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