Dealing with Information Spells in a Megadungeon

Vael, one of the PC wizards in DFRPG: Arden Vul, started with the Seek Earth spell.  For 3 mana, Seek Earth lets you find the direction and approximate distance to the nearest significant amount of any one type of earth, metal, or stone.  It also lets you exclude known quantities of the item.  It uses the Long-Distance Modifiers, which mean no range penalty for up to 200 yards away, up to half a mile at -1, and distances beyond that hardly matter in a dungeon.  Its only prerequisite is Magery 0 or Druidic Power Investiture 1, so essentially every mage, druid, elf, or half-elf can have Seek Earth for one character point.  And every skilled specialist wizard (IQ + Magery 18 or better) gets it at skill 16 or higher for one point, so they cast it for only 2 mana, and it succeeds 98% of the time (16- on 3d6) if the correct mineral is within 200 yards.  If you're rolling against 16, 17 is just a normal failure, costing you one mana point and letting you try again.  Only an 18 is a critical failure, letting the GM lie to you about the presence or location of the item (or possibly summon a demon or whatever other fun comes up on the critical spell failure table), so only once in 216 times does casting this spell really have a chance to hurt.

This is tremendously useful for loot-hunting PCs.  You Seek Earth for gold or silver, exclude whatever your group is holding and any source you previously detected and don't currently want to mess with (example: hoard of a dragon big enough to swallow you whole), and then your GM fumbles through the adventure trying to find the next closest source.  You then make a beeline for the treasure.  If it's unguarded, free gold.  If it's guarded by mooks, you get to beat them up and take their lunch money.  If it's guarded by something scary, you have a warning so at least it's less likely to surprise you, so perhaps you see it and leave, or maybe you design a good tactical plan and apply all your buffs before you alpha strike it.

So from a player point of view, Seek Earth is yes.  But from a GM point of view, it's potentially a huge pain, especially in a large dungeon where you might have to go through a lot of rooms looking for the appropriate treasure.  How do you deal with this?

First, even though the spell says "nearest", players probably won't hold you to that.  If there's some gold 40 yards to the north, and you miss that room when scanning your map and room descriptions and instead send them to a room 70 yards east, will they notice?  Probably not; if they already knew where the gold was they wouldn't need the spell.  Will they be mad if they realize weeks later there was some closer gold and you missed it?  Probably not; it still gave them a bearing on some nearby gold.  So as long as you're not deliberately hosing your players, best effort is fine.  So if you're not prepared for this spell, I think it's totally reasonable to just look at the map, eyeball the nearby rooms in approximate order of how close they are, and then check each room description for the metal in question.  Stop at the first reasonable hit rather than spending ten minutes trying to be certain you didn't miss one.

Second, it says "significant" but doesn't define it, so you can set your own threshold and ignore anything below that if you want.  (I mean, you're the GM, you can always do whatever you want, but here you absolutely need to because the rules as written are fuzzy.)  Feel free to ignore monster pocket change and focus on bigger piles if that's more fun.  Conversely, if you don't want to have your PCs zoom past every other monster straight to the big boss's loot at the end, feel free to have it pick up the 17 closer treasures first, even if they're small.  You have options.

Third, NPCs, at least smart ones, know that spells, especially common spells, exist.  Common spells should not be automatic win buttons.  If the PCs aren't the only wizards in the game, then smart powerful rich NPCs might put their best stuff in lead-lined rooms, or orichalcum chests, or no-mana zones, whatever it takes in your setting.  It's not fair to have every single gold piece in the game hidden like that and make the spell useless, and it's kind of silly to have a chest worth way more than the treasure inside it, but the players should not be able to assume that there are never countermeasures available.

For the specific case of Arden Vul, now that I know a PC wizard is going to cast Shape Earth for gold and silver a lot (and he mentioned diamonds were another possibility, and that he might take Seek Magic soon), I'm planning to make a supplemental dungeon key with just room numbers and treasure.  Here are the rooms with gold, silver, diamonds, and magic.  If I wanted to get fancy I could make a file with the (X, Y, Z) coordinates of each room (maybe automatically extracted from the coordinates of the hidden room labels I put on the Foundry maps), and then write a little program to find the distance from a room to every other room, sort ascending, then spit out the label of the first room with the appropriate treasure (for me), and the direction and distance to it (for the player).  The code is easy; most of the work is data entry.

Is that work really necessary?  No, you can guesstimate.  If most of the time they're searching for something that's fairly common in a dungeon, you can probably just check a few nearby rooms until you find some, then stop.  And if they search for something rare, you can search the adventure PDF and find all instances of the thing they're seeking and then give them the closest.  I think it's worth automating in my case because I know this PC (and probably any other future wizards or druids in the game, if this one gets eaten) is going to cast Seek spells ten times per session and this campaign could run for years, so I'd rather do some work up front during downtime to save time during play, but it's totally up to each GM how they handle things like this.

My final tip is for players: if you want to play a PC with a power that's hard for the GM to adjudicate, give them a heads-up ahead of time.  Maybe they'll ban your favorite toy, but better to find out sooner than later.  Or maybe they'll lean into it and help you make it work, but they can do a better job of preparing for it if they know it's coming.  Treasure detection is a pretty straightforward (if data intensive) case to handle.  Divination is much worse, so warn the GM before playing a diviner, and don't get mad if they only let you tell the future once per session rather than every five minutes.  Seek MacGuffin can short-circuit a whole adventure, if the point of the adventure is finding one MacGuffin, so discuss it first.  (In some cases the GM will be thrilled you can do it, because that saves them a lot of time spraying around clues for you to maybe figure out.  In others, they might need to improve their adventure so that it's still fun even if you've made the finding part trivial.)