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I want to use an unmanned aerial vehicle to tow an airborne gradiometer. Goal is to detect historic anchors buried in the sea floor. What is the effect of (sea)water depth on the magnetic signal? Any standard equation(s) I can use to determine max detection depth in relation to size (weight) of ferrous target? Sensitivity and resolution of the potassium sensor are 0.0003 nT @ 1 nT and 0.0001 nT, respectfully. Thanks!

user158037
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Seawater shouldn't have any effect, other than preventing close approach.

For equations you'd probably want Physics stackexchange. Calculate the field pattern and gradient of an iron sphere or iron rod placed in a uniform 50,000nT ambient.

Are anchors that valuable, when compared to cosmic nickel-iron, especially pallasites? Hug the ground, fly a raster across inaccessable public land (no roads, no junk cars etc.) Any out-of-place iron objects would need a hike and a shovel, not rental marine salvage ship.

Note that "Gradiometer" is a magnetometer pair: the signal is DC from the b-field distortions caused by large iron objects. (Clue: nanoTesla units.) No, oceans don't block Earth's magnetic field. Everyone search "OPM," latest cool magnetometer, like a SQUID but no cryo required, nor heavy shielding. Laser plus rubidium vapor beats proton precession or Overhauser.

But nobody has yet built a DIY hobbyist OPM instrument. How far below a hexacopter must your magnetometer dangle, to avoid motor amps and stator magnets?!

Yes, this means Christopher Walken's helmet from Brainstorm (1983) doesn't need liquid helium vapor jets anymore.

wbeaty
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Electromagnetic signals do not pass well through conductive water and it's doubtful you will be able to see more than a few inches into the water from above it.

Lloyd Butler has a paper that discusses some of the math and physics involved. See the "Underwater Radio Transmission" link on his web page. http://users.tpg.com.au/users/ldbutler/

You might be able to do this by towing the antenna along the sea bottom but you'd need to get close to the metal to detect it.

Warren
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