For a dispel to succeed, the total of the dice rolled, must equal or beat the spell's casting result.
The number of dice chosen are taken from the player's dispel pool and rolled. The results are added together, to give a dispel result, exactly as if he were casting a spell.
If the dispel result equals or exceeds the spell's casting result, the dispel is successful and the spell does not take effect. If the result is less than the spell's casting result, the dispel attempt has failed, and the spell will go through.