Magnetometer Metal Detection

Part of what motivated me to start playing with the magnetometer was an episode of Deep Sea Detectives I had recently watched on Netflix.  I believe it was the "Damn the Torpedoes" episode, where they used a magnetometer to find a ship that presumably had been buried by silt in a hurricane.  The magnetometer used in DSD was a cesium magnetometer, which is more sensitive, but probably a lot more expensive, even if it were a simple matter for an ordinary person to get their hands on cesium.  The HMC5883L that I'm using has a resolution of 2 milli Gauss, or 200nT (nanotesla).  A cesium magnetometer is probably several orders of magnitude more sensitive.

The plot shows "detection" of metal.  In this case, I moved the remains of my old internal Zip drive (which I had disassembled after viewing a "maker" youtube video along those lines) between the sensor and ground (as in the stuff beneath your feet, not the electrical sense in this case).  The off-scale measurements in the chart are probably an implementation error in the HMC5883L library mentioned in my previous post.  The maximum range of the measurements is -2047 - 2048, so I suspect that the values are being incorrectly converted from the 12-bit measurements to a 16-bit quantity.

The following list, which I unabashedly stole from geometrics.com, should give you an idea of how that rates in a real situation where a marine magnetometer is employed:

Object	flux density	measurement range
Ship 1000 tons	0.5 to 1 nT	800 ft (244 m)
Anchor 20 tons	0.8 to 1.25 nT	400 ft (120 m)
Automobile	1 to 2 nT	100 ft (30 m)
Light Aircraft	0.5 to 2 nT	40 ft (12 m)
Pipeline (12 inch)	1 to 2 nT	200 ft (60 m)
Pipeline (6 inch)	1 to 2 nT	100 ft (30 m )
100 kg of iron	1 to 2 nT	50 ft (15 m)
100 lbs of iron	0.5 to 1 nT	30 ft (9 m)
10 lbs of iron	0.5 to 1 nT	20 ft (6 m)
1 lb of iron	0.5 to 1 nT	10 ft (3 m)
Screwdriver 5 inch	0.5 to 2 nT	12 ft (4 m)
1000 lb bomb	1 to 5 nT	100 ft (30 m)
500 lb bomb	0.5 to 5 nT	50 ft (16 m )
Grenade	0.5 to 2 nT	10 ft (3 m )
20 mm shell	0.5 to 2 nT	5 ft (1.8 m)

Of course, their product is trying to do a high-level survey, where the magnetometer is tethered to a ship and is well into the water column.  In my application, I intend to have a hand-held device that I'm moving along the sea/lake bed, so I'm not going to be trying to detect a screwdriver from 4 meters, I'm going to be trying to detect a screwdriver from, maybe 1m.

I have no idea how much one would have to pay for the geometrics magnetometer, but I figure it's one of those situations where "if you have to ask...".

There is still plenty to do to even approach a usable metal detector.  Unlike traditional inductive detectors, magnetometers are measuring minute changes in the magnetic field flux density (no, really, I'm not trying to go all technobabble).  Inductive detectors can detect non-ferrous metals, but have a much lower sensitivity and therefore lower range than a magnetometer.  Obviously, then, the magnetometer is restricted to ferrous (iron-based) metals but can have a significantly higher range of detection.  The other issue, and probably the one more significant, is that any rotation of the sensor along any axis will show up in the data.  The trick will be to distinguish between movement of the detector and actual metal.