Drill core sampling for analytical purposes is a scientific process; it is the first step of various scientific experiments that involve not only the rock material, but also the porewater in the rock and the volatile gases dissolved in these porewaters. Therefore, drill core sampling necessitates proper care, diligence and precision, which by far exceeds the simple sampling of core material for archival purposes.
It is crucial that sampling is performed properly and documented precisely, as it cannot be repeated in the case of failure, unlike most laboratory experiments performed later on the core material. Sampling and samples taken for analytical purposes are therefore subject to several important considerations:
- They must represent the in-situ state of the sample as closely as possible:
- The drill core should be taken with a triple core barrel to minimise the disturbance of core retrieval. The use of hammering or water injection to force the core out of the core barrel should only be used as a last resort to remove the liner/core from the barrel.
- Unnecessary and excessive use of water during cleaning should be avoided (e.g. chemical disequilibrium with the porewater).
- Drill core sampling should be done rapidly to avoid evaporation (e.g. disturbance of the water content of the sample) or outgassing of dissolved gases from the porewater.
- Samples for mechanical investigations must be undisturbed and without indication of brittle deformation.
The number of samples and their positions along the borehole must be a deliberate result of balancing two aspects, i.e.
- Following the given sampling plan and
- Taking into account the geological situation actually encountered
Sampling at fixed intervals without considering the actual geological situation (which might be different from what was projected at the current depth in the sampling plan) is not appropriate for this type of investigation.
- Samples for different but complementary purposes should be collected in close proximity to each other.