Yup, the R/D/T parameters stay the same, it is a magical sense spell, so extra magnitudes should account for this increase, like extra magnitudes increase the size of an Individual affected.
I ask here because the Troupe is confused. x10 per magnitude is a lot, especially if you increase it twice. But +3 per magnitude seems too little. Even x2 per magnitude seems too little.
I just wondered if anyone else had thought about this
I don't think this is particularly unbalancing, and the range it stops at is just under 1.25 miles, before it starts requiring vis. At that stage it makes sense to use the vis to make the material being sought after. 1.25 miles seems like a long way off, except that people can see much farther through the air, and if there's a mile of rock and earth between them and what they want to get to, that's a lot of earth that needs to be destroyed/moved.
Or a ReTe spell inspired by ReCo base level 25 "transport target up to 1 league," +3 magnitude for R: sight, would get you a ReTe level 40 that could get you some of the mineral directly out of the earth.
As you say, adding a magnitude for size multiplies the volume by 10 and any linear dimension by approximately 2 (yes, I know it is a magical senses spell and uses T: Vision. Bear with me).
It seems to me that the base spell is calculated on allowing you to see through an Individual of dirt: ten cubic paces, which is approximately a cube of dirt 2.15 paces to a side and a diagonal of length 3.7 paces. So three paces is a reasonable distance to see through this amount of dirt.
Adding a magnitude for size increases the volume by ten, and approximately doubles any linear dimension of the cube. So if we've settled on 3 paces for the Individual, then it is 6 paces for the Ind+1, 12 paces for an Ind+2, 24 paces for an Ind+3, etc.
That's how I'd do it, anyway
Well, Miner's Keen eye is quite limited in a way. It only sees through 3 paces of intervening materials, and it only does so for the purpose of letting you spot a chosen mineral. It doesn't let you, say, see through a wall even if the wall is not as thick as the spell can let you see, but it will show you silver stashed in the wall if it's there. The base 20 is not so limited - you can see to the horizon through a mountain, if you want, and through the earth into hell (if there is hell down there). It can be quite powerful.
I think it’s because this is not a spell to se through dirt. It’s a spell to detect silver - the seeing through dirt is just a cosmetic effect. You just detect the silver, it could as easily had sung a song to convey the message of its existence in a Hearing version of the spell.
You can’t see buried skeletons or worms or anything else.
Would make sense. Spell description should include that there is a sort of fog of war that obscures everything except the silver and that you do not see anything if blocked by anything but dirt.
The cosmetic effect will probably still reveal that there is intervening material and where it is and what shape it has... Still a bit more than cosmetic I'd say but your interpretation is the best one so far I think.