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.

How geologists measure Structure in Oriented Drill Core

 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

The most common type of geological 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 my blog post HERE). However, no doubt because it is quick, easy and involves minimal mental involvement, 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 axis of the core. See figure 3. Note that in holes drilled below the horizontal (all holes drilled from the surface) the down direction points away from the hole collar. In holes drilled above the horizontal (some underground holes) the down direction will point towards the collar.

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. It is also relatively easy to do this manually by using a stereonet (for details, see my blog post HERE)

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

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 usually be taken as accurate to at least +/- 2°. Alphas numbers 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 are 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 can 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.

There are a variety of tools currently available to drillers for orienting core. I describe these tools and how they work, as well as the strengths and weaknesses of the various systems in another post HERE. The tools, although mostly reliable, are capable of producing grossly inaccurate results under some circumstances and it is not always easy for the driller or the geologist to know when this has occurred.

For the two reasons given above, mismeasurement of beta is overwhelmingly the major source of error when using the internal core angles method of measuring structure in oriented core.

Stereonet Validation

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 (see Figure 6). 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 (see definition in section below) 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. Linear structures (1D) such as fold axes, lineations or drill holes all plot as points on the net. Planar structures (2D) plot as great circles on the net, but their attitude can also be shown as a single point by plotting the line at right angles to that plane. This is called the Pole to the plane. A number of measurements of a planar structure that are plotted on the net as Poles is known as a Pole Figure.
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 role of a stereonet plot is to provide a graphical way of showing the spatial distribution patterns 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 thus be a great aid in the 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 data collection and input. I will show examples of all these effects. Thus, stereonet plots of structural measurements can be a powerful tool in validation of data.

2. Examples of stereonet Pole Figures for measured planes in outcrop or oriented drill core

 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 4, 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 4: 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 structures, then the majority of points on a pole figure will form a tight cluster, as shown in figure 5. 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 5: 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 you are plotting measurements from oriented drill core using the internal core angles method (alpha/beta), your measurements can be assumed accurate as regards to the alpha number but may be subject to random error in beta measurement (a not uncommon occurrence, especially when using electronic core-barrel orientation tools, see HERE). If this is the case. 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 6, below.

Solution: Try using a core-stub orientation system.

Figure 6: 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 (figure 7), we can draw several conclusions.

The results indicate accurate measurement of a set of approximately parallel surfaces.

Although the measurements show a wide range of orientation, these are distributed in a systematic way that shows they were taken from a geologically coherent structural domain – namely…

The measured planes have been affected by a cylindrical fold, or a set of parallel cylindrical folds (as illustrated by the insert on figure 7).

A line at 90° to the great circle distribution (which plots as a point in the opposite segment of the net) represents the trend and plunge of the fold axis or axes that are affecting the surfaces. 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 relatively planar limbs of the fold or folds. This is because random measurement across a volume of folded rocks will sample more examples of extensive fold limbs than restricted fold hinges.  The two maxima further indicates that the fold or folds tend towards similar rather than concentric in style.

Figure 7: 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.

The post Stereonet validation of structural measurement in oriented drill core appeared first on Roger Marjoribanks.

Shawn Harvey of Saskatchewan sent me this email earlier this year:

Hello again Roger, 

You previously helped me out with some alpha-beta stereonet solutions which worked great (thanks again!!). I am now looking into a slightly more complex stereonet issue. I have some semi-oriented core in which I have a “known” orientation of a foliation and want to use this plane to help calculate the orientation of a fault relative to this foliation. I have made the alpha measurement for the fault and the beta measurement relative to the bottom of ellipse mark for the foliation (i.e. Beta angle between the foliation reference line and the bottom of ellipse for the fault plane). Ideally I would use a core frame but the facility is metal rich and compass accuracy is an issue and it is -30 degrees Celsius outside; as such, I was hoping to use the stereonet to convert the internal angle relationships to geographic coordinates. I could also use Geocalculator but I would really like to understand the derivation of the results.

 For the alpha-beta solution of planes I have used your 6 step process from your 2010 publication but I was hoping you could pass on how to modify the steps for the semi-oriented core calculations. Your assistance would be greatly appreciated.

 thanks, shawn

 Good morning Shawn, 

I appreciate your problems. –30 degrees sounds pretty tough. After a lifetime of working in Australia I find UK winter temps of –1 or –2 a trial. Where are you?  Northern Canada? 

 Your other problem with the semi-oriented magnetic core is obviously susceptible to a stereonet solution, but I will have to think about it a bit.   Maybe this weekend ?  I will get back to you.

Best wishes, Roger

 Statement of the problem

Non oriented core (i.e. no bottom of hole line on core).  However, the drill hole has been surveyed and the azimuth and inclination of the core axis (CA) are known. 

The core contains two structures: a foliation (labelled s) and a fault (labelled f).

The dip and dip direction of the foliation are known from other data.

The orientation of the fault is unknown.

We have the following measurements on the fault:  (1) its alpha angle (α_f) and, (2) the angle measured around the core circumference in a clockwise direction between point E for the foliation (Es) and point E for the fault (Ef ).  We will call this radial angle theta (ϴ).  Note that the radial angle between E^l_sand E^l_f is also ϴ.

Using a stereonet, calculate the strike and dip of the fault.

A  Worked example:

CA(Core Axis): 56° to 240°;    s (foliation): 078/60 South;  α (for fault): 45°;    ϴ (as defined above): 25°

E_s and E^l_s : mark the ends of the long axis of the intersection ellipse of the foliation (s)

E_f and E^l_f: mark the ends of the long axis of the intersection ellipse of the fault (f)

Procedure

Stereonet solution to problem. The fault strikes 010 and dips 69 east

Step 1:  On the stereonet plot the information that is known; i.e the Core Axis, the core circumference plane (the plane at right angles to the CA) and the trace of the foliation (s).

Step 2:  The intersection of the foliation plane and the circumference plane is the plot of the long axis the intersection ellipse of the foliation. If this plots in the lower quadrant of the circumference plane (as in our example) then the point represents E_s.  If it plots in the upper quadrant then it represents E^l_s (E primed).

Step 3:  Along the circumference plane, in a clockwise direction from E_s (or E^l_s) , measure the angle theta (ϴ).  This will plot either point E_{f  or} E^l_f.

