Spaceborne radar cannot penetrate the earth for groundwater exploration. It is true that the higher the wavelength the higher is the potential penetration. But this applies to the first centimeters of surfaces and only works with non-vegetated, dry coarse sands or certain types of vegetation.

This page gives quite nice maps on it: http://www.webalice.it/alper78/esempi/libia/radar.html

SAR signals can not reach groundwater levels in any case. They are however sometimes used to detect the amount of moisture in filled wadis, for example. Even if the top surface is covered by sands, SAR might reveal some subsurface structures like ancient riverbeds where discharge concentrates. Nice examples are given in this paper Article Mapping Palaeohydrography in Deserts: Contribution from Spac...

So, the only thing that remains is to use SAR data (or optical imagery) to detect structure which help to describe landforms, patterns, fractures and materials which all give hints on groundwater. To detect the presence of groundwater from space is technically impossoble.

We once tried to summarize all remote-sensing aspects on groundwater exploration in one compact paper:

Article Using Remote Sensing and GIS to Support Drinking Water Suppl...

Actually the main purpose what I wanted was for groundwater table and real value of water content under surface, dont want to interpret it. Like what we do with GPR (Ground Penetrating Radar).

Between yeah, thank you for the details you have provided.