There are several places on earth where very violent and deep volcanoes bring chunks of mantle up to the surface, so we don't have to drill to the mantle to know what it is made of. Also we know a lot about the earth's interior from seismic waves-more than "average" density. As far as the exact chemical composition of the core, you're right that there's a bit of guessing involved.
They probably meant that the surface waves typically are not felt at the depth of the mine. Surface waves travel along the surface of the earth only (duh) and also happen to be the largest amplitude waves and the most destructive. If there were a large earthquake nearby, you might feel a bump or two from the body waves, but they are nothing compared to the survace waves. A seismometer, however, is quite capable of picking up body wave signals from smaller earthquakes. I'd say the scientists are interested in body waves at depth, since they will be much less attenuated than those recorded with an instrument on the surface.
Slashdot Mirror
There are several places on earth where very violent and deep volcanoes bring chunks of mantle up to the surface, so we don't have to drill to the mantle to know what it is made of. Also we know a lot about the earth's interior from seismic waves-more than "average" density. As far as the exact chemical composition of the core, you're right that there's a bit of guessing involved.
They probably meant that the surface waves typically are not felt at the depth of the mine. Surface waves travel along the surface of the earth only (duh) and also happen to be the largest amplitude waves and the most destructive. If there were a large earthquake nearby, you might feel a bump or two from the body waves, but they are nothing compared to the survace waves. A seismometer, however, is quite capable of picking up body wave signals from smaller earthquakes. I'd say the scientists are interested in body waves at depth, since they will be much less attenuated than those recorded with an instrument on the surface.
Another one I've heard is that there was a lot of sudden earthquake and volcano activity, which caused tsunamis all over the world at the same time.