This week a boat in Hamburg was carrying dangerous cargo.
It was an unexploded grenade that had been found on a building site. Now it might seem weird that this kind of cargo is being transported on a boat on the largest lake in Hamburg, but a time fuse made it impossible to defuse the bomb. The safest way to handle this, was to carry the grenade out into deeper water and explode it under controlled circumstances.
This is quite a problem in Hamburg and Germany in general, when there is a building site, it’s quite common to find blind shells from WWII. It can be quite risky to just start building somewhere without knowing the subsurface.
So geophysics knows a solution to this problemFoerster – Brochure on landmine and UXO detection. Ironically, this solution was developed for the purposes of war.
When submarines were in vogue for sea battles, people obviously wanted some way to find those sneaky machines. Since they’re built of metal there was an easy way to detect them using some form of magnetic anomaly detector. These could be used from an airplane to search the seas for a threat.
Grenades, bombs and submarines have one very important thing in common. They’re made from metal.
When searching for blind shells we could just use these planes that were used before, right?
Unfortunately we cannot. Submarines are a little bigger than blind shells. Another “problem” is that geophysicists like to be paid for their work, so a lot of building sites use an “on-the-fly prospection”, which basically means they start digging and have a look for unwanted surprises, instead of doing a proper inspection beforehand. This can be quite expensive when the entire building site has to be evacuated but many builders are willing to take the risk. However, some people take the precaution and this is how a magnetic measurements work.
We all know the comparison of the Earth’s magnetic field to one of these old magnets with two poles. However, the global magnetic field underlies some variations over time. Every five years a global model is issued to account for the newest measurements. This is called the International Geomagnetic Reference Field (IGRF)Finlay, C., Maus, S., Beggan, C., Bondar, T., Chambodut, A., Chernova, T., Chulliat, A., Golovkov, V., Hamilton, B., Hamoudi, M., Holme, R., Hulot, G., Kuang, W., Langlais, B., Lesur, V., Lowes, F., L?hr, H., Macmillan, S., Mandea, M., McLean, S., Manoj, C., Menvielle, M., Michaelis, I., Olsen, N., Rauberg, J., Rother, M., Sabaka, T., Tangborn, A., T?ffner-Clausen, L., Th?bault, E., Thomson, A., Wardinski, I., Wei, Z., & Zvereva, T. (2010). International Geomagnetic Reference Field: the eleventh generation Geophysical Journal International, 183 (3), 1216-1230 DOI: 10.1111/j.1365-246X.2010.04804.x
When a magnetic measurement differs from this IGRF we call this an anomaly, which could be an indicator for metal in the subsurface. But how do we even take this measurement?
The Proton Precession Magnetometer.
There are many different methods to measure the local magnetic field and this is my personal favorite. It uses a very simple principle, most of us know from childhood days: Spin-tops. They’re objects that cannot stand on their own but when they start spinning, they stand on their tip. However, when they lose their spin they start wobbling and eventually fall over. This wobbling is called precession and coincidentally most particles have some sort of spin too. In this magnetometer we use protons, because they all have the same “spin direction”. Some proton-rich fluids are kerosene or even just water.
A problem at this point is that all those protons in the fluid are already in the wobbly precessing phase and they’re pretty chaotic that way, but lucky us, we can use a trick to align some of those protons. We just switch on our own very strong magnetic field and a couple of those protons will line up. We wait a little and then switch off our strong magnetic field and now the local magnetic field will knock our protons over out of their nice alignment, but since they still have their spin those protons will not just tumble and fall over, they will start precessing around their axis. Another very nice thing here is that those protons will wobble depending on the force of the local magnetic field. A strong local magnetic field will knock the protons quite hard and the precession generated will be accordingly strong.
Unfortunately, we can’t just look at those protons spinning around. But when they start precessing around the local magnetic field the protons generate weak electromagnetic radiation that can be picked up and measured by a coil in the magnetometer. The radiation in the coil will create a current proportional to the force of the radiation. This can be easily translated to our local magnetic field.
The proton precession magnetometer is very accurate, but unfortunately it takes quite some energy and batteries are heavy. In 1953 Overhauser proposed an effect that we can use to significantly improve many aspects of the proton precession magnetometer.
The Nuclear Overhauser EffectNuclear Overhauser Effect
When we first used our proton precession magnetometer the protons were aligned along a magnetic field. This is a form of polarization, since we align the spins of the proton along the magnetic field. Overhauser found that one particle with a certain spin can help polarizing another particle with a different spin. In contrast to classical spin-spin coupling these particle do not have to be bound chemically, they just have to be in proximity to each other. This has two huge benefits. One benefit is the enhancement of the signal we record from our magnetometer, which leads to higher sensitivity and therefore, better measurements. But also, we can use electrons which can be polarized using a low power radio-frequency field.
So we can basically say that we get better signals from lower power consumption.TOH, H., T. GOTO and Y. HAMANO (1998): A new seafloor
electromagnetic station with an Overhauser magnetometer, a magnetotelluric variograph and an acoustic
telemetry modem, Earth Planets Space, 50, 895-903 Terrapub Great trick, isn’t it?
Personally, I think it is pretty impressive to use this kind of high tech to make the world a safer place.
The grenade was detonated under water in the middle of Hamburg, not causing too much of a hassle for the general public. This is another great situation where geoscientists can be consulted to minimize hazards for the general public.
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