Can Magnetometers miss wrecks?

[pcolaboy]

I'm curious to know what conditions, if any, may cause a magnetometer survey to not detect a 17th or 18th century vessel or at least cause the data of a site to look 'uninteresting'.

For instance, let's say someone is towing a very capable magnetometer in 40-50 feet of water with a hypothetical range of 500 yards. And lets say that they pass a 5 foot deep shoal 400 yards away that happens to have the remains of an old sailing ship. Would the data collected from that site look less interesting than it would if it were at a depth more relative to the depth that the towfish is running in?

I hope what I'm asking makes some sense. There's a wreck that I've found which is at the very edge of a shallow sandy shoal with a fairly steep drop off to about twenty feet of depth. The shoal appears to have shifted over and on top of the wreck. The last official mag survey done in the general area did not return any "hot targets" but there is most certainly a wreck lying there.

Thanks in advance,

Pcola

 

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[Ascholten]

no ferrous metals in the area or buried very deeply will cause it not to ping too interestingly.

Just because there are parts of a wreck there, it might be mainly wood. A magnetometer is really just a really snazzy sensitive compass sort of..... a nail won't set it off unless you drag it literally almost across it, you need a bit of metal to actually be detected.

Aaron

 

[ivan salis]

mags need "big" iron to work with normally iron cannons or other large iron objects like modern hulls ---- the old vessels were made mainly of wood with iron fittings and cannons --- however in some cases -- bronze cannons were carried -- that was a the mark of a rich vessel -- bronze cannons were much more costly than iron and were highly desired and coveted items -- they were safer -- they cooled off much more rapidly and thus could be safely fired more rapidly than iron cannons --who had a bad habit of bursting if overheated in a battle--- the lack of iron cannons or other large iron objects (also iron object lose their "draw" to a magnet slowly over time --thats one of the ways you can tell very old iron objects from new stuff --a magnet will really "stick" or hold on hard to the the new stuff not as well to old iron ---the lack of large iron objects (the cannons may have been salvaged or bronze) or else its iron objects may have little to no draw left in them because of their of extreme age (300 - 400 years) in saltwater--- some of the vessels in pensocola bay are extremely old----the loss of drawing power of its iron objects over time would make a magnetometer acts as though nothing is there unless it is quite close --since it works based upon magnetic draw to iron objects -- hence the name "magnet -O- meter" --- the combo of "weak signal" and being deeply buried in sand and distance -- maybe the cause of it not "hitting" on the mag.--- mag work is boring and costly you have to run close small distance over lapping sweeps -- often called "mowing the lawn" and just about as much fun---- till you get the good hitsthat is --- -- Ivan

 

[riobravo]

hi pcolaboy,
not sure what machine your using, or how many days you've been dragging or the quality of the software.
try a calm bay with a buoyed piece of metal and drag at different depths to get a feel for the readings as you pass at different depths and speeds.
the mag will give readings only within its limits, i used a proton mag for years, and when i used a cesium mag the first time thought i was king of surveys only to find a new set of variables to deal with and am still learning every job.
slow and consistant lanes with a lot of time spent analyzing data will provide results or drive you crazy.
good luck.................


P.S. the mag is on your side and distance from the target reduces readings exponentially
P.S.S. lots of good reading in the geophysics area

 

[ScubaDude]

I'm curious to know what conditions, if any, may cause a magnetometer survey to not detect a 17th or 18th century vessel or at least cause the data of a site to look 'uninteresting'.

For instance, let's say someone is towing a very capable magnetometer in 40-50 feet of water with a hypothetical range of 500 yards. And lets say that they pass a 5 foot deep shoal 400 yards away that happens to have the remains of an old sailing ship. Would the data collected from that site look less interesting than it would if it were at a depth more relative to the depth that the towfish is running in?

I hope what I'm asking makes some sense. There's a wreck that I've found which is at the very edge of a shallow sandy shoal with a fairly steep drop off to about twenty feet of depth. The shoal appears to have shifted over and on top of the wreck. The last official mag survey done in the general area did not return any "hot targets" but there is most certainly a wreck lying there.

Thanks in advance,

Pcola

I don't know of any mag that would be able to detect the small amount of iron contained on a 17th or 18th century wreck from 400 yards away. You're at or beyond the end range of detecting an steel hulled ship from that range. Read the product literature for any mag a 1000 ton steel ship would only show up as a fairly uninteresting 1 gamma hit from 800 feet away. This is the reason mag surveys are time consuming and costly because you have to run lanes that are very close together so you don't miss anything.

 

[Blacksheep 6]

Pcola...

Wholeheartedly concur w/ Ivan and Brad above...time to "mow the yard"...

Blacksheep6...

 

[rgecy]

Nothing like a mag survey and endless lanes back and forth.

There is a science to running surveys and determining lane spacing. You need to know what size object you want to detect and at what range your mag can pick up that object. Each mag is different.

But Brad and the guys posting above are absolutley right, you are at the threshold for even being able to detect a steel hulled ship, not to mention a 200-300 year old dissentegrated wooden vessel.