Step 4:  By rotating the stereonet overlay, locate and plot the Great Circle that connects points CA and E_f  (or E^l_f).

Step 5:  Along this Great Circle, starting at point CA, measure the angle (90-α)_f .  If E_f has been plotted, measure the angle in the direction away from E_f (as in the worked example).  If E^lf has been plotted, measure 90-α in a direction towards E^lf.  This plots the pole to the fault plane (P_f)

Step 6:  From the pole to the fault read off its strike and dip :  in this example strike 010° dip 69° E

Longitudinal section of core in the plane of the Core Axis, and the long axis of the intersection ellipse of a cross-cutting plane.

The post Stereonet solution for non-oriented core appeared first on Roger Marjoribanks.

The attitude of a surface in oriented drill core can be determined by the measuring two internal core angles known as alpha (α) and beta (β). These numbers are then normally entered into a software program which calculates the strike and dip of the surface[1].  There is a simple and quick stereonet procedure which produces the same results.

How to use a stereonet to convert alpha and beta angles in to strike and dip is the subject of this post. 

Oriented drill core is core which meets three criteria:

1. A down-hole survey has established the azimuth and inclination of the core axis  along its length.
2. A core orientation survey has established the intersection of the original gravity vector  with the core surface. This is usually shown as a line, called the Bottom of Hole Line, marked along the original bottom surface of the core.
3. The down direction of the core is marked by an arrow placed on each piece of core. For holes angled below the horizontal (that is, all surface holes) these arrows will point away from the hole collar towards the hole termination.  For holes angled above the horizontal (some underground holes) the arrows on the core will point towards the hole collar and way from the hole termination.

Figure 1: The geometry of a planar structure in drill core. (a) is a perspective view of a piece of drill core containing a penetrative planar structure (bedding or cleavage) along which the top of the core has broken. (b) is a view looking down the core axis. (c) is a longitudinal section through the core containing the core axis, the long axis of the intersection ellipse of the planar structure and the pole to that structure.

The internal reference lines and planes in oriented core

These are:

 1. The Core Axis (i.e. the imaginary line along the centre of the core), labelled CA.

2.  The plane at right angles to the core axis, known as the circumference plane (by some, the propeller plane).

3. The bottom of the hole line – labelled BOH.

4. The long axis of the intersection ellipse of the planar structure. This line requires some further explanation:

The trace of any planar rock structure on the surface of cylindrical drill core defines an ellipse, known as the intersection ellipse.  The long axis of the ellipse is marked on the core surface by points of maximum curvature on the trace of the plane. These are called inflection points.There are two inflection points marking the opposite ends of the long ellipse axis.

The inflection point that makes an acute angle with the down direction core axis is known as point E. The inflection point that makes an obtuse angle with the down direction core axis is known as point E^l ( pronounced E primed).

Note the two special cases:

1. Where a hole is drilled at right angles to the planar structure, then the trace on the core surface is a circle and no ellipse long axis is definable.
2. Where a hole is drilled parallel to a structure, the trace of the structure trends along the along the length of the core and no inflection points can be defined – at any rate, for as long as this particular geometry holds good (which in real rocks is seldom more than a meter or so). 

The orientation of a planar structure in core is defined by two angles (figure 1)

1. The acute angle between the core axis and the long axis (E-E^l) of the intersection ellipse. This angle is known by the Greek letter alpha – whose symbol is: α   The alpha angle can be measured in any core, irrespective of whether the core is oriented or even whether the hole is surveyed. 
2. The radial angle between the BOH line and the point E.  This angle is measured from BOH around the core circumference in a clockwise direction.  Note that ”clockwise” refers to a view looking down the core (i.e. in the direction of the arrow marked on the core).  The angle is known by the Greek letter beta – whose symbol is: β

Now for the stereonet procedure…

Step 1:  Plotting the core reference lines and planes

Figure 2: Stereonet plot of reference lines and planes for a hole drilled at – 45° to 225° azimuth or + 45 to 045 (click for larger image).

On a stereonet lines plot as points and planes plot as great circles (i.e. planes which pass through the centre of the sphere). The azimuth and inclination of the core axis (at the depth of the measured structure) enables it to be plotted as a point on the net. The core circumference plane is a great circle at 90° to the plot of the core axis. A vertical plane is a straight line passing through the centre of the net. The circumference of the stereonet is the horizontal plane. The BOH line is the point where the vertical plane passing through the core axis intersects the lower half of core circumference plane.

A plane can also be shown by plotting the line that is normal to the plane (this point is known as the pole to the plane). The core axis is the pole to its circumference plane. 

Remember that a stereo net is a projection of the lower half of a sphere and the centre of the net represents the down direction of the vertical (the line pointing to the centre of the earth: the gravity vector).   To a stereonet, a drill hole is a line oriented in space.  The direction in which the hole was drilled is immaterial.  This means that an underground hole that is inclined at +45 degrees above the horizontal towards azimuth 045 ° will be plotted on the net as a line at -°45 (below the horizontal) to 225° (the reciprocal of the azimuth).

 Step 2: Plotting the beta (β ) angle.

This is the step where most care needs to be exercised.

The beta angle defines the position of the long axis of the intersection ellipse on the net. This  line ( E –E^l ) is represented by a point on the net.  The angle is measured in a clockwise direction along the trace of the circumference plane, starting at point BOH.  For all beta values between 0° and 90°, point E lies on the net (see left diagram of figure 3).  When beta is 90 ° (figure 3, right) point E lies on the circumference of the net, indicating a horizontal line. The opposite end of that line is point E^l , which now lies on the net circumference at the diametrically opposite side of the net from point E.

Figure 3: Plotting the beta angle for the range β=0° to β=90°

For all beta angles between 90° and 270° , the E end of the long axis of the intersection ellipse is rotated out of the net projection and cannot be shown (it may help to think of E as now pointing “up in the air” out of the plane of the page). However, for this range of beta angles, the opposite end of the axis, E^l , is now rotated on to the net and its position can be plotted by continuing to measure beta around the circumference plane.  The measurement is now made from the point where the left edge (“left” when looking in the direction of dip) of the core circumference plane meets the net circumference. This point is β = 90° (figure 4).

