Stereonet validation of structural measurement in oriented drill core

SUMMARY

Measuring the attitude of structures in drill core requires fully oriented core. But the tools for orienting core that are currently available to drillers often fail, especially with small core diameters (NQ or less) and where the rock has fissile surfaces within it. As these failures are not always apparent at point of core recovery, geologists can make incorrect measurements which are then entered to data bases and become input for computer programs.

This post details how these failures can occur and outlines stereographic techniques which enables these problems to be identified and quantified.

 

PART I

1.Oriented Holes and Oriented Core

 An oriented drill hole is one where the inclination and azimuth of all points along the hole are known, usually by means of a special down-hole survey. This data provides the orientation of the longitudinal axis (the Core Axis, CA) of the cylindrical core of rock extracted from the hole. However, once extracted this core is not fully oriented because it has been rotated by some unknown amount about the core axis and the attitude of structures within it cannot be accurately measured.

In fully oriented drill core, an additional survey procedure, called a core orientation survey, has been carried out to determine the line of the original down gravity vector across the length of the core. Drillers use special core orientation tools to do this.

The difference between oriented and non-oriented core is graphically illustrated below.

 

Figure 1: Although the orientation of the core axis may be known, the core has rotated by an unknown amount around that axis. RM, 2015.

Figure 2: The core is now fully oriented in 3D space. RM 2015

2. How geologists measure Structure in Oriented Drill Core

The most common type of structures measured in oriented drill core are planar (bedding, cleavage, veins, joints etc.). Assuming that the core has been correctly oriented (more on this assumption below), the best way to do this – one that produces fewest errors and creates the greatest geological understanding, is by using a geologists’ compass to directly measure structure in core pieces that have been set up in their original orientation by means of a Core Orientation Frame (for further discussion on this subject see HERE). However, because it is quicker and easier, it is my experience that most geologists today measure and record the attitude of planes in oriented core by the Internal Core Angles Method. This technique involves measuring the angles which the structure makes with lines of known orientation in the core. These lines are the Core Axis (known from a down-hole survey) and the Bottom of Hole line (provided by the driller using a core orientation tool). These angles are:

Alpha (α) – the acute angle (0°-90°) between the core axis (CA) and the long axis of the intersection ellipse (E-E^I) defined by the trace of the planar structure on the cylindrical core surface. See figure 3.

Beta (β) – the radial angle (0°– 360°) measured in a clockwise direction about the core circumference from the Bottom of Hole Line (BOH) to the down-hole end of the intersection ellipse.  Clockwise is determined looking down the core. See figure 3.

Alpha and beta measurements numbers are then subsequently crunched by computer, along with surveyed hole orientation data, to produce a standard strike and dip (or dip and dip direction) measurement, which can then be displayed as a stereonet plot, a histogram or as short lines of intersection on a drill section.

 Figure 3: The angles which define the orientation of a planar structure in oriented drill core. Click for sharper image.

3.  Potential Errors in Measuring Alpha and Beta

 Measuring alpha is quick and easy using any standard protractor. The core does not need to be oriented. You do not need to know which end of the piece of core points up the hole and which points down. All values of alpha from 0 to 90 degrees can be measured with the same level of accuracy. Where the planar structure is well defined and reasonable care is taken by the geologist, measured alpha angles can be taken as accurate to at least +/- 2°. Alphas numbers thus are seldom a source of error in computer input.

Errors in measuring beta angles cause most of the errors when using the internal core angles method.

These errors occur in two areas:

a. In identifying point E

The trace of any planar structure on cylindrical core is an ellipse. The long axis of the ellipse defines points E and E ^I on the core surface, where E points down hole and E ^I points up hole (see figure 3, above). E and E ^I can be recognised as inflection points (points of maximum curvature) on the trace of the plane. Where the alpha angle (the acute angle that E-E ^I makes with the core axis) is low, the resulting intersection ellipse is elongate, with sharp inflection points easily defined by eye. However, with increasing alpha angle, the ellipse becomes fatter and tends towards circularity until, at alpha = 90°, the “ellipse” is a circle with no definable axes.  As alpha increases, inflection points become broader and harder to accurately define and errors in correctly locating point E increase. Since measurement of the beta angle is dependent on being able to define point E, high alpha angles lead to significant beta measurement error. For all alpha angles over 65°, I recommend that a core frame be used to measure structure in core rather than the alpha/beta method. But in my experience, very few geologists taking structural measurements in oriented core do this.

b. In the BOH mark placed on the core by the driller.

Drillers use one of two methods to determine the position of down gravity vector on the surface of the drill core.

