It is an odd series of steps to go through. You are first aware of the 4 classes of asteroids, S class, M Class, C Class and Ice.
When prospecting you find Dense, Crystalline and Exotic on the chart on page 75.
If you find Exotics (by rolling 12's) you then roll on the Exotic Chart and can find C Class asteroids or Radioactives, or Artefacts if you roll another 12.
Survey and Logistics: You determine the class of asteroids as C, M or Ice, and ignore Silicate asteroids as they are not mentioned again.

You roll on the composition chart on page 76, then roll on the Resource Presence chart below it and that is your asteroid surveying done.
Roll twice on page 77 for size and yield of useful metals. The remaining tonnage of the asteroid is 50% Common Ore, 30% Uncommon Ore and the rest is useless material. (pg 76)