Figure 4: Plotting the beta angle for the range 90° to 270°

When the beta angle is exactly 270°, the line E – E^l is again horizontal and its two end points lie at diametrically opposite locations on the circumference of the net as shown in figure 5, left. In this case, point E is on the left, and E^l is on the right (compare to the plot of β=90° on figure 3, right diagram).

For all beta angles in the range 270° to 360°, point E will plot once again on the net (right diagram of figure 5). The left intersection of the circumference plane with the net circumference represents 270°.

Figure 5: Plotting the beta angle for the range 270° to 360°.

 Step 3: Plotting the Alpha angle

By rotating the overlay, locate the great circle which contains the points CA and E (or E^l).  There is only ones such great circle. This is the trace of the longitudinal core section shown at (c) on figure 1.

On figure 6, the great circle has been drawn as a purple dashed line. Along this line, starting at the point CA, measure the angle 90-α °.  If point E has been plotted on the net, the angle 90-α° is measured in a direction away from point E (as in figure 6, left).  If point E^l is plotted on the net, the 90-α° angle is measured in a direction towards E^l (figure 6, right).  The logic of this step should be obvious from figure 1(c). The measurement locates point P - the pole, or normal, to the planar rock structure being measured.

Once the pole to the plane has been plotted, the net scales can be used to read off the its dip and dip direction, strike and dip, or apparent dip on drill section, as required.

Figure 6: Plotting the alpha angle and locating the Pole

Speed and Accuracy

On a standard 15 cm stereonet, the thickness of a pencil line or point is 1-5 degrees, depending on where the line is on the net. As a result, most stereonet measurement can be considered, as a rule of thumb, to be within one or two degrees of the correct value. That is to say, a stereonet measurement of 45° could be anywhere between 43° and 47°.  By contrast, mathematical manipulation of spatial data would give exact numbers, to a fraction of a degree if required, and limited only by the accuracy of the input numbers. But calculating a figure with this sort of accuracy would be both misleading and spurious. Plus or minus 2 degrees is an entirely appropriate and acceptable range of accuracy for geological measurements. In fact, most geologists would consider themselves favoured if their result is within 2° of the notional “correct” answer. This is because “planar” rock structures are seldom perfectly planar or constant in orientation over distances of more than a few tens of centimetres. In addition to this, orientation lines marked on core are generally considered acceptable if the match of the BOH orientation line from run to run, or from core piece to core piece, is less than +/- 5°.  Finally, measurements of alpha and beta angles (and especially of beta angles) taken by geologists on core typically have similar levels of accuracy. Fortunately, these various sources of error are not cumulative – that is what a +/- designation means.

The long verbal description and many figures that I have had to use in describing the stereonet reduction of internal core angles probably has given the reader the impression that this is a long and complex process. So that the logic of the solution and the plotting techniques can be understood I have shown all the construction lines. However, after a little practise, it will be found that most of these lines can be omitted. Almost everything can be done by eye using the pre-printed net scales, and just two points need to be plotted on the net overlay. Here is how it is done:

1. Plot the core axis and BOH points on the net overlay with permanent inked marks.  A single plot of these points will usually be good for a large number of measurements taken on core with that axial orientation (deviations in the hole azimuth and inclination of less than 2 degrees can be safely ignored).

2. Rotate the BOH point on to a principal net diameter. Using the degree divisions of the net, use angle beta to locate point E (or E^l) and mark this onto the overlay with a pencil mark.

3. Rotate the overlay again so as to bring points CA and E (or E^l ) on to a great circle, then, with the overlay in this position, measure the angle 90 minus alpha to locate the pole to the plane. Mark this point on to the overlay with a pencil.

4. Rotate the pole to the plane on to a principal net diameter:  read off the dip and dip direction of the plane from the scales of the net.

Now erase the two pencil marks to be ready ready for the next calculation.

Total time – 20 seconds.

Try beating that, from cold, with a computer.

The post A stereonet solution for alpha beta angles in oriented drill core appeared first on Roger Marjoribanks.

In a previous post (see here) I described how quantitative orientation data can be collected from from a single drill hole, even where the core is not oriented.  In this post, techniques for collecting orientation data on planes are described when more than one non-oriented hole is available from a prospect.

The need to determine the strike and dip of a planar structure from a number of drill intersections is one that occurs very frequently – this is often called the three-point problem[1] and every geologist should be familiar with the simple solutions to it (Marjoribanks, 2007).

The attitude of any plane is fully defined if the position in 3D space of three or more points on that surface is known. Where three separate holes intersect the same marker bed, they provide three points of known position on that surface. From the intercept data, there are three ways of calculating the strike and dip. The first makes use of structure contours. The second involves the use of a stereonet. In the presentation of the solutions below, it is assumed that the bed to be measured has the same attitude in all three holes.

 There is, of course, the third way: feed the numbers into a computer program such as the freeware you can download at  www.edumine.com ; then click the button marked “Answer”.  The “answer” in this case is useful strike/dip/dip direction information from intersection coordinates.  But what matters almost as much as information is the means by which you arrive at it.  Graphical manipulation is a mental process that engages your brain in the 3D realities of your measurements, and it is this pre-conditioning that helps you turn mere information into knowledge and understanding.  In this case, “Understanding” is the ability to create predictive models for ore, or to cut losses, walk away, and try your luck elsewhere.  For this kind of understanding, easy shortcuts can be self-defeating.   

Remember the mantra, the theme of many of my posts:

Data is not information

Information is not knowledge

Knowledge is not understanding

Understanding is not wisdom

 Solution using structure contours.

 In figure 1, the intersections of three holes into a common marker bed are projected onto a plan. Proceed as follows:

 Figure 1 Using structure contours to determine the strike and dip of a planar surface intersected in three drill holes. On the map three drill holes have intercepted a common bed. The intercepts have been projected vertically onto the plan and labelled with their height above a common datum. Click for full size figure.

Step 1

Determine the three-dimensional coordinates (i.e. northing, easting and height above the datum) of each intersection of the marker bed in the holes.

 Step 2

Plot the three bed intersection points on a map using the northing and easting coordinates for each intersection. Write the depth (often called the Relative Level, or RL) of the intersection beside each point.

