“But” said the driller “it’s a great system. Works every time, always gives a perfect answer. And look – it’s got an LCD readout” Behind him the Project geologist nodded in mute agreement. They were talking about the Reflex ACT tool for orienting diamond drill core… and I realised that if I were going to convince them to try an alternative system I would have to marshal my arguments…
Some background
Before a geologist can measure the original orientation of structures in a piece of drill core, he (or she, of course) needs to know the orientation of that piece of rock before it was cut out and dragged from mother earth. This means that in addition to the E-W, N-S and up-down coordinates of the drill hole itself (established by routine down-hole surveys) the position of the original gravity vector across the core must also be established. Today, this is usually done by the driller using one of two competing proprietary tools. These are the Reflex ACT™ (Reflex was a Swedish company but taken over by the Australian IMDEX company in 2006) and the Australian designed and made EzyMark™ (also taken over – in 2014 – by IMDEX). These tools carry out the task in two very different ways and this affects their ease-of-use, accuracy and reliability. But before I can discuss ease-of-use, accuracy, reliability etc..
How the Reflex ACT works
The Reflex ACT core orientation tool appeared on the market around 2005. It is a tube-shaped device that is attached to the top of the core barrel – the extractable inner tube of the wire-line DD system into which the core passes as it is drilled. Every minute the Ace tool uses in-built electronic software to determine the direction of the down gravity vector across the tool, and records this information in digital memory against a time clock. Subsequently, the gravity data for any chosen time (entered via push buttons) can be graphically displayed on a Liquid Crystal Display. The tool thus orients core barrels. The gravity vector that is chosen is that for the short time interval after a run of core has been drilled and immediately before the core lifter grips the lowest piece of core in the barrel prior to breaking it free from the ground. How does orienting the core barrel provide an orientation for the core within the barrel? The key is this: it is assumed 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 (established by the Reflex ACT electronics) can thus be transferred to the piece of core locked in the core lifter after the core has been extracted. This provides a Bottom of Hole mark which can be transferred along the length of the core run by matching broken ends.
Reflex ACT orients core barrels very well: for all intents and purposes with 100% accuracy 100% percent of the time. It is also easy to use. The advanced electronics housed in a sealed case and the liquid crystal read-out it is so 21st Century. What’s not to like? And drillers love it. In my experience, if you allow a driller to specify the core orientation system, he will usually opt for the Reflex tool.
But hi tech is not necessarily high quality. Because the orientation is done after drilling the core, there is the chance that the core has been broken free and rotated by some unknown amount in the core barrel before the ACT tool takes its reading. When this happens, any assumption that the orientation of the core is the same as the orientation of the barrel leads to major error and the read-out is meaningless. It is easy to show theoretically why this is so, but it is also the widespread experience of many geologists. When drilling massive, competent homogeneous rock the error does not happen very often: where a penetrative fissile structure is present, it can happen frequently and unpredictably. There is no easy way of knowing when this has occurred, for the Reflex tool always successfully orients the core barrel. That’s all you can know because that’s all that it does. Everything else is assumption and hope. The error can only be picked up post hoc, by comparing adjacent oriented runs, after the laborious process of marking up the core is complete (and then only sometimes, if you are lucky). In all too many cases, structural measurements made on miss-oriented core are entered into the data base.
How the EzyMark core orientation tool works
The EzyMark™ tool is designed and made by the Perth, Western Australia based company 2icAustralia PL and has been in use since 2002. This is a mechanical system that, like the Reflex, is an integral part of the core barrel. Before drilling, the tool is armed and fitted into the bottom of the barrel. It is lowered down inside the rod string, part-projecting from the leading edge of the barrel and makes contact with the rock at the bottom of the hole. The tool then determines a gravity vector for this position. The rock being oriented is thus guaranteed to be 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.
Note the contrast – the EzyMark orients core before it is drilled: the Reflex ACT orients core after it has been drilled.
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With the EzyMark, the down gravity vector is established with great sensitivity by means of steel balls that are free to move in three, annular, oil-filled races. The positioning of the tool against the rock exposed at the bottom of the hole is established by a number of spring-loaded pins (see picture) which press against the core stub. As a backup, a wax pencil also makes a mark on the exposed rock. Once the template has been formed and the mark made, the steel balls are locked in position and the marking elements are withdrawn into a protective sheath. After the core has been drilled and extracted, the original core stub – now the top surface of the new run – can be matched with the orientation tool using the template pins and wax mark. Matching tool and core allows the down gravity vector (recorded by the position of the locked weights) to be transferred to the core.
Occasionally, a piece of rock will fall onto the core stub and the EzyMark will not work, but this is rare. In any case, the driller is almost always aware whether or not he has achieved successful orientation and will not put a misleading orientation mark on the core.
With EzyMark, errors in establishing the gravity vector are obvious – the three locked weights do not line up. The template and wax mark either match the core-stub or they do not. In other words: the system is auditable at the time of drilling.
If the EzyMark is used in conjunction with the OriBlock™ system (made by the same company) then the transfer of the Bottom Of Hole point from the EzyMark tool to the core can be done by the geologist not the driller, and can be done at any time after the core is drilled, thus removing another common source of error. (Note: this is not a comment about the reliability of drillers vis-a-vis geologists, but merely that drillers are often required to mark up core under less than ideal conditions at the drill site – mud, dust, rain, artificial light, 2 o’clock in the morning etc…).
What other tools are around?
In this methodology, Ace is one of a family of core orientation tools that include the BallMark™ and the CoreTel™devices. The BallMark – made by an engineering shop in Orange, NSW – developed the original idea and has been around for a couple of decades. Before BallMark, there were only tools – such as the Spear and the Craelius™– that were lowered down the hole on the end of the wire line after the barrel of core had been extracted in order to orient undrilled bedrock exposed at the bottom of the hole. Spear and Craelius tools are simple, unsophisticated devices that require a separate time consuming procedure between each drill run. The accuracy of most tools of these classes is also not very good and for these reasons they are seldom used nowadays.
Much more could be said about the Spear and Craelius systems, by means of which much historical core and a not inconsiderable amount of present day core was/is oriented, but I might leave that for another blog post.
The Swedes adopted the basic idea of the BallMark, by converting Ball Mark’s clunky mechanicals into a slick electronic system.
CoreTel is the most recent entrant to the field of core barrel orientation tools and is effectively an Australian-made clone of the Ace with some minor tweaks in the operating system (which admittedly make it slightly easier to operate, but the gain is marginal). So close a copy, in fact, that in 2009 Reflex took CoreTel to court for breach of their patents (I confess, I do not know if they won or not).
Now for the recommendation
So, in conclusion…. In my experience, the EzyMark is more reliable than the ACT and its results are auditable. Like all tools, it sometimes gives inaccurate results but these are usually obvious at the time the measurements are made and no time need be wasted on them. The ACT is slightly easier to use, and for most of the time produces reliable results, but when it stuffs up, it stuffs up big-time. And you may never know.
I recommend the EzyMark core orientation tool.
But that is my opinion. And I could be wrong.