The need to recognise the limitations of equipment and how best to manage these risks
Drill core orientation is a fundamental process for obtaining orientation data from diamond drill holes (Davis, 2014).
It is essential to the mining and resources sector because it attempts to mark and determine the orientation and position of the core while in the ground.
A number of tools are spruiked by companies who purport to have devices that accurately define the original orientation of the core prior to extraction from the bottom of the drill hole. To do this, the instruments being marketed must establish the dip and azimuth of the hole and marry it with geographical orientations (x, y, z via downhole survey). Critically, the tool must also establish the gravity vector across the core so that the top and bottom sides of the core are established. If this is not done, the core can be reoriented with the correct dip and azimuth but it may have been rotated an unknown amount. The definition of the downhole side of the core by the core orientation device allows the core to be positioned in its original unrotated orientation.
Core orientation devices – electronic tools
Core orientation devices fall into two main categories – electronic and mechanical. Examples of the electronic devices include the Reflex ACT tool, Devicore™, and Boart Longyear’s TruCore™ system. In recent years, the electronic core orientation tool revolution has seen the minerals industry favour these electronic devices over mechanical ones. In the majority of cases, the electronic devices can deliver genuinely good orientation marks, which provide usable orientation data for mine planning, resource definition, etc. However, in my experience, many marks are deemed excellent if the display on the instrument, and its companion PDA, indicates that there have been no errors in the operation of the orientation tool. When the driller sees this, they can ‘confidently’ place a bottom-of-hole or top-of-hole orientation mark on the core (note that the general convention globally is to mark the bottom of the core – if only this were universal). And, of course, the marks must be excellent if the geologist never comes back and suggests to the drillers that the marks might be a little bit ‘suspect’.
Praise for the electronic instruments is overwhelmingly by drillers and drilling companies. On its website, Boart Longyear proudly states that its TruCore™ system ‘delivers instrumentation designed for drillers, by drillers’.
Unfortunately, drillers are not the people who work with the core to get orientation data, and are not the people to carefully audit core orientation marks. This is fine as, generally, drillers are not paid or trained to do this. But someone has to see if the orientations are valid so that we can feel comfortable in the quality of the structural orientation data we take. Once this is done, we ideally should be able to put our hand on our heart and say ‘this resource model, which is what will make or break this mine, is the best possible based on the data we have extracted from the core’. But what if it isn’t? Where are the potential problems?
It may seem unlikely, but most issues actually stem from the orientation mark itself, which has been placed on the core based on the readout from the electronic tool. If these tools are so great, why are there errors? There are a couple of reasons.
Firstly, sensitive internal accelerometers collect gravity vector data via time stamping, allowing the measurements from the instrument to be married with depth and time. However, in all cases, the electronic instruments fit on the back-end/top of the core barrel and the orientation taken by the device pertains to the last piece of core drilled. The core break that receives the orientation mark is the face that was in contact with the remaining core stub at the bottom of the hole.
Effectively, the electronic device is taking an orientation of the core barrel, not the core, and transferring this to the final piece of core drilled. That is, orientations are placed on the core after completion of a drilling run, based on the reading at the time when drilling of the run was completed. Consequently, the core in the barrel could have rotated an unknown amount during drilling, especially if intervals of broken ground are encountered. A mark will always be made on the bottom end of the core (ie the end of the last piece drilled) by the driller, which reflects the orientation of the barrel. Some devices now boast that the core readings (ie orientations) can be taken without having to break a joint in the inner tube. In this case, the core is not visually assessed before the orientation mark is made. So, we will always get an orientation on the final piece of core, regardless of how much broken ground, gravel and rotated core are present in the core tube above it.
Placement of the orientation mark on the last piece of core drilled is based on the assumption that, when the drilling is complete, the core in the bottom of the barrel is still attached to the country rock. At this moment, therefore, the orientation of the core barrel with respect to the country rock is the same as that of the lowest piece of the core in the barrel. The orientation of the barrel at that time can thus be transferred to the piece of core locked in the core lifter after the core has been extracted. This process excludes the possibility that any of the pieces of core have rotated prior to marking. So, such tools will always successfully orient the core barrel; we should never assume it automatically pertains to the core as well.
