3D acoustic imaging of small bodies of water - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: January 26, 2025, 02:50:21 am
content: Oddly, in different spaces (here and HaD), the subject of DIY 3D acoustic imaging has come up. Mostly very naive, but it's a complex topic of strong personal interestI spent my career in seismic research in the oil industry. The software to do all the processing has been around for many years and is completely FOSS. I spent 15 years maintaining the major package FOSS seismic package, SU.The primary obstacle to a DIY system for mapping the bottom of a small body of water is combining someone able to build the acquisition system with someone able to process the data.I don't think that there is a problem with finding processors. I know plenty of guys who would love doing it. I can easily find guys in Houston who would love to search for cold case missing cars in ponds and quarries. Learning the processing is a major effort, but collecting data has a much faster learning curve. If you can build and operate the UMV it is a viable project.So I put forward the following proposition:Design and build an autonomous marine vessel which can achieve cm location performance and carry a recording system.The differential GPS IMU for the boat and the boat are the hard part. The processing software s already FOSS and has been for years. The ability to use it is a bit more demanding.For the receivers, you can use FET microphones in a thin soft vinyl tube filled with mineral oil. But R.T. Clark would probably give you a great deal on a small lot of professional hydrophones.Zoom recording deck for however many channels you want to acquire. Make the vessel as small as possible and still be able to run to 8+ hours. 4 with a battery swap at a minimum.Key UMV requirements:Real time differential control of vessel location to 1-2 cm accuracyAbility to carry a Zoom 8-24 channel DAW to record data with suitable phonesAbility to operate unattended for 4+ hours without recharge or fuelingQuiet hull and propulsion system designAny competent RC boat person can easily do all of the above. I know plenty of seismic processors who would love to process such data once or twice a year.So if yo happen to be or know such a person point them my way.Have Fun!Reg

author: Analog Kid
author_rank: Super Contributor
timestamp: January 26, 2025, 03:48:13 am
content: High TLA*count in your post:HaD (??)DIYFOSS (??)GPS (well, everyone knows that one)IMUFET (that too)UMVDAWRC (yeah, we know that one too)BTW (another one!), I once had a glancing involvement in the seismic oil exploration industry. The company I worked for was one of the only ones, or maybe the only one, that had a system capable of reading "seismic" 9-track tapes on a PC, which used extraordinarily long blocks that were unreadable by any stock tape drive controllers. I seem to remember our customer in this case was Schlumberger.*some FLAs and some 2LAs.

author: nctnico
author_rank: Super Contributor
timestamp: January 26, 2025, 08:02:11 pm
content: The location performance is the hard part. Forget about GPS. Though, if you can drop 3 beacons in the water at known locations and record their pings together with the data, you can triangulate the exact position.

author: rhb
author_rank: Super Contributor
timestamp: January 29, 2025, 12:26:34 am
content: Don't get me started on 9 tracks. I arranged for the only remaining 9 track at a big oil company to put in a room along with some DLTs and a 3570. Had card key access because sticking a 3480 cartridge in a 3570 meant a service call and head replacement.The company had a lay off and all the tape drives I had scrounged around the company were thrown away. Then someone brought me a tape and I went down to the room that held the drives, they were all gone. i don't know what they paid to read that tape, but I'm sure it was not cheap.My introduction to 9 nine tracks was a Seismograph Service Corporation Phoenix which took punched card input generated by my IBM mainframe VM/CMS system. I became widely renowned in the NOLA office for my ability to crack hex dumps of weird tape formats. Several people asked for my binder of examples. I gave it to the guy who taught me the Amoco seismic processing system.You have to do differential GPS. You cannot provide real time control of position any other way which is not cost prohibitive and not as accurate. You have a local base station which sends its current location to the vessel which then adjusts it's position estimate accordingly. This is how professional kit works. I don't know the precise mathematical details, but mm level is possible if you spend the money. The dominant error is humidity. It's actually used to measure atmospheric moisture. First system used 1500 Trimbles. That was 20 years ago. Not sure what they use now.I've been discussing acquisition system designs with various friends for 40 years. More different systems than you can imagine, both land and marine going back to 1990 and before.A quick check shows that a 96 kSa/s 24 bit audio CODEC would give 5-10 centimeter target resolution without significant technical issues. One can do better, but it gets to be a lot of work. Enough work to pay for better acquisition. For ~$800 USD you can have a Zoom L-20 24-bit/96kHz 22-track Recorder. Realistically 20 tracks. So a 300 cm sensor array for 20 channels. That's reasonable for a hull mounted sensor array.Commercial hydrophones are cheap, but maybe not as good as FET microphones in mineral oil. RT Clark is the goto for professional stuff. Call and tell them what you want to do. They are sharp.So, bottom line is if you want to image ponds and quarries at 2-4" resolution it is very practical, albeit a demanding learning curve. And time consuming data acquisition. Cost is well under $3k all up for materials.Have Fun!Reg

author: nctnico
author_rank: Super Contributor
timestamp: January 29, 2025, 05:16:22 pm
content: How does GPS work under water? If you bring the sensors closer to the bottom, I'd expect the image resolution to improve drastically. Every now and then I watch the professional treasure hunters use autonomic, submersible sonar drones which work under water. Surely they must have some way to determine their location.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 29, 2025, 05:38:02 pm
content: For post processing data and building maps, there is already a range of tools from roboticsROS uses the SLAM algorithm for 2D, not sure if it can map 3D, too, and AFAIK it's FOSS.

author: nctnico
author_rank: Super Contributor
timestamp: January 29, 2025, 05:48:07 pm
content: Quote from: RoGeorge on January 29, 2025, 05:38:02 pmI already guessed that much. Modern day computational power enables simple solutions for problems which where hard to do 20 years ago.

author: rhb
author_rank: Super Contributor
timestamp: January 30, 2025, 02:10:50 pm
content: Quote from: nctnico on January 29, 2025, 05:16:22 pmResolution is controlled by wavelength, not proximity. It *is* the wave equation. Ever wonder where the 1/10th wavelength rule of thumb came from?The problem with this thread is that the use case has never been stated clearly. The techniques vary widely and are use case dominated. I described a viable system for surveying small bodies of water to a depth of 1-2 m below the water bottom. It is normal seismic, not side scan which is the synthetic aperture radar math.Location below the surface is “interesting” as it relies on integrating multiple position techniques. Modern seismic ships can tow as many as 10-12 streamers. Each hydrophone position must be known to a fraction of a meter and there are thousands per streamer. The streamers are as much as 10-20 km long.I’ve never seen an autonomous underwater side scan ROV. Do you know of one? EGG has always been the ne plus ultra of side scan. All the examples I’ve seen were towed.Have Fun!Reg