Ranged attacks against a civilian is always 40 damage.The base combat strength of a tile improvement is 0 (same as say, a civilian unit). However the net damage is the damage done after all air defense modifiers are tallied up. In other words, it would be the same as if you targeted a worker unit with an air strike. If there is no air defense at all, the unit will do its usual full damage (though I think this damage is modified by the HP of the attacking air unit, but either way it will function the same way as current air-strike mechanics).