Originally posted by lookingforanswers01
Well said! It happens all too much on these, and probably any forum.
What about underground radar imaging? You know, like the kind used to look under the ground, find dinosaur remains and the like. Couldn't that be
applied in a satellite application? Or would the radar signal be too weak by the time it hit the earth?
That's part of it. Actually GPR is more suited to finding voids and metallic objects than bones. People do use it for that, but there are a few
problems. One is that Maxwell sort of limits the depths you can penetrate the earth with a high frequency signal. But you need high frequencies to get
good resolution. So the better the resolution, the shallower you can search. The attenuation is bad enough in dry non-conductive soils, but this is a
real problem with wet soils or soils with salt content. It would be really tough to search beneath a salt flat.
Another issue is that you're really seeing either the metallic content of the target, or you're seeing a significant discontinuity in material
characteristics, such as a void, or maybe a concrete bunker in sand.
Therefore to consistently find fossil remains with GPR, you have to have large bones as targets, and you have to have them in a dry, non-salty soil
that is significantly different in material characteristics than bone, so limestone's probably out.
GPR also doesn't really give you pictures, it's more like an old depth sounder.
They usually emit in the thousands of Watts up to tens of kiloWatts at ground level in order to get enough of a return, even for depths of just 50
meters or so, and that's under ideal conditions. The soil really eats up the signal strength.
So from a satellite, you don't have any signal left to start with. You might
be able to drum up a few microWatts with a really high power
transmitter. By the time you get done losing all but a hair of that to the soil, the signal would have to make it back to the satellite.
Then you have the problem with keeping the signal tight. The GPR frequency is usually in the 150MHz range. Emitted from what's effectively a point
source in the scanning truck, it still spreads pretty badly. From a satellite, how tight could a 150MHz footprint be? You've seen those pictures of
the old SAGE radars at 440MHz, it takes a whacking big parabola to focus a wavelength that long. I don't think you could get a big enough effective
aperture on a satellite to get any resolution at all.
GPR is right out for a satellite.
Oh, X-rays do go through concrete, not efficiently, but they do. The problem is that you have to have your imaging material on the other side. So you
can't use it to examine underground. You can use it to check piers or supports for voids, if you can get to both sides and it isn't too thick.