So, to summarise what I've been writing over the last week:
- Lava tubes are natural tunnels formed by flowing lava.
- In some places, they have collapsed to form rilles.
- On the Moon they are roughly an order of magnitude larger than their equivalents on Earth.
- They offer considerable protection for a manned base from a variety of environmental dangers.
- Lava tubes would provide an ideal location for a the base for a mission lasting longer than a couple of days.
- Satellite photography can be used to look for intact lava tubes indirectly by looking for discontinuities in rilles.
- GPR can see lava tubes directly, but has serious problems associated.
- Geophones can generate precise data on lava tubes, but require a network of sensors and a number of shock waves in different places.
- "EMP thumpers" combine the advantages of geophones and GPR, but is still under development as a technique.
By the way, a disclaimer: writing these articles is part of a learning process for me. I don't suggest that they're even factually correct (although I do my best). If you find them useful, then great! And if you find any problems with them please let me know.