 Step 3

On the map draw a lines joining the hole intersections (figure 2). The height of the intersection at the beginning and the end of each line is already marked. Using a ruler, scale off along each line to identify the positions of all intermediate depths: identify and mark even-number depth divisions.  

Step 4

Draw the lines joining points of equal depth on the surface. The lines correspond to the height contours on a topographic map and are known as structure contours. They represent the plot of horizontal lines on the bed and thus mark its strike.  This strike can then be measured on the map using a protractor. 

Step 5

Use the map scale to measure the horizontal distance (h) between any two contour lines – the further apart the better. Since the vertical separation (v) of the contour lines is known, the dip of the surface (d) can be calculated according to the formula:

 Tan d = v/h

Figure 2 Constructing lines between each plotted intersection, the position of different heights along the lines can be scaled off. Structure contour lines (dashed) for the bed are constructed by joining points of equal height. These lines define the strike of the surface. From the map scale, the horizontal distance between lines of known height can be measured – simple trigonometry then allows the dip to be calculated. Click for full size figure.

 Solution using a stereonet

 Proceed as follows (see figures 3 and 4):

 Step 1

Determine the absolute position coordinates (i.e. northing, easting and height above a common datum) of each intersection of the marker bed in the three holes.

Figure .3 Using a stereonet to determine the strike and dip of planar surface intersected in three drill holes. The intercepts are projected onto a plan and labelled with the height of each intersection above a common datum. The line that joins any pair of intersections is an apparent dip on the bed and can be described by its dip and dip direction. Where v is the vertical height difference between each pair of intersections, and h their horizontal separation, the apparent dip ( a ) can be calculated using Tan a = v/h. The dip direction is measured directly from the plan with a protractor. Three apparent dips can be calculated in this way. Click for full size figure.

Step 2

Plot the three intersection points on a map. Use a protractor to measure the trend (bearing) of the lines joining the three points. Use a ruler to scale off the horizontal distance between the points. Knowing the horizontal and elevation difference between any pairs of intersection points, simple trigonometric formulae (see step 5 above) will provide the angle of plunge (the angle which the line makes with the horizontal, measured in the vertical plane) for the line that joins any two pairs of points.

Step 3

We have now calculated the trend and plunge of three lines lying on the surface of the marker bed. Mark these lines on to a stereonet overlay. They plot as three points, as shown on figure 3.

Step 4

Rotate the overlay so as to bring the three points to lie on a common great circle. Only one great circle will satisfy all three points[2]. This great circle represents the trace of the bed that was intersected by the drill holes.

 Step 5

From the net, read off the strike and dip of the surface (or dip and dip direction, or apparent dip on any given drill section).  

Figure 4 On the stereonet, the three apparent dips plot as three points. The net overlay is rotated to bring the points to lie on a great circle girdle. This girdle is the plot of the bed and its strike and (true) dip can be easily read off. (Actually, only 2 apparent dips are necessary to define the surface, but using a third line provides extra accuracy.) Click for full size figure.

 An elegant stereonet solution to determining the attitude of planes in non-oriented core

Where there is no single marker bed that can be correlated between adjacent holes, it is sometimes still possible to determine the orientation of a set of parallel surfaces (such as bedding planes, a cleavage, or a vein set) provided that the surfaces have been cored by a minimum of three nonparallel drill holes (Mead, 1921, Bucher, 1943). The same technique can even be extended to a single hole, provided that the hole has sufficient deviation along its length for the differently oriented sectors of the same hole to be considered in the same way as three separate holes (Laing, 1977).

In the example illustrated in the following figures, three adjacent but non-parallel angle holes have intersected the same set of parallel, planar quartz veinlets. None of the core is oriented, but the average alpha (α) angle (for definition of alpha angle see here) between the veins and the core axis has been measured in each hole: it is 10° in Hole 1; 56° in Hole 2 and 50° in Hole 3.

In our example, Hole 1 is drilled at -50° to 270°; Hole 2 at -65° to 090° and Hole 3 at -60° to 345°. On the stereonet, the orientation of each drill hole plots as a point.

When plotting planes on a stereonet it is always much easier to work with the pole[3] to the plane rather than the plane itself. If a plane makes an angle α with the core axis, then the pole to the plane makes an angle of 90 – α to the core axis, as illustrated in figure 5.

Figure.5  The angular relationship of the alpha angle (α) to the pole (i.e. the normal) of a surface intersected in drill core. Click for full size figure.

 Let us consider Hole 1 (figure 6).  Angle α for the vein set is known. Because the core is not oriented, the poles to the veins could lie anywhere within the range of orientations that is produced as the core is rotated one complete circle about its long axis.  This range defines a cone, centered on the core axis, with an apical angle of 2 x α. From figure 5, we see  that the pole to the plane will describe a cone with an apical angle of 2 x (90 – α). That is all we can tell from one hole, but this information can be shown on the stereonet, because a cone centered on a drill hole plots as a small circle girdle around that hole. In Hole 1, 90 – α is 80°. The vein set in Hole 1 can therefore be represented by a small circle girdle at an angle of 80° to the hole plot. 

 Figure 6  On the stereonet a small circle girdle at 90-α degrees from Hole 1 traces all possible orientations of the poles to a vein set making angle of α with the core axis. Click for full size figure.

Now the same procedure is carried out for Hole 2 by drawing a small circle at 90-α (34°) to the plot of that hole on the net (figure 7). The small circle about Hole 1 and the small circle about Hole 2 intersect at two points (P1 & P2) – these points represent two possible orientations for the vein set. Already, with just two DD holes we have reduced the problem of determining the orientation of the vein set from an infinite number of possibilities to just two possibilities.

Figure 7  On the stereonet, the poles to all possible planes making an angle of α  degrees to the axis of  Hole 2  inscribe a small circle girdle at 90-α (34°) to the plot of the hole.  The small circle about Hole 1 and the small circle about Hole 2 intersect at two points (P1 & P2) – these points represent the only two possible orientations for the vein set, given the data plotted so far. Click for larger image.

