How many measurements in drill core are enough?

As little as ten years ago, before orienting drill core became routine, geologists hardly ever measured structures in their core. Much useful information relevant to understanding geology and hence finding and defining ore bodies was ignored. Now the tendency is to fill vast data bases with hundreds or thousands of measurements. It is as though the volume of numbers collected provided a measure of the thoroughness and effectiveness of the structural logging. This practise is encouraged by the use of the quick and easy internal core angles method of measuring structure (alpha-beta angles) and justified by the hope that the meaning of a measurement will become clear if only enough of them are taken. Like the tourist who is so involved in taking photographs that he only sees the sights through the lens of his camera, these geologists only see structures through the scales of their core protractors.

Why anyone should believe that a structural orientation, whose significance is unclear when the core is in their hands, will become significant and meaningful weeks later on being viewed along with hundreds of other measurements on a statistical plot – when memory of individual structures actually seen is long gone -  requires explanation.  Actually, it is not a mystery, geologists who do this sort of logging lack confidence in their ability to understand what they see in the core. They have never been properly trained – indeed, their supervisor expects little more of them than that they be geological operatives – collectors of numbers, fillers-in of data bases. It is hardly the role for which they spent four years or more at university training as a professional.  It is a lack of preparedness to even try.  But more than all these, it is a hope that the geological significance of the numbers will emerge from a  statistical treatment.  This ignores a cardinal rule of any statistical treatment data – you can only meaningfully analyse data if you already have a specific hypothesis to test. A hypothesis cannot emerge from the data, rather data is collected in order to test a hypothesis.  Charles Darwin expressed much the same idea more than 140 years ago when he wrote: “ How odd it is that anyone should not see that all observation must be for or against some view if it is to be of any service!”

 How does a geologist logging drill core develop a hypothesis as to the meaning of the structures that she sees? She does it by acquiring a small number of quality measurements and analysing what these measurements mean in regard to her evolving geological model at the time the measurements are made.

 So how many measurements are enough?

 Once a qualitative idea is gained of the structure present in the core, significant or representative examples of these structures are selected for accurate measurement of their attitude. These measurements are then used to construct accurate drill sections and maps and facilitate the precise predictions that are necessary to target additional holes. The purpose of measuring structures is not to compile impressive tables of numerical data, but to help provide answers to specific questions that arise as the core is being logged and interpreted.

 The number of measurements that need to be made depends upon the variability of the structures present. If the attitude of a structure is relatively constant through a hole, then a representative measurement every 10-30 metres or so down hole, would be quite sufficient to define it. As well as obtaining an even spread of measurements down the hole, at least one measurement should be obtained for each major lithology in the hole, with particular emphasis on features of economic interest such as vein orientations, or any banding or linear structure in ore.

 Where the attitude of a structure is rapidly changing, more measurements are required to define this change – perhaps as many as one measurement of the structure every 3-5 metres. Such detailed measurement would only normally be taken over limited sections of the core. Routinely collecting hundreds of measurements from each hole according to some invariable rule, generally adds nothing to understandingIt is always far better to collect a small number of high quality measurements than a large number of low quality measurements. By “high quality” is meant that each measurement is carefully selected to be representative of a section of core, and the nature of the structure, its position and relationships with other structures, with mineralisation, alteration, host lithology etc. are all carefully observed and noted. A high quality measurement is also one that can be understood and recorded in geologically meaningful terms (i.e. as a strike and dip or a trend and plunge) at the time that it is made. Numbers that have meaning only after subsequent computer processing, when memory of the rock that was measured has faded, are, in this context, considered to be low quality measurements.

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