I plan on integrating a magnetometer into a Blue Boat setup for archaeological survey, mostly in shallow water. The mag would be paired with side-scan sonar, single-beam echosounder, and possibly a sub-bottom profiler. This would be a smaller version of the setup we typically use for manned-vessel surveys, where a magnetometer towfish (often a Geometrics 882) is surface towed well behind the vessel to avoid magnetic interference. Thus far, my research has led me to the Geometrics MagElement, the Marine Magnetics Explorer AUV, and the Sensys FGM3D. If anyone has any experience using these models or any other model with the Blue Boat, let me know.
Hi @bhaley
I have a had a Marine Magnetics Explorer since January 2023 (and also investigated the Geometrics at that time) I have found it a simple instrument to use and interpolate the data (I had a history of using Proton Mags as well as SSS an MB’s).
I am currently modifying the Explorer to run headless (no top box or laptop and just 2 18650 batteries to power it) and just a ESP32-S3 datalogger with 3” screen and have found this quite easy to do (with some handholding from Marine Magnetics)
What I saw as the disadvantages of the Geometrics unit was the (?from memory) 15 minute warm up prior to use and the 18W power usage compared with the (say) instantaneous start up of the Marine Magnetics and 2W power usage as they basically had the say accuracy / repeatability (your call but I don’t believe the Sensys unit is in the same accuracy as this pair).
I am assuming you would still be towing the fish behind the Blue Boat (say 10m) to be out of the boats motors magnetic field. It should be a somewhat easy integration into the Blueboat (just a RS232 input), which you should be able to send back to the ground control station or record direct to the vessel and post process.
Typical Data stream (Date/time, Field Strength, Signal Strength of reading, Towfish depth, Sample cycle, Data quality)
*23.221/03:06:06.2 F:052821.565 S:179 D:+006.5m L0 1278ms Q:99
Let me know if you have any specific questions
Scott
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Thanks. Good info. I assume you are talking about the Explorer AUV-specific model? So what’s your motivation for wanting to run headless? My thought is to just go straight into the Rasberry Pi and either write an API (I can handle the Python) or sending it to the remote laptop. Do you find the Explorer to be fairly robust? I often rent the Geometrics G-882 for manned survey and it is a tank, where as I’ve had some reliability issues with the Marine Magnetics SeaSpy. Yes, I would plan on towing at 8-10 meters using a layback offset (as is common in Hypack).
A possible problem with the Geometrics MagElement is that it really is not a towfish, according to Geometrics. They have sent a photo that uses a PVC boom extending back to keep it stationary. I could probably make it work but it’s a little different than what I’m used to with manned surveys. But I can rent the MagElement from them to test it, but am not finding an Explorer rental.
The Sensys system is a 3-axis fluxgate and I’m nervous about that for marine work, although I use fluxgate gradiometers on land. The sample data Sensys sent me was very clean but it’s hard to really know without testing. It is the lowest price solution (MagElement the most, Explorer AUV a little more than half of that, and the Sensys about half of the Explorer), but we would likely have to build a DIY towfish.
Thanks again for your comments and info!
Bryan
I think you are running the regular Explorer, based on your link. The AUV model is here: Explorer v.AUV | Marine Magnetics. I’ll have to research the differences but it may be weight and power draw. It’s good to know you made the Explorer work though.
Hi @bhaley
Yes, I have the stock standard Marine Magnetics Explorer
My main use case is in offshore waters (I have a 35m and a 135m towcable often used with a heavy weight fixed partway along the cable to get greater depth/ better coverage due to the invert cube issue with mags). I have used the unit in many smaller boats of opportunity (think local pacific islander power canoe) and this is where I have run into problems (basically the need for a top box and laptop – Which I have killed at least 1 laptop when the local guide put it down in the water at the bottom of the boat -I travel with a back up). Thus, why I was looking for a somewhat headless unit (100mm x 80mm x 50mm waterproof / submergible box)(also to allow the unit to be used either from kayaks or wading through the shallows without a laptop)
I don’t believe there is any “significant” difference between the standard Explorer and the AUV model. The fish will run on +9 to +28VDC and output and just pumps out RS232
The topbox is nothing more than power filtering (which Marine Magnetics have confirmed is NOT required for battery operations just if there is a AC transformer) and opto isolation on the signal so I think (99% sure) the AUV model is nothing more than the base fish (and then the AUV/ROV/Blueboat does the RS232 coms and power supply).
