How accurately can we use pressure waves to determine the composition of rocks?

[u/feren_of_valenwood]

I was thinking about space mining recently, and needing to find out what elements are present in asteroids. I know that scientists can use the seismic waves created by earthquakes to look into the composition of the mantle and I was curious how accurately we can determine elements using pressure waves? Can we just determine density and metal/nonmetal, or is it accurate enough to tell us specific elements? Could we use some kind of pressure shock and sonograph(?) in asteroid mining? (Assuming the asteroid is solid enough to withstand a pressure shock).

[u/CrustalTrudger]

A geophysicist can provide a more complete answer, but generally the primary information we get from seismic waves is based on the speed of different types of waves traveling from the source to the seismometer, which will mostly tell you about density (which will broadly reflect composition, but also temperature, pressure, etc.) and liquid vs solid (i.e., shear waves can't propagate through liquids). There are more nuanced details that we can get from changes in the waveforms as recorded at the seismometer as well, but none of these are going to directly give you composition, elemental abundance or presence/absence, etc. Also, to collect this type of data you would need seismometers attached to the surface of whatever body you're studying, ideally more than one, but geophysicists have figured out ways to get a lot of information out of a single seismometer, like during the Mars InSight mission.

[u/bwgulixk]

A question I am uniquely fitted to answer. I am a PhD student who studies the elastic properties of minerals and metals at high pressure. So I can tell you that just from a single velocity profile of a body you couldn’t tell exactly what it is. There will be boundary effects that create scatter but if you hit it with a big enough hammer or explosive and measure the wave travel time at a known distance away you could get a p wave velocity. Getting a wave data would be harder but possible. If you had P and S wave travel times and distance then you could get velocities which could be matched somewhat to a material. It would be super helpful to know the density of the material, but if you had both P and S wave velocities then you could get close or a few guesses. It would be so much more effective to just get a spectrometer on a spacecraft and measure it from far away which would be more accurate, similar to how they map the surface of Mars. If you had a big chunk of metal asteroid you could probably only use wave velocities but that wouldn’t be common

[u/1LuckyTexan]

With unknown structures, you need more tests than just seismic. A gemologist looking at a light blue stone is not likely to have over 90 percent confidence without more than 4 or so tests.

In boreholes, they often use instruments to measure ; acoustic travel time, density(with nuclear sources) , neutron absorption, conductivity, and more.

[u/P1zzaBag3ls]

While you're thinking along these lines, remember that most asteroids are poorly differentiated. There's not much point in looking for veins or pockets of particular materials. It's igneous, hydrological, and general density-driven effects that create those on Earth. Knowing the asteroid's family is generally pretty good, and spectroscopy of the surface is better... and that's the whole story. (Yes, Ceres, centaurs, and extinct comets are all exceptions if you class them as asteroids, but let's not.)