles must survive to operate at this depth are simply astronomical. At full depth a square inch would have 8,400 lbs of pressure on it. A square foot would then have one million two hundred nine thousand six hundred pounds of force exerted on it. The rough area of the vehicle in total is about 90 feet. That means the entire vehicle, when at full depth, is under one hundred eight million eight hundred sixty thousand pounds (108,860,000 lbs), or, if ya wanna round it off, just call it an even 54,000 tons. When I tell people these things are bullet proof I ain’t lying. On this job we’re only working at about 3,700 meters, so the vehicles are only suffering 33,000 tons of force. A walk in the park to the girls.
What’s that you say? “Alright, they’re damn tough but what else can they do? After all, as any cook worth his salt will tell you, you’re only as good as your last meal.”. Well listen up my friend and listen well. About the only things these things can’t do at full ocean depth is carry a tune or pick something up.
The primary sensor on the AUVs is side scan sonar but ours can carry a suite of additional equipment as well. I won’t go much into sonar here (detailed information about that can be found in my logs from the Search for Amelia (http://log.searchforamelia.org) but in essence they are like sonic flashlights shined out to either side of the vehicle, effective out to 600 meters in each direction. You “see” what the flashlight in the metaphor would light up and the shadows created give you a lot of information, help define the shape of the “lit” item. Depending on the reflectivity of the surface you can also tell a lot about its nature (for instance sponges are less reflective, steel much more so).
Mounted permanently on the bottom of the girls are Doppler Velocity Loggers which can be set to act as Acoustic Doppler Current Profilers (used to measure water currents). This sensor looks a bit like a hat with 4 red circles on it and acts as a bottom sensor. When the vehicles get to depth this ‘sees’ the bottom and then helps the vehicle maintain altitude. It also works with the system’s computers and a pencil beam sonar mounted on the nose that looks forward and slightly downward, effectively providing the system collision avoidance capabilities. The AUV is slightly buoyant after it releases its descent weight and if these sensors tell its brain that it’s about to run into something it tries to turn up and avoid it; if that doesn’t work it stops the prop from turning and floats upwards until the obstacle is no longer detected, then it purrs merrily along doing its thing. This is invaluable in a search like the Air France flight 447 effort which is taking place above a submerged mountain range. A towed system is very difficult to handle on that type of search (deep with rugged terrain) because you are often trailing equipment well behind and far below your support vessel and if you are constantly coming too far off the bottom you are not making good data. The girls can come to a near full stop as they float upwards so when they start moving again they haven’t missed so much. It is a signature capability of the REMUS AUV’s.
The Waitt Institute also owns a multi-beam unit that can be mounted on the girls; this is like a downward looking sonar that can be used to produce a 3 dimensional representation of what the vehicles ‘see’ below them. One reason we use sonars instead of multi beam is that the swath, or area covered, is so much smaller when using the multi beam. The vehicles have what we refer to as an “Eco Puck” which acts as a CTD (conductivity, salinity and depth) sensor, the data from which it uses in its navigations. This can be rigged with a sub bottom profiler which gives readings that inform the user of the make up of the bottom (sand, gravel, coral, etc). I really do not know much about this sensor but we plan to use it out here so more on that one later as I learn.
Finally the vehicle carries a camera. Now that may not seem like much but this, too, is a signature sensor of the Waitt Institute AUVs. Imagine with me for a moment…. you are looking for something on the ocean bottom and you are towing a side scan sonar (as the Institute did working in conjunction with Naticos in ’06 looking for Amelia Earhart’s plane). You are sitting in front of a screen watching data drop down about an inch every 15 minutes and shazam, you get a hot target. Naturally you want to get another look. If you are towing at ocean depth a turn alone may take a couple hours and you may not have the ability to switch to a higher frequency for better resolution. But we’ll say you do, so you make a two hour turn and switch freq.s and go past it again. If you are lucky enough to trail your tether, which is towing your sonar 4 miles below you and four miles behind you, directly over your target (and trust me, that ain’t easy) you still are going to be looking at a sonar image and that is nothing like a picture. It is a representation based on sonic energy and it needs interpreting. Moving on, your target still looks hot and you decide you want to take a real look at it. So you haul in your sonar which takes hours, rig your AUV, more hours, send it down and assuming you don’t have any ground fault issues and get directly onto your target (ASS U ME, remember) when you find it you will know. It could take days if everything works, weeks if not, months to years if you don’t have an ROV that can handle the job and have to go back to port, raise money and get back out there; all based on a interpreted image. Good luck with that pitch.
Now, cut to the girls in the same scenario. One of them was on a 24 hour mission; after recovery, data download and an hour or two of a quick initial look at the data the sonar analyst sees something he likes as a target and wants another look. On her next run she is programmed to re-acquire target for assessment and 4 hours after she came to the surface she is headed back to the bottom. She heads into the next box up the line and when she gets close to the target area she breaks off to head to the target and remember, our navs are flawless as they rely both on Deep Ocean Transponders and a Kearfott Inertial Navigation Unit in the AUV. It’s the kind of technology the military uses to put missiles into basketball hoops hundreds of miles from launch. We don’t miss. When she gets to the target zone she drops in altitude, switches to high frequency, high resolution sonar and begins clicking away with her camera. 24 hours after launch you know whether or not you have found your target. Don’t take my word for it; ask anyone in the industry, it’s a huge capability. To be able to ground truth within a day of detection at full ocean depth? It’s revolutionary to the world of underwater survey, an unparalleled, cutting edge technology that I truly believe will put towed systems out of business in the next 20 years. Don’t get me wrong, I mean no offense to my brethren in the field who use towed systems. Those systems work and they work well but, I believe the improvement in capabilities represented by AUVs is an exponential leap in technology and is the way of the future.