Personally I think starbases and other static defenses need a buff and mines should just be taken out entirely.
Barring that, limit each grav well to 3-5 fields a piece, which seems like it would be a simple thing to do (but sitll buff the other defenses).
Barring THAT, each side should get a ship, or an ability added to an existing ship (say the fighter defense frigates) to minesweep; that is, if the mines are detected by your scout, the ship can detonate the mines from a distance, within a certain radius.
As it stands, mines are far more annoying than cool and strategic.