Now, in the same manner, we draw the third small circle about Hole 3 representing the alpha angle measured in that hole (figure 8). We now have three small circle girdles on our net, centered about each of the three drill holes. Since the assumption behind this procedure is that all measurements are of the one vein set with a constant orientation, the single point (P) where the three small girdles intersect must represent the pole to the one orientation that is common to all three holes. This pole defines the attitude of the common vein set seen in the holes. Of course, with a real set of measurements it is unlikely that three small circles plotted in this way would meet at a single point. Rather, the intersecting lines will define a triangle whose size reflects the accuracy of the measurements (and the assumptions made that we are dealing with a single parallel set of surfaces). The true pole position (if there is one) will lie somewhere within this triangle of error.

 From the point P, the strike and dip (or dip and dip direction, or apparent dip on drill section) of the vein set can be simply read off from the net.

Figure 8  On the stereonet, the poles to all possible planes making an angle of α  degrees to the axis of  Hole 3  inscribe a small circle girdle at 90-α (40°) to the plot of the hole.  The small circles about Hole 1, Hole2 & Hole 3 intersect at a single P. P is the pole to the unique plane which satisfies the measurements made in the three holes.. Click for larger image.

REFERENCES

 Bucher WH (1943) Dip and strike for three not parallel drill holes lacking key beds. Econ Geol 38, 648-657. 

Laing WP (1977) Structural interpretation of drill core from folded and cleaved rocks. Econ Geol 72, 671-685. 

Marjoribanks RW (2007) Structural logging of drill core. Australian Institute of Geologists Handbook 5 (2^nd ed.), 68p.

 Mead WJ (1921) Determination of the attitude of concealed bedding formations by diamond drilling. Econ Geol 21, 37-47.

The post The Three point problem: Calculating strike and dip from multiple DD Holes appeared first on Roger Marjoribanks.

My drill core is not oriented. How do I measure structure?

Down-hole orientation surveys record the deviation of a drill hole from its initial azimuth and inclination. However, the solid stick of drill core (sometimes, not so solid) recovered from a hole is not fully oriented by a down-hole survey. Although such surveys give the azimuth and inclination of the core axis at any given down-hole depth, the core itself has another degree of freedom in that it has rotated by some unknown amount about its long axis. This does not affect a lithological log made of the core (the down-hole depth to any given point on the core axis can still be measured), nor the ability to measure true thickness of beds (see figure 6, below), but the original attitude of structures (i.e. their dip and dip direction relative to geographical coordinates) cannot be directly determined. Techniques are available to fully orient the drill core (see previous posts in this series) and in all cases, when drilling into unknown rocks, it is recommended that these are employed[1]. However, orienting core adds expense and time to any project; some managers are just not prepared to spend the money and some drilling companies are not equipped or have the knowledge to do this.  In any case much historical core that geologists have to deal with is not oriented (and orientation can never be done post-facto).

But all is not lost: it is still possible to make useful quantitative structural measurements on non oriented core.  

How to measure structures in a single non-oriented core is the subject of this post. Much of the material is taken and adapted from Marjoribanks, 2003 and 2011[2].

In many cases rocks contain a penetrative planar fabric such as bedding or cleavage whose orientation is known from surface mapping. In these cases, if this surface can be identified in the core, and the assumption can be made that its attitude is constant, the surface can be used to orient the core. A cleavage is a better surface to use for this than bedding because the attitude of a cleavage, being a later tectonic surface, is generally more constant than that of bedding (Annels and Hellewell, 1988[3]).

Place a piece of core containing the structure to be measured in a Core Orientation Frame set up at the appropriate azimuth and orientation for the drill hole. Rotate the core about its long axis until the known structure is in the correct orientation according to your compass and clinometer. Now the orientation any other structures present in the core (such as mineral veins, fold axes etc.) can be directly measured with your geologists’ compass.

A common situation occurs when a hole is drilled at right angles to the strike of a planar structure whose dip is either not known or else is suspected of changing through the length of the hole, perhaps as the result of folding. This is a situation that often arises when drilling surface geochemical or geophysical anomalies. The situation also commonly arises when surface mapping is able to establish the strike of extensive planar rock units, but accurate dips cannot be determined. In non-oriented core all that can be measured of the dominant planar structure is the angle alpha (α) – the angle that the surface makes with the long axis of the core. Since the strike is known (or confidently assumed), and the hole is drilled at right angles to strike, the surface can be plotted on the drill section in only two possible attitudes, that is, symmetrically disposed at angle alpha about the hole trace (see figure 1, below). In many cases simple inspection of the two geometric possibilities for the attitude of the measured surface will lead to one of them being discarded as unlikely.

Figure 1:  How to plot the attitude of surfaces in non-oriented core where the strike of the bed is known, but the dip is not (i.e. assuming the hole azimuth is normal to strike). Of the two possibilities plotted, one can often be discarded as unlikely. Click for full size figure.

In the general case where neither strike nor dip of a surface is known, then the angle between that surface and the core axis (the angle α) defines half of the apical angle of the cone that would be produced if the surface were rotated 360° around the core axis. This is illustrated in figure 2, which shows the range of possible attitudes of a surface on rotating the core about its axis. In this case, no absolute measurement of the attitude of the surface is possible, but it is still worthwhile to plot the boundaries of the cone of lines on to the drill section to represent the limits to the range of possible solutions to the true orientation of the surface. Small ticks placed on the ends of these lines on the section indicate the limits to the domain within which the surface will intersect the section.

Figure 2: How to plot the attitude of a surface where neither its strike nor dip is known. On the trace of the drill hole on section two lines with ticks indicate the range of possible attitudes defined by the alpha angle. Click for full size figure.

Two special cases exist. One, where the surface is normal to the Core Axis (α = 90°), and two, where the surface trends parallel to the Core Axis (α = 0°). With real core and real world measurements, of course, these situations can be taken as any α alpha over 85° or under 5°. 

1. Where (α = 90°) you are drilling at right angles to the strike of the surface and the true dip is 90°- the hole inclination. Rotation around the Core Axis gives no apparent change in attitude. The surface can therefore be plotted unambiguously on to the drill section as a short line at right angles to the hole trace (figure 3). However, note that the core is still not fully oriented, and other planar or linear structures that might occur within it (that are not normal to the core axis) still cannot be fully measured.

Figure 3 How to plot the attitude of surfaces in non-oriented core where the surface is at 90° to the core axis (α = 90°). This situation can only arise when drilling at right angles to strike. The dip shown on the section is therefore a true dip. However, the core is still not fully oriented, as the attitude of any surfaces not at 90° to the core or any linear structure still cannot be measured. Click for full size figure.

