Routine core handling is a role for a suitably trained and experienced field technician, working under the supervision and direction of a geologist. If the technician’s job is done properly, the geologist is free to concentrate on observation and recording of the core. The duties of the field technician are set out below [1].
- Carrying out down-hole orientation surveys.
- Measuring core recovery.
- Measuring RQD (if required).
- Supervising the drillers’ core-handling procedure by ensuring core is correctly placed in trays (e.g. not jammed in too tightly or too loosely; ensuring that pieces of core are not rotated in the tray with respect to each other, etc.).
- Ensuring that the drillers’ core blocks, which mark the down-hole depths at the end of each drilled interval, are correctly placed and made permanently legible. When the blocks become misplaced (which readily happens when core is transported), the correct position for the block can sometimes be spotted by the parallel grooves that the core-lifter sometimes leaves on the base of each barrel of core.
- Measuring hole depth at the beginning and end of each tray.
- Marking hole depth, hole number and tray number on to each tray.
- Marking the core in even one metre intervals as an aid to subsequent logging and sampling. The measurements are taken with a flexible steel tape from the nearest drillers’ core block. The only accurate way to do this is to remove the core from each drilled interval and then reassemble it, piece-by-piece, by carefully fitting the broken ends together in a separate V-section channel. When laid out in this way, and provided there has been no core loss, measuring depth intervals, or marking a line along the core to indicate the proposed cutting line for sawing, is both easy and accurate. Reassembly of the core in this manner is essential when dealing with oriented core, or where complex structural relationships need to be resolved. Although time consuming, the technique is strongly recommended, even for non-oriented core.
- Drawing a straight line along the length of the core as a guide mark for subsequent sawing. This position of this line must be determined by the geologist. In non oriented core the cutting line is positioned so as to make a high angle to any dominant planar structures within the rock. Where the core has been oriented, the cutting line is the Bottom of the Hole (BOH) line– that is, it marks the line of intersection of the vertical or drill section plane with the core (see Sect. B.2 for a description of the BOH line). When drawing the line along the core, the aim as far as possible is to ensure that it is consistently oriented for the entire length of the hole.
- The half core to be retained after sawing and sampling should be identified by the geologist. This half is then marked by placing a small arrow or angled line on one side of the saw line on each core piece. The arrows point down-hole and serve as a vector for each piece of core.
- Cutting or splitting the core and collecting the sample between the intervals nominated by the geologist.
- After cutting, it is important that the same half of the core is always taken as a sample. There are two reasons for this. If the technician sometimes takes one half of the core for assay, and sometimes the other, the retained pieces of half core will no longer fit together, and it may not be possible to replace the core in the core tray. The second, more important, reason is that by preserving a consistent view of the structures seen on the sawn surface of the retained core, geological interpretation is greatly aided. If the core is oriented, the saw cut along the Bottom of Hole line corresponds to the drill section. One half of the cut core will therefore show structures as they appear on the normal view of that section: this is the half that should be retained after sampling. For example, an E–W section would normally be viewed looking north, so the north half of the core should be retained after sampling.
- Marking the sample number on to the tray where intervals have been taken for assay. Adhesive sample number tags carrying the same sample numbers as those in the sample books are available and can be used for this purpose.
- Permanently marking and sealing the drill hole collar. This is necessary, not only because open holes can be dangerous, but also because hole collars often need to be re-located, sometimes many years after they were drilled. It may also be necessary to re-enter an old hole in order to deepen it or to carry out a down-hole geophysical survey. For these reasons dirt must be kept from entering the hole.
- Measuring the specific gravity (SG) of the core, where required. SG measurements are necessary in order to calculate the weight (tonnage) of a volume of rock. It is also an important parameter that is used in interpreting the results of gravity surveys.
Reference
[1] R. Marjoribanks, Geological Methods in Mineral Exploration and Mining. Berlin, Heidelberg: Springer, 2010. doi: 10.1007/978-3-540-74375-0.
