G-882TVG Cesium Mag w/Transverse Gradiometer... is it worth it?

[Marine Archaeology Int.]

It has been recommended to me the dual mag system from Geometrics with the transverse gradiometer is probably the best marine survey system on the market today.... but the whole enchilada come with a $100K+ price tag... my question: is it worth it?

ftp://geom.geometrics.com/pub/mag/DataSheets/G-882TVG_DataSheet.pdf

 

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[Salvor6]

Definitely not worth that price. The Sea Spy is just as good for $15,000.

SeaSPY2 | Marine Magnetics

 

[enrada]

I agree. I can't see the need for a gradiometer when searching for a wreck. Save your money. If you have no navigation software you can just buy a $5000 program to use for running a grid system and post processing.

 

[enrada]

You may want to check out <jwfishers.com> proton4 magnetometer. It comes with Tracker navigation software. I have bought a number of their products and dealt with Chris Coombs(sp?) as well as the now past Jack Fishers. I can tell you their customer service is excellent when I have contacted them from South America and asked them to send me some parts. I think around $14,000.

 

[Marine Archaeology Int.]

Thanks Enrada! I'll check them out

 

[signumops]

The SeaSpy is Overhauser compliant, so you can tow it in any pattern you like, however, maintaining a pattern is essential. If looking for real old targets, you have to keep the fish as close to bottom as possible, and go as slow as possible. Not easy to do, but there is a software, free to use in basic form called GPS Trackmaker that is ideal for the job. Using this package and a NMEA interfaced GPS, you can pre-plot your own survey in advance, place it in world space in GPS Trackmaker and zoom in to max level and plot you track in real time, with position update every 4 seconds if using a serial interface at 4800 baud. I recently completed a mag survey using this method. The data collector for the SeaSpy was run on a separate computer while I piloted the boat using GPS Trackmaker on an older laptop. Zoomed in max on GPS Trackmaker, the built-in grid display is about 50 foot N to S and E to W. Regardless of what anybody tells you, it is difficult to maintain exact lanes while trolling the fish, so exact updating at legible scale is almost mandatory, and you have to have the display at the pilot's seat. GPS Trackmaker exports a KML file of you track so you can see it in Google Earth, and it also imports Shape files as necessary. Once you have saved your tracks you can display them as separate colored features in the program. I have included a screen shot of the coroborating SeaSpy data and you can see that the waypoints match the track data. BTW, isolate your power sources for the mag, the mag recording computer, and the tracking computer from ship's power. I use a separate 12 volt car battery, and the mag is isolated to a 24 volt battery. All three systems should run on independent power sources, otherwise, you will have errors in your mag data.

 

[ScubaFinder]

Neat stuff Terry. I do something similar with Open Captain and the SAR plugin. I generate the search grids and then import the GPX files straight into my Humminbird 998SI. That way I can watch sonar data right next to my search lines. :-)

 

[signumops]

Never tried Open Captain... sounds very interesting. I will look into that!

 

[Marine Archaeology Int.]

Thanks Signumops. Pretty impressive. I will take all your advice into consideration!

 

[ropesfish]

I have been giving this very question a lot of thought over the last few months so for what it's worth- and that may be just a little less than nothing--here's my considered opinion:
Assuming an experienced operator who is obsessive about laying out and making perfect lines on the chart and takes into account all the best practices in running a mag...
If you are surveying an area that has not been magged, has not been magged with modern equipment or has been magged by someone using poor technique, then any good quality magnetometer/GPS/software package combined with high precision navigation will give you adequate results. If ferrous materials are present, you have a good chance of finding them as long as you do everything right.
On the other hand, if you are surveying an area that has been magged numerous times with state-of-the-art gear by experienced professionals (as most of the East Coast of Florida, da Keys and a good bit of the Caribbean has been) then in order to expect superior results (finding that which previous efforts did not discover) you either need to use superior equipment or superior technique (or get lucky). The "easy" targets have been identified, dove on and/or dug up. Many of the cannons and anchors that went down with those ships have been salvaged and some moved in order to conceal the location of a particular area. It's a tough game out there.

