Humans on the Moon would be protected from solar and cosmic radiation by building habitats inside lunar lava tubes. Radiation shielding will need to be at least three meters thick to protect residents from cosmic radiation and solar flares. That is why an intact lunar lava tube is an ideal place to build the first lunar base. They are naturally occurring self-supporting radiation shields that have roofs which are tens of meters thick. Lunar lava tubes are much larger than those on Earth and are large enough to accommodate a large base. They are hundreds of meters wide. They can be thousands of meters long. A base in a lava tube could be easily expanded. An intact lunar lava tube will provide ideal protection from meteors, radiation, extreme temperature fluctuations and the abrasive lunar dust. It is also possible that the lava tubes will contain life support volatiles since they are a stable -20 degrees Celsius (Mendel) any water vapor which entered the tube would freeze and still be there. Intact lava tubes could be found near riles on the border between mare and highland. Partially collapsed lava tubes can be seen in photos of the Moon. They have been considered for use as a location for a lunar base by Coombs and Hawke. These lava tubes have questionable structural integrity because they are partially collapsed. It would be much better to find an intact lava tube. They will be larger since they are whole. They are probably deeper. Also in the future an entire tube could be sealed air tight so there could be earth-like sub-selene cities.
The only problem with using an intact lava tube is finding them. Two possible ways of finding lava tubes are radar or active seismic sounding. There are problems with both. Radar may not work. Seismic sounding only covers a small area a time. Radar on Earth only penetrates about 10 meters because the Earth is very wet. The Moon is extremely dry but other things, such as metals which, may not allow radar to penetrate the 1 kilometer needed for a complete survey for lava tubes. Even if radar does penetrate, large satellites may be required.
Seismic sounding will only cover a few square kilometers at a time. This would mean we would have to have a good idea where to look. Areas with partially collapsed lava tubes and areas with similar geology are a way to narrow the search. The area could also be narrowed by doing a very accurate gravity survey of the Moon. Areas which have too little mass could be areas with lava tubes. One complication with doing an active seismic survey of the Moon is that the dry lunar rock and regolith do not attenuate seismic waves. The Moon rings like a bell when it is impacted. There is no water to absorb the sound vibrations. When the Apollo 12 Lunar Module was crashed into the Moon, the Moon reverberated for over two hours. This means we can not simply use terrestrial analysis techniques on lunar seismic data. Some one will have to develop new techniques.
Both radar and active seismic survey were done as part of the Apollo program. These experiments only obtained small amounts of data. Also the equipments used were primitive by today’s standards. All of the Apollo experiments were designed with limited understanding of the lunar environment. The radar experiments where hindered by the limited data storage technology. This should not be a problem with modern computer equipment. The seismic experiment data had larger error bars then the magnitude of the data. More data and better designed experiments should solve this problem. The experience of Apollo and the technology developed since then along with experiments in the future will give us everything we need to find intact lunar lava tubes.
Once lunar lava tubes are found to use them all we would need to do is to drill an elevator shaft. Then floor of the lava tube will need to be cleared of debris and leveled. A dust-lock could be used to allow a cleaning area for equipment and space suits before entering the base. The ends of the base area could be sealed by stretching the same material used for the inflatable habitats over the opening sealing the section of the tube air tight, it would be a buffer between the lunar vacuum and the high internal pressure of the base. This would reduce the rate of air leakage from the habitat. Once prepared, the tube will be ready to receive inflatable structures.