Use the Kursk Magnetic Anomaly. It is apparently easy to notice (humanity noticed it as early as 1773!) and is interesting for any future technical civilization for practical purposes, as it is one of the most massive iron ore basins on the planet (some sources claim it is about 50% of the surface iron ore reserves). So they will start digging.
Apart from that, it has the advantage of being part of the east-european craton, so it has higher chances to survive plate tectonics.
Anything directly on the surface will still get destroyed by glaciation, but there are huge iron ore mines at the area, which may leave some traces, and grab some attention.
There are also big underground mines.
So, here is somewhat practical proposal: put big blocks of neodymium-iron magnets in some of the possible more noticeable places of KMA, like centres of the largest mines, places of the strongest magnetic anomaly, or places with the largest not-yet-mined Fe deposits. Bury them somewhat. These could work as beacons, and are likely to be noticed by magnetic measurements (which are likely to be conducted either to determine where to dig for iron, or out of scientific curiosity) or when digging iron ore.
(information would be inside of the huge iron blocks, so they would work also as a shield)
Burry the message also in KMA in some of the deep mines, and give coordinates to it relative to the beacons. These will work regardless of big plate movements.
How large a magnet would you need to be noticeable at a reasonable distance? (E.g. how large a magnet would you need for us to have noticed it at a depth of 30 feet or whatever?)
How expensive is the cheapest magnet of that size, and will they last 500M years?
Intuitively, I wouldn’t expect that you can use magnets to significantly increase the visibility of the beacon.
Somewhat more precise estimate, with magnetite, as it is apparently the cheapest material (more on that later).
The mineral magnetite has a volume magnetic susceptibility ca 1x10^6 − 5.6x10^6 (roughly, that is the ratio of the created field compared to the “background” field.
In far regions, the magnetic field of a sphere with radius $a$ behaves as magnetic dipole with moment $ m = 4⁄3 \Pi a^3 M $, where magnetization M is roughly external field x susceptibility.
Dipole field decays (the important quantity being “magnetic flux density”) in the leading order approximation as $ \mu / (4Pi) m r^-3 $
So, a 1m magnetite sphere will create larger than 10% disturbance in the external field as far as 100m away. Earth’s surface magnetic field strength is between 0.25x10^-6 and 0.65x10^-6 Tesla, fluxgate magnetometers discovered in ca 1936 have precision of order 10^-8 Tesla.
So by this simple account, something which can be noticed by surface magnetic measurements is not that difficult to construct. As I see it now, the bigger problem is creating field disturbance “strange enough” that it will be noticed as something not natural, in comparison to the natural background. There is huge amount of magnetite in the area.… E.g. straight lines of material are bad, as there is a lot of magnetite deposits of this kind.
Also the limiting factor is probably not the depth of burial, but the spacing of the measurement grid—according to this source, “Magnetic surveys are usually made with magnetometers borne by aircraft flying in parallel lines spaced two to four kilometers apart at an elevation of about 500 metros when exploring for petroleum deposits and in lines 0.5 to one kilometer apart roughly 200 metros above the ground when searching for mineral concentrations. Ground surveys are conducted to follow up magnetic anomaly discoveries made from the air. Such surveys may involve stations spaced only 50 meters apart. ”
To get to the practical proposal.
KMA is so big that it is easily measurable from space, or noticeable by relatively primitive technology. It seems likely that it would be interesting for any technical civilization looking for sources of iron. It also seems likely such civilization would do higher resolution survey with lines spaced between hundreds of meters to ~ kms, as it is the way how to find the highest mineral concentrations.
The question is what would be noticeable pattern. My first guess is that rather than create something completely new, it may be better to use the largest existing open pit mines, or deposits of waste material. I’ll try asking some geologist how strange would these look in future magnetic surveys, when buried. Than the next step could be 1] trying to shape the mine or the deposit so it looks more un-natural 2] creating some higher resolution structure, such as big cube of magnetite in the waste deposit 3] create some non-magnetic beacon in the area
The advantage may be that 1] and 2] may basically mean moving some materials withing an existing mine. How much does it cost is a bit unclear, in practice would depend on negotiations with a mining company in Russia.
(some caveats of the whole proposal are that the whole area at some unknown point in the future may be glaciated, or turned into a desert, or have some other property making it unattractive for exploration)
Also it may be worth looking into other types of mining efforts as a place for beacons: the general pattern is technical civilizations will be looking for some raw minerals, so the largest deposits are something like “Shelling points”. And at the same times mines & waste deposits are some of the largest “structures” humanity creates.
I’ll do a more precise calculation in a day or two, but magnetic materials are noticeable when buried. The intuitive reason is the spins in ferromagnetic materials align themselves with the external field, increasing the field inside the magnet so strongly, that it changes earths field noticeably even at somewhat large distances, and even when buried.
The thing to optimize for is likely something like field change when buried of an object large enough to be visible on some realistic grid size, per dollar.
As a quick guess, something like a spiral or other shape 1km wide made from ferite magnet 1x1m cross-section would have been noticed by a survey using WW2 instruments when buried 10m deep, in an area where some survey was done.
Nice.