 

[Marine Archaeology Int.]

Thanks Ropesfish. This is actually a very thoughtful answer to my question. In my particular case, the area to be surveyed has never being magged before, at least to my knowledge. The site is also very large (aprox 50 sq miles), far off and difficult to reach. Therefore, in an effort not have to go there more than once, a large expedition to sea is being put together. Considering all the other costs (vessel, fuel, personnel), a state of the art mag system that can run wider lines and not miss anything, even at a $100K price tag, might be worth it.
We will give your opinion, as well all others, a careful thought. I certainly appreciate your help in this regard.

 

[ropesfish]

Good to hear that someone, somewhere gets to do a big expedition again!
As I understand the reasoning behind the gradiometer concept, one of the advantages is getting a pretty precise location on the first pass, which is not always the case with a single fish. On a long range expedition where your onsite days are limited by the amount of fuel, gear and food supplies (assuming a watermaker) that you can safely take aboard, the less time spent retracing steps with the survey boat the better.
Don't forget...there is always this option:
"

• Combine as many as 6 systems for increased coverage – the internal CM-221 Mini-Counter allows multi-sensor data concatenation. Multi-sensor systems allow dense coverage that maximizes detection of small targets."
• 

So...depending on your daily survey costs, maybe streaming multiple gradiometers like that or like the petroleum companies geophysical survey crews do for seismic surveys, using planers to get horizontal separation.

Equipment like a top-line mag is never a sunk cost as long as it survives the trip anyway. Resale on new gear has to be at least 60% of cost and there is always the option to do surveys for hire (or let me 'store' it for you.
I do have a couple of ideas about how to make survey faster, cheaper and more efficient for a fairly reasonable cost, but that would be best discussed in a PM.


<Flight of ideas time> The quality of autopilot (or lack of one) aboard the survey RIB could also be very significant as far as the quality of survey lines...would it be reasonable to check the autopilot's lines with a GPS tracker or two?

 

[Marine Archaeology Int.]

OMG! That thing is towing 5 fish all at once! And I was concerned about maybe two. HA!

Thanks Ropesfish. I might reach out to you before we deploy whatever system we decide to use. I always enjoy your insights in this forum.

 

[ropesfish]

An update on a couple of things in this old thread:
Since we bought one of GME's Seaspy magnetometers and it appears that I am the new "mag guy" on the crew, I am spending a good bit of time familiarizing myself with Open Captain, Humminbird PC, Raytech Planner and GPS Tracker. We have 3 boats - a 19' Pro-Line, the 26' Stamas and the 38' Bertram, any of which could be used to pull a mag, a sidescan or a sub-bottom profiler. (Yes....I have plans ) Jason and Bill Seliger have done some very good work with these Seaspy mags and were kind enough to send along some great info on how they managed their mag surveys. Thanks to both of them...I can see that I have hope!
Some observations:
There is a lot to be said for having the same electronics on every vessel, as each manufacturer has their own proprietary operating systems and interfaces. At the moment Showdown has a Raymarine setup with it's own radar, sounder and nav gear, the Carib has a Humminbird 998 and the Proline will be getting a Garmin 4612 and sounder. I am not a great fan of Raymarine at the moment so I suspect we will be all Humminbird or all Garmin within a year unless I find a way to integrate it all into a PC based system that is reliable, customizable and less expensive. In the interim, I'll be looking around for an 1198 head unit to use as the portable survey navigation unit that stays with the mag and is powered by a separate battery (as Terry suggests above).
I have a couple of laptops with GPS pucks and a Surface tablet with an integral GPS. When I get time I will be experimenting with GPS Tracker as mentioned above.
So many of these areas have been magged in the past, I am hoping to refine my technique so we can get better results. Time will tell.
and now...if you are planning to use Open Captain and the SAR plugin...well...Open Captain is not offering that plugin as of today (01Jan2018) so I hunted it down and snagged the original off of Google Earth. Here's the download page link: https://sites.google.com/site/navigationalalgorithms/

I'm reading "A Field Guide to Geophysics in Archaeology" and just ordered a book that I am looking forward to reading as well...a cheery volume called: "Ship of Death-A Voyage that Changed the Atlantic World"
Here's the link: https://read.amazon.com/kp/embed?as...linkCode=kpe&ref_=cm_sw_r_kb_dp_Ze5uAb5M1XZBY

Now...enough frivolity...I have to go fix a boat.
More as it happens.