Secondly, the internal electronics are sensitive. If the instruments are thrown about like other rig-tough tools, there will be problems. Some companies champion the use of two tools, such that one is always down the hole. For example, Chris Lambert, the global product manager for Boart Longyear, is quoted on the company website as saying ‘The ability to send a second TruCore™ tool down immediately after the first tool is retrieved, combined with wireless communication, means core readings can be taken without having to break a joint in the inner tube. Which leads to a faster drill site operation.’ However, if one tool is damaged and is being alternated with a second one, there is little chance that the marks will line up when the core is reassembled.
Thirdly, the electronic tools are unable to be audited at the time of taking the measurement. We do not know if the tool took a bogus measurement or not. We have to believe the electronics.
Finally, there is the human factor. For all our attempts at doing a perfect job, there will always be times when human nature conspires against us. The biggest problem, which is acknowledged and stated by drillers, is poor execution of procedure due to lack of care or lack of comprehensive operator training. Such issues can only be mitigated by regular checks of equipment, audits of procedure, and careful adherence to training of the crews.
What are the alternatives?
In addition to the electronic devices, there are mechanical tool options and then there is the long-lived, unfairly maligned downhole spear. I won’t mention the spear here, as it has been discussed elsewhere (Davis, 2014), but I will discuss the main mechanical instrument in use in the industry currently, which is the Reflex Ez-Mark™ system. This mechanical system, like an electronic instrument, is an integral part of the core barrel. However, it is fundamentally different to the electronic ones in that it is lowered down inside the rod string, and makes contact with the core stub still attached to the rock at the bottom of the hole. The tool then determines a gravity vector for this position, largely ensuring that the core stub, which will be the first core to enter the barrel, is in its original orientation. After the tool has made the orientation measurement, it automatically unlocks and moves passively up inside the core barrel ahead of the advancing core. This represents the fundamental difference between the Reflex Ez-Mark™ and the electronic instruments – the Reflex Ez-Mark™ orients core before it is drilled, the electronic ones orient core after it has been drilled.
The Reflex Ez-Mark™ system is auditable. The downward gravity vector is established by means of three freely moving balls moving in separate races that encircle the instrument. The front of the tool has a series of steel pins that conform to the profile of the end of the core stub. When the pins press against the stub the tool locks and records the profile of the core stub, as well as marking it with a wax pencil. The locking of the tool freezes the previously rotating balls in the lower position of their races, such that they align parallel to the bottom side of the core. Matching the tool with the core allows the down gravity vector (recorded by the position of the locked balls) to be transferred to the core. Thus, three independent recordings of the core orientation are made and must all conform to give an accurate orientation, ie the orientation mark is auditable immediately. Furthermore, the pin system is temporary, removed after each orientation, and placed in the core tray as a proxy core block. This allows future reference of the pin profile back to the core profile on- or off-site, or even years later.
Ultimately, we are trying to find, understand, design and economically mine our mineral resources. Oriented core is a critical tool in this regard, and wrong orientations can lead to disastrous results. Increased efficiency and usability does not always translate to accurate orientations, and an audit system is essential. In all cases, potential errors need to be checked by comparing adjacent oriented runs. Only then, if three or more marks align, can the core be confidently marked up with top- or bottom-of-hole orientation lines. Some orientation processes make this audit system easier than others.
Suffice to say, this is not a forum to discredit one system over another. All tools, electronic and mechanical, usually give great data and, more often than not, it is the human in the equation that causes the mistakes. That said, when devices fail, they commonly fail spectacularly. We need to know how to identify the mistakes and how to rectify them. As professionals, we have to be cognisant of the limitations of our equipment, and understand the implications of these limitations.
Davis B K, 2014. Use and Abuse of Oriented Drill Core. AusIMM Monograph 30: Mineral Resource and Ore Reserve Estimation – The AusIMM Guide to Good Practice, second edition, p 121-134.