 2.  The second case (where α = 0°) means that your drill hole lies within the surface you want to measure. The trace of the bed on the drill section can be shown as a single short line parallel to the drill trace (figure 4): but note – this line represents an apparent dip only: it would only be a true dip if you were drilling at right-angles to strike[4]. But you still know something about the dip: remember, apparent dips can never be greater than true dips. So you know that the true dip of the surface cannot be less than that shown on your drill section.

Figure 4 How to plot the attitude of surfaces in non-oriented core where the alpha angle is 0°. Note that this is an apparent dip only, unless the hole is drilled at right-angles to strike. Click for full size figure.

For the general case where no assumptions can be made about the attitude of structures in the core, absolute measurement can only be made if the core is mechanically oriented by means of a core orienting device. From oriented core you can measure the strike and dip of the surface. If it turns out that you are not drilling at right angles to the strike of the bed (the most likely situation) then the intersection of the measured surface on the drill section will be an apparent dip. This is illustrated in Figure 5. Knowing the true orientation of the surface, its apparent dip on section can be calculated using a stereonet or by looking up a table of apparent dips[5].  

Figure 5  How to plot the attitude of a plane on a drill section when both the strike and dip are known. Click for full size figure. 

Even where core is not oriented, it is still possible to make a large number of useful structural observations and measurements. For example: 

• Qualitative observations on the style and nature of structures. 

• The relative ages of structures and their relationships to lithology, veining and alteration. 

The only orientation measurement on a surface that can be made in non oriented core from a single drill hole is the alpha angle. Taking and recording alpha angles for planar structures is quick and easy to do and should be a routine procedure in all core logging.  From the alpha angle is possible to use simple trigonometry calculate the true thickness of any tabular bed or vein intersected by the core. How to do this is illustrated in figure 6, below.

Figure 6 How to calculate the true thickness of beds in core. All that is needed is the alpha angle. It is not necessary for the core to be oriented, or even to know the azimuth and inclination of the drill hole. Click for full size figure. 

In this post I have described methods for obtaining quantitative structural data from a single non-oriented drill core. If a number of drill holes have been drilled into a property then an additional range of techniques are available to acquire this data. These will be the subject of the next post under the heading “The three point problem”

Note this blog does not accept comments any more (there was too much Spam coming in!). However you can use the CONTACT ME tab and comments, criticisms questions etc. are welcome.

The post Measuring Structure in Non-Oriented Drill Core appeared first on Roger Marjoribanks.

It is always better to know the orientation of planar structures at the time of logging rather than at some later date when the structure that was measured is long forgotten and the core returned to its stack in the yard.

The six rules are mostly common sense. With a little thought, and the help of an actual piece of core to play with, you could work them out for yourself. But employing them will greatly speed up the process of understanding the attitude of planar structure in oriented drill core at the time of logging.

Measurement of planar structures – such as bedding, veins or cleavage – in oriented diamond drill core can be done in two ways. The simplest way is to set up the core in its original orientation by means of a core frame and then measure structures within it using a geologists’ compass.  The second way is to measure the angles which the plane makes with lines of known orientation in the core and then, knowing the hole orientation, use these angles to calculate the strike, dip and dip direction. This is called the internal core angles method. I described these methods in some detail in an earlier blog post and discussed the advantages and disadvantages of the two methods (Measuring structures in oriented core, Oct 19 2013). 

But there is a third way: just eyeball the data and work it out for yourself. My six rules show you how to do this.

Before I present the rules, I will quickly recap the internal core angles method. The orientation of a planar surface intersected by drill core can be defined by two angles called alpha (α) and beta (β). The definition of these angles is shown in the diagram below: 

To be geologically useful, these alpha and beta numbers for a plane have to be converted into the more meaningful dip and dip directions. This conversion is usually accomplished by mathematical calculation (computer software) or less frequently by graphical means (i.e. a stereonet). How to use a stereonet to reduce alpha/beta angles is detailed in my blog post here.

In the discussion of Rules 1-6 that follow, note that no alpha or beta measurements made in core can ever be considered accurate to more than +/- 2 degrees.  Therefore for 180° you should read 178° – 182°, for 0° read 358°-002° and for 90° read 88°-90°. If the alpha and beta angles are more than +/- 2 degrees away from the key numbers of 0, 90, or 180,  the more we are into the area covered by Rule 1 – estimating approximate values for dip and dip direction. The further measured alpha and beta are from 0, 90 & 180, the more approximate that estimation will be.

 Rule 1

When logging oriented core it is a simple matter to hold a piece of core in your hand and roughly orient it to its original in-ground attitude – your hand acting as a crude core frame. Most experienced geologists logging structure in core would do this automatically. By doing this one can eye-ball any planar structure in the core and quickly estimate its dip and dip direction. However, very often structures are analyzed ex post-factum on the basis of recorded observations long after the original structure is forgotten. All there is to work on is the record in the structure log. This is not an ideal situation, but all is not lost. We can recover the same quick qualitative assessment of dip and dip direction by visualizing the core in its original orientation and peopling it with a plane which would give the recorded measurements.

But why would you want an approximate dip and dip direction when you can use a computer program to calculate an exact one? A good question, but there are at least four  good answers:

1. The process is quick.

2. An approximate answer may be all that you need to answer your structural question.

3. If mass data has been entered into a spread sheet for computer processing, knowing the approximate solution for any given measurement provides a quick way of checking the accuracy of the computer output (or more particularly of the data entry process, where errors are most likely to have occurred). This step is the essential one of quality control and validation.

4. For particular values of alpha and beta (as explained in Rules 2, 3,4 & 5) simple mental calculation involving adding or subtracting two-digit numbers can provide an accurate dip and dip direction.

Except in the few specific cases referred to in Answer 4 above, it is impossible for any normal person to mentally calculate an exact dip and dip direction from alpha and beta angles.  However, by visualizing the angles on a mental image of the core, it is usually possible to produce an approximate estimate of the dip and dip direction of the plane – one good enough to be in the right ball park (1). I illustrate this with a random example (one sent to me by a reader):

a hole drilled at -44⁰ (dip below horizontal known as hole inclination In) to 031⁰ (the direction of drilling known as Azimuth  Az) ; a plane in the core with internal angles alpha 09⁰ and beta 227⁰. 