The first method is to measure the orientation of the core barrel after a run of core has been drilled, but before the core is broken free from the ground and brought to surface inside the rod string. The position of the down gravity vector across the core barrel is determined by an accelerometer built into a tool screwed onto the top of the core barrel. This data then stored in a memory against a time stamp. After the barrel with its contained core has been pulled to surface, the barrel orientation recorded during the time interval between core drilling and core extraction is recovered by means of an LCD readout and transferred to the last piece of core to be drilled, which, at this stage, is the piece of core still gripped by the core lifter.

The key idea behind this method of core orientation is that the last-drilled piece of core was attached to Mother Earth at the time the gravity direction across the core barrel was made and recorded. In most cases, this assumption is valid, and the orientation of the barrel (or rather, the core lifter, an integral part of the barrel) is the same as the orientation of this piece of core. The gravity vector shown on the LCD read-out can thus be transferred by making a mark on the core. The whole drilled core run, with the drillers mark at one end, is then extracted from the barrel, placed in a core tray, and delivered to the geologist to make his or her measurements. However, where the core is broken or contains a fissile surface (such as cleavage or bedding) the end piece of core may have been broken free by the rapidly turning bit and rotated by some unknown amount prior to its being gripped by the core lifter. In this case, the orientation mark placed on the core is meaningless. There is no way for the driller, and no easy way for the geologist, to know when this has happened.

Core barrel orientation tools work perfectly every time at orienting core barrels, but the orientation of the barrel is not necessarily the same as the orientation of the core inside the barrel.

The second method, and generally a much more reliable one, is to orient the core stub before it is drilled. The core stub is the broken rock surface at the bottom of a drill hole which then becomes the top surface of the next drilled run. The match between the orientation tool and the core stub is made by a percussion or wax pencil mark, a shape template, or some combination of these techniques. The gravity vector at the moment of first contact between tool and core stub is recorded by built-in mechanical system (level bubbles) or by an electronic (accelerometer) system.

In the simplest (and oldest) tool, a narrow but heavy steel rod with a pointed tip (known as a spear) is allowed to slide down the rod string on the end of the wire line after a run of core has been extracted. In angle holes the weight of the spear keeps it in contact with the lower surface of the rods. The spear then impacts the core stub making a mark (percussion or wax pencil) on the lower edge of the stub. The technique is absurdly simple, but in the hands of an experienced driller (he has to control the speed of impact) it could give excellent results. Although seldom used nowadays, much legacy drill core was oriented by this technique. The technique fell out of fashion not because of poor results (quite the opposite) but because its use required a separate down hole procedure after each drill run – a procedure which could take 30 minutes or more depending on the depth of the hole.

More sophisticated core-stub tools operate on the same basic principle but use a template to record the shape of the stub rather than (or perhaps as well as) making a mark on the core. The template can then be matched to the shape of the stub after it has been drilled and pulled from the ground.

Core stub tools can fail if mud or disaggregated broken core obscures the core stub, but in my experience, their failure rate is less than that of core-barrel tools. But the really important difference between the two systems is this: when core-stub systems fail, that failure is almost always obvious. In other words, results from core-stub tools are auditable at the point of core recovery and no time need be wasted marking up failed runs and making inaccurate measurements from it. And because the ways in which core-stub systems fail are different from the ways in which core-barrel systems fail, core-stub tools can produce accurate orientation in rocks where the core barrel tools fail.

A discussion on the currently available core orientation tools is given in footnote [1]

 Figure 4: The REFLEX ACT electronic core barrel orientation tool. This is screwed on to the top of the core barrel. The gravity vector across the tool is measured by an accelerometer and recovered by means of a graphical LCD display. Image from the current Reflex website.

Figure 5:  Core stub orientation tools.

The upper pic is the Reflex Verti-Ori tool. It fits within the bottom of the core barrel. Steel pins and wax pencil record the shape of the core stub. The tool then slides up the tube ahead of the advancing core. Gravity vector is determined by an inbuilt accelerometer and recovered by means of an electronic user interface.  See LINK.

The lower picture shows the pointy end of the EzyMark core-stub orientation tool. It fits inside the bottom end of the core barrel and slides up inside the barrel ahead of the advancing core. The pins and wax pencil record the shape of the core stub. Lockable level bubbles inside the tool record the gravity vector across the tool at moment of contact. This all-mechanical tool is no longer available. The image was accessed in 2013 from www.2icAustralia.com. 

PART II

Mismeasurement of beta is overwhelmingly the major source of error when using the internal core angles method of measuring structure in oriented core..

Once a set of measurements have been made on oriented drill core, there is a simple test to determine if inaccurate beta numbers are a significantly affecting your results. Plot your dip and dip direction results from measured planes as poles on a stereonet. For a set of measurements through a volume of rock, the distribution of poles (known as a pole figure) can enable deductions to be made about the accuracy of your measurements or whether or not they are made from approximately parallel surfaces. As a bonus, stereonet plotting of structural measurements can also enables useful geological interpretation of your results.

