Things don’t get less complex once the Earth gets involved. The acoustic shock waves may strike soft soil, hard rock, various forms of human infrastructure, and so on, each of which will affect how energy gets transmitted. Some of the energy gets converted into what are called Rayleigh waves, where the energy is transmitted as a wave that moves along the Earth’s surface. The rest go deeper, forming waves that may move through some combination of loose material or the underlying bedrock.
To extract information about the Earth’s structure, you have to understand what the seismic waves from a thunderclap would normally look like. Which, to an extent, requires modeling all of the above processes. Since each thunderquake is going to be unique due to the different locations and conditions, this model is going to be, at best, an approximation. The fear that any approximation wouldn’t be good enough to generate usable data probably kept people from trying to analyze thunderquakes sooner.
To get their approximation, the team started with a software package called SPECFEM3D Cartesian, which is dedicated to 3D reconstructions of seismic waves. Already, that choice necessitates a few compromises. For example, the software treats the atmosphere as a 3.6km-thick homogeneous layer, even though the atmosphere near a thunderstorm is anything but. The model also updates events at a frequency that’s slower than the waves moving through the Earth-air interface. So, to compensate for that, the researchers simply stretched the top 20 meters of Earth out to cover 200 meters.
These and other factors mean that there were plenty of reasons to think that the model wouldn’t be sufficient to handle real-world data. So, the people who developed it tested it against the real world, using thunderstorms that passed by their campus.
Passing the test
One of the nicer discoveries in seismology has been the realization that the same fiber-optic cables that rush cat pics to your LAN can act as seismometers. And, conveniently, the Penn State campus has a 4 kilometer fiber line that has been set aside for seismic sensing running under the campus. And said campus happens to be located in a part of the US where summer thunderstorms are a regular occurrence.