Reflectors
Base permafrost is normally a strong seismic reflector. Often there are multiple interfaces and lenses of liquid/frozen pore water that complicates the picture but at low frequency you get a good reflection.
On Mars, we're more interested in detecting natural Marsquakes than in active seismic sounding. You can learn a LOT from quakes:-
geographical distribution of activity - stress of crust
depth of activity - thermal structure
transit time and secondary waves - Internal structure of planet
etc...