For versatility (without knowing 100% your use case) I would suggest the standard unit (so you can use it behind standard boats) and then an integration AUV/ROV/Blueboat (no need for a Pi [on a Blueboat just get the RS232 into the onboard Pi and then send 2 data streams {Mag data stream and Boat data GPS stream} back to the shore based laptop where you should be able to port those 2 streams to the BOB software com ports]
I have found the unit to be extremely robust (it’s the ancillary equipment laptop and power supply basically which is your supply where the overall system can fall down, as with anything on the water). I typically when collecting data (Mag/Multibeam/Sidescan) do not usually watch the data stream on the boat but just run the lanes, but check the data (SonarWiz) back on shore (see Note). I have travelled internationally (large and small local turbo prop flights) numerous times and would suggest NOT getting their standard travel case (I think it was over 20kg’s empty) and is a bit of a dog, I grabbed an Air Pelican and machined up some closed foam and now it is 16.5kg full ready to go (no excess baggage fees).
Note For mag data I usually use a instantaneous deviation from a base moving point average (typically a 2 minute rolling average) to look for the magnetic deflections (so a poor man’s gradient unit)
My take on cost is about survey time
| Marine Mag | GeoMag | Sensys | |
|---|---|---|---|
| Absolute Accuracy | 0.1 nT | 0.1 nT | say 1nT |
| Sensor Sensitivity | 0.02 nT | 0.01 nT |
Broadley the Marine Magnetics and the Geomagnetics are around the same accuracy but the Sensys (sort of hard to exactly define as it is a % or range) is about 10X worse (still better than a proton Mag)
So, assuming (just for comparison) object detection 10X Absolute Accuracy, 1Tonne, in 10m of water the Marine Magnetics and the Geomagnetics search would be based around 60m lanes whist the Sensys unit would (under the same 10X, 1T, 10m) would be based on 23m lanes. So broadly 3 times the time and distance on water for the same deflection/intensity coverage
Let me know if you have any other questions, happy to help
Scott
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@Scott_W It looks like the AUV is a slightly modded towfish that is slightly smaller diameter / slightly less weight (2.5 pounds less and neutrally buoyant) and with a lighter cable. Specs seem the same otherwise. You’re right that I’d like something to double dip and use from manned vessels too. Mostly the standard model would cost some battery life on the Blue Boat.
So you are diving your Explorer from the Blue Boat? This is a question I had since we occasionally work in deeper water. You don’t get too much drag and weight with the longer cable? Like I said, we mostly float our mags for surveys in water depth of 10-15 feet or less, so this is less of an issue for us. I could probably put some flotation on the cable in our case.
I will probably not consider the Sensys fluxgate system. I do get .05nT resolution with our terrestrial fluxgate system but we are 1/2 meter off the ground. What we do for cultural resource survey is regimented with survey specs dictated by fed and state regs. Usually we’re required to use 10 meter and 20 meters transects. But I’ve used Geometrics cesium and Marine Magnetics overhauser mags (the SeaSpy) and they are accepted by the community and project reviewers. The Marine Magnetics seems like the sweet spot all around.
Thanks again. I might check back in as things develop. Good luck with your surveys.
Bryan
Hi @bhaley
No that’s for when operating off “real”/full size boats, I typically start using depressor weight with anything over (say) 10-15m of water to get it closer to the source. The depressor weight (typically 20-40 3lb dive weights mounted on 2 stainless 50mm x 2mm x 500mm strips see image - it’s on a starfish SSS but it shows the concept) is mounted on a rope with a streaming short (1m) tail line connected to the umbilical (usually via a rubber band breakaway)
Remember resultion is not accuracy its just how many digits it shows and not necessarily significant digits
Yes I understand, I have had to use a mag whilst looking for a Aluminum target (for others remember a magnetometer is not a metal detector it is a device to measure magnetics fields of which ferrous items (steel) can affect the field)
Scott
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Thanks. Yes, I use magnetic gradiometers on land to image features such as prehistoric pits that have a +/-1nT range, but then all the way up to large ferrous tsrgets. We only care about very local contrast, so an absolute magnetic field value is not really important. OK, more soon. I have a rental supplier for the regular Explorer now and will be testing at some point in the near future.
Bryan