 

[Marine Archaeology Int.]

On our end we are developing an all aerial mag system to be deploy as a stinger, for both fix wing or helicopter, and pretty soon as a drone application as well. Considering the advances in the new miniaturized mag units (not commercially available yet, but soon) I have no doubt this is the future of this marine surveys (at least for shipwrecks down at 40' or less). I'll be happy to share results once I test and understand what the hell I'm dealing with here.

 

[signumops]

Bill:
As you know, I have added the GPS tracking utility to DigTrakR, so I would not use GPS Tracker anymore. The advantage to using DigTrakR, in your case anyway, would be that you could isolate iron targets from the existing dig charts, and build a quick grid atop them to use for navigation as you towed your SeaSpy. I did a screen shot of the GCB cannons and anchors with such a grid as seen below with 40 foot lanes:

View attachment 1535254

The problem using the SeaSpy in these areas is bottom strike, which throws a false signal. Basically you want your boat driver to follow the lanes and keep you prompted for actual depth every few seconds. Meanwhile you operate your own computer using SeaLink and if your depth sensor on your SeaSpy is working properly you will have some idea of how far you are off the bottom... the idea is coordination where you can tell the boat driver to speed up to keep your SeaSpy from striking the bottom. This depth issue is critical for maximum sensing as is the layback if you want to accurately map your survey. You are looking for very weak signals on these old wrecks. The slower you can go, and the closer to bottom you can maintain the fish, the better will be your results. Also, since most of these mapped targets were fixed using sextant shots, you can't get too excited to see your SeaLink data does not match up with what your boat driver tells you as you pass over the target symbols found in the original maps. You will probably find them to be way off. Meanwhile, I am sure you already have iron targets in the field you have exact coordinates on for test purposes. I have three in the area of Wabasso I can furnish to you within six feet by GPS fix that I know will show up with the SeaSpy.
I have not made mention of DigTrakR on Tnet so far, but I can say that its previous version was called MagTrakR and that version has full support for SeaSpy and AquaScan mag mapping and editing. I did not add that module to the DigTrakR program, but I can, however I don't want to distribute it. The mag module has an editor built in so that you can make corrections to your SeaSpy data file and use the remaining good data for mapping. The mag module will map your survey for you, and I have used it very effectively at the Cabin Wreck, the Unknown Wreck, and elsewhere. Editing the SeaSpy data and mapping it is not a simple process, but it is definitely worth the effort.

 

[seekerGH]

OpenCPN is a great program! https://opencpn.org/ Have not heard it called open captain before...

It works very well with a WAAS and Linux.

It is useful to combine a sidescan with the mag array, that way you can track both the mag signal and correlate it with the sidescan data. This also helps you keep it off the bottom and optimize the search pattern.

 

[Whitt]

Hey guys, new to this thread & looking for some advice. We run either a JW Fishers Pulse 12 or Proton 4 depending on our survey needs. Most of the time it's the Pulse 12. We used to have a dedicated survey boat and the mag was connected with the plotter. Search grid was placed on the Garmin manually using waypoints. As you can imagine, this was very time consuming (although it worked very well and we had great success). It's been a few years now and we sold the survey boat. I'd like to run the navigation through a laptop now and find the best navigation software/application to do the job. I'm also looking for a much less tedious way to set up the search grid. Going to check out OpenCPN when I get home. Thanks!

 

[seekerGH]

Be sure to check out the addons. https://opencpn.org/OpenCPN/info/downloadplugins.html

With some programming, you can add it all up, even couple it with some autopilots to set up the search pattern.

The mag can output in .kml to Google Earth.