With just introspection it is possible to say that the plane must dip  broadly to the east (the beta number tells you that) with a dip that is a bit steeper than the hole inclination (the alpha number tells you that).

The exact answer (which I calculated using a stereonet): is 54⁰ to 094⁰

If my calculation of had come up with a plane dipping at, say, a low angle to the NE, I would have instantly known that this was BS and I had made a gross error in my stereonet manipulation.

RULE 2

If β = 180º, then you are drilling at right angles to strike of the plane and in the direction of its dip. In this case… 

                                    Dip direction = Az

                                    Dip =  In – α    

Rule 3          

 If β = 180° and angle α = In, then..

 Dip = 0° (i.e. horizontal)

No strike can be defined (see footnote 1)

 Rule 4        

 If   β = 0º, then the hole is drilled at right angles to the strike of the beds. There are 3 possibilities for the dip:

 (a)Where In + α < (is less than) 90º

                                           Dip Direction = Az

                                           Dip = In + α 

  (b)  Where In + α > (is greater than) 90º, then…. 

                                           Dip Direction = Az + 180º

                                           Dip = 180 – α - In

   (c)  Where In + α = 90º, then… 

                                          Strike = Az + 90º (Note: no dip direction can be assigned)

                                          Dip =90º             (i.e. Vertical) 

 Rule 5    

 If α = 90º, then the hole is drilled at right angles to the strike of the plane in a direction that is opposed to the dip of the beds. There is no useful definable beta angle (if you think you can measure a beta angle then the number you get is meaningless – see Rule 6). The relationships are these: 

                                                Dip direction = Az + 180º

                                               Dip = 90º – In    

 Rule 6                       

As the angle alpha approaches 90°, the intersection ellipse on the core surface approaches an intersection circle on which no unique long axis can be defined (see Rule 5). For large α angles it is therefore not possible to define the point E (or E’) on the core surface with sufficient accuracy to enable an accurate measurement of the beta angle to be made.  Small errors in measuring beta can produce large errors in the calculated strike and dip for that bed. How large does alpha have to be make errors in the corresponding beta angle unacceptable?  There is no fixed cut-off point: every increase in alpha above 50° causes potential measurement  error in beta to rise exponentially. Experience suggests any alpha angle greater than 65º is large - but how much error you are prepared to tolerate is a matter for judgement and is to some extent dependent upon how well the plane is defined in core and how many parallel planes of that orientation are present.

 Where large alpha angles are encountered the alpha/beta method should not be used and core should be set up in a core frame and the surface measured with a geologists’ compass.

***** ****

(1) Computer software will calculate an exact strike (or dip direction) for a plane, even if the difference between the alpha angle and the hole inclination is a fraction of a degree. However, if the difference between these two angles is 10 degrees or less, the calculated dip direction is essentially meaningless, since even the smallest error in measuring either α or In will lead to an order of magnitude greater error in the calculated strike. This is the same problem as trying to measure the strike of near-horizontal beds exposed at surface: but the problem is much more acute in drill core because of the small area of exposed surface.

******

Please note that this blog no longer accepts comments (there was too much spam coming in!) However, I welcome your opinions, comments even criticisms.  You can contact me direct through the email form on the Contacts Tab.

The post Six rules for alpha-beta measurements in drill core appeared first on Roger Marjoribanks.

Using Stereonets

Cheap, versatile, reliable, compact, robust, ultra lightweight and does not need any batteries: there should be a stereonet in every geologist’s field kit.

A stereonet is a tool (a type of nomogram[see footnote 1]) that allows the attitude of planes and lines in three-dimensional space to be shown on a specially-constructed, two-dimensional, pre-printed graph.   Planes appear as straight or curved lines on the stereonet graph: lines appear as points. The attitude of a plane is uniquely defined by the attitude of the line that is at right angles to it. This line is known as the normal or the pole to the plane.  The attitude of any plane on a stereonet can thus be plotted as the single point that represents its pole.

This is a Rolls-Royce model stereonet set up -  most are much simpler.  The graph is mounted in a storage box with room for a supply of tracing sheets in the base. The net is printed on a reversible board with an equal angle net on one side (Wulff Net) and an equal area net (Schmidt Net) on the other. A metal spike through the centre point allows articulation of tracing paper overlay. For details on how stereonets are constructed and used, see any good geology textbook (a list is given below).

 Stereonets are used in structural geology in two ways:   

1. As a means of solving simple problems in 3-dimentional trigonometry. Exploration and mining geologists are constantly facing problems like these and their solution is critical to understanding the structure of the earth – the 3-D arrays of beds and faults and fold axes and ore bodies that are the matter of their profession. At what depth will this bed that I observe here intersect that fault there? What will be the apparent dip of an NE-dipping ore shoot on An E-W drill section? Will this plunging linear ore shoot pass below the drill hole that I plan? How do I convert the alpha and beta angles measured in core into strike and dip? (For the answer to that last question see here).
2. As a means of graphically presenting the attitudes of a large number of field structural measurements on one plot. This is a statistical technique: the pattern of a large number of points on the graph can contain information about the processes operating in the domain from which the measurements were taken that may not be obvious from the raw measurements alone.

 Now it is true that solving problems of 3-D trigonometry can be done mathematically. There are readily-available (albeit usually expensive) computer programs which will do this for you.  I am often asked – why use a stereographic net when everything that it does can be accomplished with less mental effort by a software program? Well, here is a related question -  why should I memorize multiplication tables when pocket calculators will give me the answer by just pressing a few buttons in sequence? The answer to both these questions is much the same.

 There are four main reasons – they can be summed up as: mental effort is good for you, it helps you find ore bodies. 

1. For simple manipulations of angular relationships, using a stereonet is simply quicker than any computer program (allowing time to find your program, go through a series of pull down menus and enter data).  Moreover, the stereo net can be used in the field, on top of the core tray and so on where using a computer is not always very convenient. With a little practice, stereonet lots and manipulation take only a few seconds. It is laughably-simple technology – but not therefore to be despised. It involves a re-printed graph, a thumb tack and a sheet of tracing paper.

