DARPA to build nothingness detector for tunnel sniffing Renowned US military tech bureau DARPA has issued a request for an ultrasensitive gravity-measuring instrument which could be mounted in a low flying aircraft and detect underground tunnels. The project, known as Gravity Anomaly for Tunnel Exposure (GATE), was formally announced on Friday. In essence, it will detect an absence …
This is how you detect subs nowadays - they are too silent so the trick is to scan for a void (and allows you to see that it is a sub and not a whale - Jonah's trip aside, whales are not empty). Since boats have this (for a least two years to my knowledge), it was just a matter of time before it gets put on a plane/copter.
So, come on DARPA, pull the proverbial out and think up some proper loony ideas.
so the technology detects that there is a bit missing by looking at the gravity,
sounds good.
mass of ground creates gravity, take mass away = less gravity...
so what if you line the tunnels with lead?
take air filled dirt mass away and replace with air filled denser than dirt mass surely a ton of dirt and a ton of lead have the same mass, and same gravity, or an I just missing something in pointing out a fairly obvious flaw.
(I assume the same is true of steel as well, though you may have to opt for smaller passages).
...is unlikely to work. You'd need some nontrivial engineering and scientific know-how, some extremely sensitive gravitometers, and a handy supply of large quantities of lead (plus the tools and machinery to work with it) to get it right. And then, after you've hired a team of physicists, metallurgists, and construction workers and spent months and months getting it exactly right, simple ground penetrating radar would show your network as if it had flashing Christmas lights all over it.
Airborne gravity surveys are pretty routine in the oil and gas industry, so it is possible to build gravimeters that can survey being buffeted in a plane and compensate for any movements relative to the terrain.
BUT the differences they're looking to identify here are insanely small - in the tenths or even thousands of a milliGal (i.e. not-a-lot). This sounds only borderline doable, so they'd better turn their beanie-caps up to 11.
So what you end up with is a linear line of a tunnel with a specific density. So while you may have approximated the density of the surrounding surface, you'll have a line with the same specific density so you'll still be able to 'see' the tunnel.
Of course you'd have to know the specific densities of the surrounding earth, or just put in random amounts of mass to try and randomize the readings.
I do wonder if the device could be used to actually try and find oil, gas, or other precious minerals....
My rough estimation and wiki research suggests that lead is about 4 times as dense as your average rock. Even if the cost of material isn't prohibitive, it does seem like shipping enough of the stuff to maintain a 4:1 space to lead ratio would be a bit of a nuisance.
Any detection measure like this can be broken with enough effort. But I seriously doubt that every secret-cave-owner we might be looking for will suddenly jump on the opportunity to gravity-detector-proof his lair.
As Robert Forward & a team developed a highly sensitive g detector in the mid 60s. Spinning weights (not particularly big) machined from a solid block (not uncommon aerospace practice) spin backward & forward at a mechanical reasonsance frequency. g shift gave a resonance frequency shift IIRC.
I was always weary of this. They claimed it could detect the distortion (in gravity) caused by the presence of a hand nearby.
Either 1)I read the report wrong. 2)It was a mistake that was corrected (but I don't have the correction) or 3) This is the real deal and the hardware won't need much of a spruce up for the 21st century.
Pedant's note. Should it not be called by anology with the sting on ASW aircraft a "Gravitic Annomaly Detector?" Or GAD.
I though DARPA only did completely bonkers, almost zero chance of pulling it off. This is nearly off the shelf.
as someone who has been involved in gravity surveys and also with geophysical surveys to attempt to locate shallow subsurface cavities such as old mine workings. Detailed gravity surveys to locate even shallow cavities, where the depth to the top of the cavity is of the same order as the diameter of the cavity, require the elevation of the gravimeter to be known (relative to a local base) to within a few millimeters. The anomaly associated with a cavity decreases rapidly with the depth of the cavity, and increasing the height of the measuring equipment will have the same effect. There is also the problem of geological and topographic "noise".
Certainly airborne gravity surveys can detect major geological features, but these are orders of magnitude larger than even a large tunnel. The use of gradiometers will probably allow greater resolution, but I suspect that even with extremely sensitive equipment this could only have a chance of working if the equipment is flown very close to the ground, which would probably require the use of a helicopter, with the actual sensors in a bird on the end of a cable to allow the helicopter itself to operate at a moderately safe height.
Look at the amount of work put into the Goce orbiter, which is working at a much coarser level and only has to deal with drag from the outer edge of the atmosphere. These guys want something that can measure the gravitational effect (or lack thereof) of a couple hundred kilograms, somehow stabilized against surface-level winds and vibrations from the aircraft's engine. It's not going to happen.
Whenever I encounter a new Lewis Page article mentioning DARPA, I immediately click through to find his latest description of the military boffins (eg "the military research bureau so far beyond the bleeding edge it serves moon-on-a-stick as canapés.").
But alas, it seems as if he's run out of pithy metaphors.
Looks like the Hughes thingy was a g radiometer, rather than an absolute value device.
So putting one of these on each wing-tip, nose and tail flying over a tunnel should give a definite (but small?) dip across at least one pair.
Flying at constant height above ground would probably be a very good idea so terrain following radar is likely to be on board (unless you want to do post flight data reduction based on some kind of recorded ground profile) and some fairly decent vibration isolation. Foam and shock absorbers might do it but probably some kind of active system to null out aircraft movement.
Somehow though it's just not up to DARPA's usual standard. There's none of the usual crazy-as-as&*thouse-rat-with-rabies but if its even 20% there it'll be pretty impressive quality.
OTOH if it were to be artillery launched
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