But first…

1. A quick Primer on the Stereonet…

 A Stereonet is a pre-printed net of intersecting lines which allows the three-dimensional attitude of measured linear or planar rock structure to be shown as points on a two-dimensional graph. The scales of the net then offer a quick and easy way to provide approximate solutions to problems in 3D geometry, in much the same way as the scales on a slide ruler allow numerical solutions to math problems. Cheap pocket calculators, which first appeared in the 1970’s, have now replaced slide rulers. Computer software can solve math problems in 3D geometry too, but as a cheap, quick, low-tech and always available tool, the stereonet still has a useful role to play in this area. In structural studies, approximate solutions (i.e., to the nearest few degrees) are usually all that can be expected and all that is required.

But an equally important function of stereonet plots is as a graphical way of showing the distribution pattern of a series of orientation measurements taken through a volume of rock. Our brains are analog computers, fine-tuned for recognizing visual patterns (sometimes too fine-tuned). Patterns of plotted points on a stereonet can be a great aid in interpretation of underlying geological processes. But these patterns need to be distinguished from merely coincidental aggregations of random numbers or from the effects of systemic problems with the input data. I will show examples of all these effects. Thus, stereonet plots of structural measurements can be a powerful tool in validation of data.

2. Stereonet Validation and Interpretation

 Example 1

If your measurements of planar structure across an area or a through a volume of rocks are completely random, their stereonet pole figure might look something like that of figure 6, below, a plot constructed using a random number generator. You may see partial patterns of lines or circles or ellipses or clumping of points, but these are coincidental and have no meaning.

If you get a random distribution of points such as this from a real set of measurement, it most probably means that your measurements were collected across several distinct structural domains.

Solution: Identify the different structural domains. Group your measurements by domain and plot each group separately.

Figure 6: A stereonet plot of poles to bedding created using a random number generator. Any patterns or concentrations of points that a visual inspection might suggest are purely coincidental and have no real world meaning. If this was a real set of measurements across an area, then the most probable interpretation would be that the measurements were taken across several distinct structural domains.

Example 2

If your measurements are accurately made from a set of parallel, or approximately parallel, planar structure (bedding, cleavage etc.), then the majority of points on a pole figure will form a tight cluster, as shown in figure 7. If the measurements were from oriented drill core, then the centre of the pole cluster will lie at an angle of 90-α° to the plot of the core axis.

Q: What is the logic behind this number 90-α°?

A: This is a plot of poles to planes measured in oriented drill core. If you refer to figure 3, you will see the poles to these planes lie at 90-α° to the core axis (CA).

 Figure 7: Poles to planes measured in oriented drill core by the internal core angles method. The orientation of the core axis is shown as a red circle. The results indicate the planes are approximately parallel with only minor, acceptable, error in both alpha and beta measurements. The centre of the pole cluster lies at 90-α° to the core axis. Click for a sharper image.

 Example 3

If your measurements of alpha angles are accurate (as is usually the case) but are subject to random error in beta measurement (a not uncommon occurrence, especially when using electronic core-barrel orientation tools), the pole figure plot will show a partial or complete distribution about a small circle at 90-α° to the core axis. There is no known geological process which will produce such a pattern. This pattern is shown in figure 8, below.

Solution: Try using a core-stub orientation system.

Figure 8: Poles to a set of planes measured in oriented drill core by the internal core angles method. The consistency of alpha indicates accurate measurement on planes that are approximately parallel. However, the scatter of points around a small circle at 90-alpha degrees to the core axis indicates that large random errors have been made in the measurement of beta.  Click for a sharper image.

Example 4

If the pole figure for of large number of orientation measurements taken from scattered surface outcrop or oriented drill core shows distribution about a great circle on the net (as shown on figure 9), we can draw several conclusions.

The results indicate accurate measurement.

The measurements are from a coherent structural domain.

The surface has been affected by a cylindrical fold, or a set of parallel cylindrical folds (illustrated by the insert on figure 9).

A line at 90° to that great circle (which plots as a point in the opposite segment of the net) represents the trend and plunge of the fold axis or axes. This point is conventionally labelled pi (π).

The two weak bedding-plane maxima which can be seen on the great circle of figure 7 can be interpreted as the two planar limbs of the fold. This is because random measurement across a volume of folded rocks is much more likely to have been taken on extensive fold limbs rather than on restricted fold hinges.  The two maxima further indicates that the fold or folds tend towards similar rather than concentric in style.

Figure 9: Poles (n=50) to a set of bedding planes measured across scattered surface outcrop or oriented drill core. The great circle distribution indicates folding about a cylindrical fold, or a set of parallel such folds. Click for a sharper image.

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