It was in the old THB FAQ. It's still available online, just not linked from anywhere I can see. Look here:
http://uo.stratics.com/thb/faq.shtml#q13
13. What about mining?
Mining is not a required skill, that is to say you can find and dig up any level of map with zero mining skill. However, mining does provide a few advantages. It increases your dig radius (from the treasure spot) as follows:
- 0 to 50.9 mining gives a 1 tile radius (in other words, you must dig the exact correct spot of the chest)
- 51 to 80.9 mining gives a 2 tiles radius
- 81 to 99.9 mining gives a 3 tiles radius
- 100 mining gives a 4 tiles radius
Note: These numbers refer to enhanced skill, not "real"
If you use a map archive to locate the treasure, you may not need the advantages of mining. However, it also allows you to stand back from the chest spot a little, which can give you a bit of a head start if you plan on making a run for it once the chest is up.