Tremorsense question

[Tarmor]

The part that I have a question about is the tremorsense. It allows you to pinpoint things, and as far as I can tell gives a general idea of the size, but not shape for things in contact with the ground.

So there's a part of the campaign where I'm trying to create an ambush on the party with possibly buried skeletons. Would tremorsense notice them before they are actually able to ambush the party? The biggest reason why I'm curious is cause they're unmoving skeletons buried in the ground. But I don't want to just say it works or it doesn't without a good reason why. Cause I know if the player can't detect them, he will want an explanation.

3.5: The creature automatically senses the location of anything that is in contact with the ground and within range. It must itself be in contact with the ground, and the creatures must be moving.
Based on that your player is not aware of them until they move, but I'd say he's not surprised.
The Pathfinder SRD doesn't mention movement, but it does say its sensing vibrations - undead don't breath. There won't be vibrations until they dig themselves out.
In both cases I'd only be telling the player that he's aware of X things emerging from the ground at Y distance. Depending on the distance involved, he may still have time to alert the rest of the party. If they are right underneath the party, the ambush will 'work', but not on the tremorsense player.