On earth, there exists large amounts of water underground in aquifers. Curiosity has proven that water used to exist on the surface. If this is true, Mars still may have underground reservoirs of water...
Has there been any research/missions to search for underground water reservoirs?
One technique used on earth to search for underground water reservoirs, without digging, is to use Proton Magnetic Resonance (PMR). Is there a reason such a technique has never been applied to Mars, to search for underground reservoirs up to 150 meters?
Side note: Can PMR also be used for discovery of caves/lava tubes too?
Finding a cave, or underground water reservoir would be a place of high scientific interest and/or possibly a good place to colonize...
Clarification -- My main question is in regards to looking for underground resources, such as water an/or caves, any technique is a welcomed answer. PMR is simply one example that my research has found.