2. The Stereo net presents orientation data in a graphical way that enables the data to be visualized. Making a stereo plot by hand forces the geologist to think in three dimensions. A mental 3-dimensionmal model is essential if the relationships between raw measurements and their geological meaning are to be understood.

3. The stereonet presents 3-D information in a way which enables any mistakes in measurement and plotting to be immediately apparent.   Garbage is garbage, but computer -processed garbage can acquire a life of its own and be hard to recognize.

4. Using a computer encourages a menu-driven approach to problem solving.  It tends to encourage a large number of low-quality measurements to be made in the expectation that quantity will make up for quality.  In structural geology, this approach seldom works.

For these reasons, every explorationist should be familiar with simple stereo net manipulations.

For details on how stereonets are constructed and used the reader should refer to any good geology textbook. The classic (and best account, though now long out of print) is: The use of stereographic projection in structural geology by F C Phillips (Edward Arnold, 1960, 86p). A good detailed treatment is in An outline of structural geology by Hobbs, Means & Williams (Wiley, 1976, 571p).  A useful and practical treatment for several stereonet procedures that are of particular value to the explorationist can be found in Geological methods in mineral exploration and mining by R W Marjoribanks (Springer, 2010, 238p). See also my own blog posts under the Tab; Stereonet solutions in structural geology.

The post Using Stereonets appeared first on Roger Marjoribanks.

 The Rocket or the Protractor – which is the best technique?

 For most applications, it is recommended that a core frame be used to measure the orientation of structures. Core frames are a cheap, low-tech and readily available tool.  One is illustrated below. The frame permits direct visualization of structures in their original orientation, and allows them to be measured in terms which have geological meaning at the time they are made. Setting up core in a frame permits not only measurement of individual structures but, importantly, allows the relationships between sets of structures in the same piece of core to be observed.  Measurements can be constantly compared against the evolving geological model for the deposit as the core is being logged. Significant changes in attitude of structures can be identified and examined in more detail while the core is still laid out. Mistakes in measurement (or problems with the core orientation line) can be quickly spotted.  In short, Core Orientation Frames are GEOLOGIST FRIENDLY.  Even where the method of measuring internal core angles is being used, a core frame must also be available in order to measure planes with high alpha angles[1]. In addition, a core frame is the only way in which non-penetrative linear structures – such as fold axes – can be measured.

A Core Orientation Frame – sometimes called a “Rocket Launcher” It enables a piece of oriented core to be set up in the same orientation as it was when part of mother earth. Structures can now be measured with a geologist’s compass in the usual way. For most applications in logging core, the use of a core frame is recommended.

The internal core angle technique involves measuring two angles referred to as alpha and beta. These two numbers (meaningless in themselves) are then fed into a computer where they are crunched with down-hole survey data to produce the attitude of the measured structure in terms of a standard dip and dip direction measurement. This number processing can also be done manually using a stereonet — It’s not hard and takes only a few seconds once you have mastered the technique. You can find full details of the stereonet technique elsewhere in my blog (LINK) and also in my book “Geological methods in mineral exploration and mining” (Springer 2011).

Definition of the angles that a planar structure makes with the Core Axis (CA) and the Bottom of Hole (BOH) lines in surveyed and oriented diamond drill core. The alpha angle is the acute angle between CA and the line E-E (the long axis of the intersection ellipse). The beta angle is the angle between the Bottom of Hole (BOH) line and E. This measured around the core circumference in a clockwise direction when looking down the hole. The beta angle can range from 0 to 359 degrees.

Using a simple home-made protractor to measure the alpha angle of a planar structure in a piece of oriented drill core.

Measuring the beta angle of a planar structure in a piece of oriented drill core. The black line marks the bottom of hole (BOH) : it represents the intersection of the original vertical plane with the core and has been determined by a special down-hole core orientation survey procedure. The protractor is a basic school plastic 180 degree model ($2.80 – any good stationer or newsagent) cut to fit the core circumference. More elaborate beta-angle protractors are sold commercially, but none are as accurate or as easy to use as this design.

The alpha-beta method was developed in 1979 by the geotechnical engineer R E Goodman as a means rapidly taking a large number of measurements that could be plotted in bulk and analysed statistically. Statistical handling of bulk plots has value in geotechnical studies but tends to obscure geological relationships by promoting quantity over quality. Nevertheless, the speed and ease of making alpha/beta measurements as digital computer feed, and the fact that it is a mechanical, mindless process which obviates the geologist from actually having to think about the meaning of what he or she is actually looking at, has made the technique very popular. It is fair to say that in most drilling programs using oriented core, the alpha-beta method is now the technique of choice from the very first exploration hole through to final ore definition drilling. The reality is that the transfer of this technique from the geotechnical engineering field to the structural geology field has been, with few exceptions, a disaster for geological understanding. It encourages a culture of filling data bases with thousands of meaningless numbers in the hope (seldom fulfilled) that somewhere there is a computer program that will tell the geologist what it all means.

 Geologists should always resist being turned from professionals whose brains are actively engaged with the material they are examining into mere operatives – collectors of endless numbers to be entered into computer data bases. 

Data is not information

Information is not knowledge

Knowledge is not understanding

Understanding is not wisdom 

 POSTSCRIPT:

Well he would say that wouldn’t he? (He sells core frames, after all.)

 Lest I be thought biased, I must point out that the internal core angles method of measuring planar structure in core does offer some advantages over using a core frame:

•  Measuring alpha/beta angles is much faster than using a core frame.
•  If the requirement is to make a very large number of measurements of a set of planar structures, whose identification and significance are otherwise well understood, with a view to a statistical treatment of the results, then the internal core angle technique, with computer processing of the results provides the quickest and most effective option. This situation can arise, for example, in a mine application, in geotechnical logging or in the advanced infill drilling stages of prospect exploration. In this case, the internal angles would normally be entered directly onto a spread sheet type of log form (or directly into a portable computer) for subsequent processing. Presentation will usually be in the form of a computer generated section or map, a stereonet pole figure or a histogram. However, even for these applications, the inherent limitations (often impossibility) on the use of this method, for planes with particular orientations, or for measuring non-planar structure, need to be borne in mind.
•  Measuring internal core angles may be the most efficient method of recording structure attitude onto analytical spread-sheet type logs during the advanced drilling stages of a prospect.

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