Borrow it from somewhere:Trenching involves digging by hand or...

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Trenching involves digging by hand or by backhoes, usually to depths up to 1-2m channels that can be systematically sampled. This gives and idea of the witdths of the mineralisation and the variability of grade, but only in the very shallow depths.

Trenching is important if the results from this can lead to a drilling program, and it is a systematic sampling that should be free from too much selective bias. However, it is stiill just scratching the surface. It is all part of the process required for geologists to get a budget allocation to drill. There is always the risk of surface enrichment. In the case of calcretes it is possible that the entire mineralised horizon lies within the top few metres.