So a more specific proposal:
Use the Kursk Magnetic Anomaly. It is apparently easy to notice (humanity noticed it as early as 1773!) and is interesting for any future technical civilization for practical purposes, as it is one of the most massive iron ore basins on the planet (some sources claim it is about 50% of the surface iron ore reserves). So they will start digging.
Apart from that, it has the advantage of being part of the east-european craton, so it has higher chances to survive plate tectonics.
Anything directly on the surface will still get destroyed by glaciation, but there are huge iron ore mines at the area, which may leave some traces, and grab some attention.
There are also big underground mines.
So, here is somewhat practical proposal: put big blocks of neodymium-iron magnets in some of the possible more noticeable places of KMA, like centres of the largest mines, places of the strongest magnetic anomaly, or places with the largest not-yet-mined Fe deposits. Bury them somewhat. These could work as beacons, and are likely to be noticed by magnetic measurements (which are likely to be conducted either to determine where to dig for iron, or out of scientific curiosity) or when digging iron ore.
(information would be inside of the huge iron blocks, so they would work also as a shield)
Burry the message also in KMA in some of the deep mines, and give coordinates to it relative to the beacons. These will work regardless of big plate movements.
How large a magnet would you need to be noticeable at a reasonable distance? (E.g. how large a magnet would you need for us to have noticed it at a depth of 30 feet or whatever?)
How expensive is the cheapest magnet of that size, and will they last 500M years?
Intuitively, I wouldn’t expect that you can use magnets to significantly increase the visibility of the beacon.
Somewhat more precise estimate, with magnetite, as it is apparently the cheapest material (more on that later).
The mineral magnetite has a volume magnetic susceptibility ca 1x10^6 − 5.6x10^6 (roughly, that is the ratio of the created field compared to the “background” field.
In far regions, the magnetic field of a sphere with radius $a$ behaves as magnetic dipole with moment $ m = 4⁄3 \Pi a^3 M $, where magnetization M is roughly external field x susceptibility.
Dipole field decays (the important quantity being “magnetic flux density”) in the leading order approximation as $ \mu / (4Pi) m r^-3 $
So, a 1m magnetite sphere will create larger than 10% disturbance in the external field as far as 100m away. Earth’s surface magnetic field strength is between 0.25x10^-6 and 0.65x10^-6 Tesla, fluxgate magnetometers discovered in ca 1936 have precision of order 10^-8 Tesla.
So by this simple account, something which can be noticed by surface magnetic measurements is not that difficult to construct. As I see it now, the bigger problem is creating field disturbance “strange enough” that it will be noticed as something not natural, in comparison to the natural background. There is huge amount of magnetite in the area.… E.g. straight lines of material are bad, as there is a lot of magnetite deposits of this kind.
Also the limiting factor is probably not the depth of burial, but the spacing of the measurement grid—according to this source, “Magnetic surveys are usually made with magnetometers borne by aircraft flying in parallel lines spaced two to four kilometers apart at an elevation of about 500 metros when exploring for petroleum deposits and in lines 0.5 to one kilometer apart roughly 200 metros above the ground when searching for mineral concentrations. Ground surveys are conducted to follow up magnetic anomaly discoveries made from the air. Such surveys may involve stations spaced only 50 meters apart. ”
To get to the practical proposal.
KMA is so big that it is easily measurable from space, or noticeable by relatively primitive technology. It seems likely that it would be interesting for any technical civilization looking for sources of iron. It also seems likely such civilization would do higher resolution survey with lines spaced between hundreds of meters to ~ kms, as it is the way how to find the highest mineral concentrations.
The question is what would be noticeable pattern. My first guess is that rather than create something completely new, it may be better to use the largest existing open pit mines, or deposits of waste material. I’ll try asking some geologist how strange would these look in future magnetic surveys, when buried. Than the next step could be 1] trying to shape the mine or the deposit so it looks more un-natural 2] creating some higher resolution structure, such as big cube of magnetite in the waste deposit 3] create some non-magnetic beacon in the area
The advantage may be that 1] and 2] may basically mean moving some materials withing an existing mine. How much does it cost is a bit unclear, in practice would depend on negotiations with a mining company in Russia.
(some caveats of the whole proposal are that the whole area at some unknown point in the future may be glaciated, or turned into a desert, or have some other property making it unattractive for exploration)
Also it may be worth looking into other types of mining efforts as a place for beacons: the general pattern is technical civilizations will be looking for some raw minerals, so the largest deposits are something like “Shelling points”. And at the same times mines & waste deposits are some of the largest “structures” humanity creates.
I’ll do a more precise calculation in a day or two, but magnetic materials are noticeable when buried. The intuitive reason is the spins in ferromagnetic materials align themselves with the external field, increasing the field inside the magnet so strongly, that it changes earths field noticeably even at somewhat large distances, and even when buried.
The thing to optimize for is likely something like field change when buried of an object large enough to be visible on some realistic grid size, per dollar.
As a quick guess, something like a spiral or other shape 1km wide made from ferite magnet 1x1m cross-section would have been noticed by a survey using WW2 instruments when buried 10m deep, in an area where some survey was done.
Excellent idea about KMA. However, needed Niodim magnets should be to be very large and expensive