Radar satellite come in two varieties. Downward looking and
 sideways looking. Sideways looking radars use the motion of
 the spacecraft to synthasize (sp) a very large antenna in one
 direction. A lunar version of the Earth resources satellites with
 a long slab antenna might be a way to get the resolution required.
 Incidently knowing where the satellite is essential to reconstruction
 of the radar data, so the satellite could also act as a gravity probe.
 However sideways looking radars use back scatter from the surface and
 are not that good at deep penetration surveys, however such a survey
 might reveal the surface signatures (slumping) that indicate a lava
 tube.

 A first crack at that problem might be to examine existing photographs
 of the Moon close to the terminator and look for shadow lines.

 For deep penetration of the Moon you want as much power as possible going
 down, so a dish pointed at the surface would be required, synthesis is
 not going to get you sensitivity, but some smart electronics. (spread
 band, encoded waveforms) will help. Once again it will be essential to
 know where the spacecraft is for reconstruction.

 As for geophones. You do not need explosives. Do what was done during
 the Apollo missions. Bombard the Moon with space junk. If your mission
 is not a direct landing on the Moon, but involves a lunar orbit bus,
 then once the geophones are in place. Deorbit the Bus, it should make
 a very nice bang when it lands. And if more data is required simply
 send something heavy to crash on the Moon. This will allow you to
 taylor the mission to your budget.

 "Look we have a couple of million bucks left in the account, and
 the end of the financial year is coming......"

--
Dave Stephenson                  
Geological Survey of Canada            *       BEWARE!
Ottawa, Ontario, Canada                *
Internet: steph...@ngis.geod.emr.ca    *Bill Gates is lurking you!