There is no practical method for measuring "the ball on dirt lower than standard surface level of the fairway". For example, fairways have slopes, mounds, depressions etc... which make them anything but level surfaces. Also, even on a perfectly flat level grass surface measuring a difference of a millimeter or two would require some sort of precise device.
I honestly feel like this should not be hard. We either A. accept divots and hitting out of them is going to happen.
Or we can all decide that we all honestly know a divot when we see one and can get relief from it. I feel like anyone that has played golf for a reasonable amount of time can tell the difference from a Divot caused by a club versus a depression in a fairway.
but. We could really take it a step further and call any small break in the fairway where dirt is exposed ground under repair and take relief. The game is hard. Lets make it a little less hard. the game is unfair. We all accept that but lets make it a little bit more fair..