The Tarong high-grade thermal coal deposit, in the South Burnett area of Queensland 180km NW of the State Capital Brisbane, was discovered in 1968 by a geological team from Conzinc Rio Tinto of Australia (CRA).

In the remainder of this essay, I will refer to the company as Rio Tinto [1].

Rio Tinto did not waste time. By 1970, following a massive drilling campaign of mostly vertical open drill holes, they had proven 163 million tons of coal within an open pit mine profile with low stripping ratio [2]. Much, much more has been discovered since in the immediate area (see: South Burnett Coal Reserves 2015). They named their discovery after the Tarong pastoral station in which it was located.

The deposit is now owned by the Queensland State Government utility, Stanwell Corporation. Coal has been continuously extracted from Tarong since 1984 through the open cast Meandu Mine. The mine’s entire output of 2 million tpa is sent by conveyor belt to the adjacent, Government owned, Tarong Power Station, which burns it to produce 1.4 GW p.a. of electricity, fed directly into the Australian East Coast Grid [3]. Tarong does not just contribute electrons to the ECG. As with all turbine systems, it contributes critical grid stability (firming) at no extra cost.

With present known reserves and present consumption rate it will be at least 250 years before the Tarong Power station runs out of coal.

With the closures and projected closures of coal mines across Australia [4], the price of electricity from Tarong is set to become the lowest in Australia [5].

About the geology

The Tarong deposit is located within a north-trending, 60x10km, outlier of thick Late Triassic sediments. The outlier is now called the Tarong Basin [6]. The Basin is fault bounded within Early Paleozoic metasediments and assorted intrusives that make up the Australian east-coastal Tasman Orogenic Belt of the Great Dividing Range. Major steep dipping, north-trending faults divide the Belt into elongate tectonic slices called “blocks”. It is probable that the Tarong Basin is a pull-apart basin caused by a late Triassic strike-slip re-activation along one or more of these faults. The sediments that fill the basin were laid down in a swampy environment of slow meandering streams, shallow lakes and, most importantly in this context, a riot of tropical vegetation whose remains locally accumulated to fossilise as coal.

Rio Tinto (and I) appear on the scene

In 1964, Rio Tinto discovered the giant copper-gold deposit of Panguna in Bougainville Island, Papua New Guinea. By 1968, they were advanced in the process of developing a major mine there. They hoped to find a similar deposit in Eastern Australia. Stream-sediment geochemistry – then a new exploration technique – had worked well for them in the islands but had not previously been employed at scale in Australia. It was a smart exploration play.

In the early months of 1968, as a junior geologist with Rio Tinto Exploration, I was transferred from the Northern Territory to work on base metal exploration in the Lower Paleozoic rocks of the Queensland coastal ranges. Geologist W H Johnstone (hereinafter, Bill) and I were tasked with collecting stream sediment samples from across an approximate 100km by 30km area between the regional inland centers of Towoomba and Kingaroy. Bill had been working in the area for some months prior to my arrival and was much more experienced than I. He mentored me in the techniques of stream sediment sampling.

The area is dominated by forested north-trending ranges, the intervening valleys partially cleared for grazing and for horticulture on the rich alluvial flats. Access was generally good: paved roads and farm tracks in the valleys, logging tracks through the intervening hills. Some of the most beautiful country in eastern Australia, baking under the hot summer sun.

We based ourselves at the Grand Hotel (today, the Grand Old Crow Hotel) in the small agricultural town of Crows Nest (pop. around 1000), central to the area. I was based there for over three months. One of the best exploration bases I ever had.

Rural Town Interlude

On weeknights there was little entertainment to be had in Crows Nest (no rural TV in 1968). We spent our evenings in a shared room at the Grand (bathroom at the end of the corridor, telephone in the public bar downstairs), sorting samples, writing up notes, planning next day’s activities. I did not own a camera back then so attempted to record my day’s experiences in a sketch book.

Long summer weeknights in the Grand Hotel, Crows Nest

But on Saturday the town was busy, even roisterous, as the farmers came into town for shopping, a meal at The Grand and to socialise after a hard-working week…

Saturday night in the Ladies Lounge at the Grand Hotel. 

Back to the Story

Early in our acquaintance, Bill told me that sometime previously when he was staying in a hotel at Nanango (50km north of Crows Nest), he was approached by a local man called Nobby [7], who owned and ran a contract water drilling service for farmers. Nobby had told Bill that around the headwaters of the Yarraman and Meandu Creeks (30km north of Crows Nest) many of his boreholes had encountered seams of coal. At that time, drilling for agricultural water was subsidised by the State Government, but drillers had to inform the Department of their results and provide a geological log for each hole.

Bill decided to go to the State capital Brisbane to check this information at the Department of Agriculture. He would not have done this without the permission of our immediate supervisor, Gladstone-based geologist Ian Witcher. It turned out that, over the general area described by Nobby, almost every water bore sunk over a period of many decades had recorded coal in their logs (even rural water drillers had little problem identifying that black stuff in their cuttings). By contouring the results, an area prospective for coal mining in the southern Tarong Basin was outlined. Although many other coal occurrences were known in Queensland at that time, the attraction of Tarong was its good existing infrastructure and proximity to the population centers of SE Queensland.

Me (smoking a pipe), Nobby [but see footnote 8] and Bill Johnstone in the front bar of a Nanango Hotel

Ian Witcher notified the Head Office of Rio Tinto Exploration in Melbourne. The company’s Exploration Manager – the legendary Australian mining Geologist Haddon F King – made the strategic decision to pivot exploration in the South Burnett area from base metal to coal. The first diamond drill hole to test the coal potential of the Tarong area was planned for the center of the best legacy water bore results.

For whatever reason (I was the most junior geologist on site), I was asked to geologically log this hole [9]. It was collared on the top of a low hill in the Meandu Creek area: a hill clothed in dry grass, scrub and stands of Eucalypt trees with some clearing for pasture on its lower slopes; now long swallowed up by the later mine.

Prior to drilling, although outcrop was poor, I attempted to make a geology map. A conglomerate was exposed on the hilltop – an outlier of the basal unit of a late, overstepping formation. On the lower slopes, some surface float of sandstone. Along the new drill access, track the dozer blade had exposed outcrop of a soft, rubbly, buff-coloured rock. I confidently identified this as mudstone. All units were flat lying.

The 200m vertical drill hole presented the easiest logging exercise of any I have ever attempted. Layer cake stratigraphy; well-bedded sedimentary lithologies dominated by sandstone, siltstone, mudstone and coal; sharp contacts between units; little post-depositional deformation. In first 100m, the drill hole encountered at least five coal seams ranging from 1 to 25 meters thick, the first seam only 20m below surface.

After the ancient metamorphic rocks of the Territory, I thought coal geology a piece of cake. But the “mudstone” I had observed at surface turned out to be heavily weathered coal. So much for my surface rock recognition skills.

My association with the Tarong project ceased at this point as I was transferred to another project in Papua New Guinea (see HERE). Someone else was brought in to replace me. Hopefully someone with more knowledge and experience of coal exploration than I.

The Moral of my Story

The discovery of Tarong is a good example of a general principal for the exploration geologist: when you are exploring an area, always seek out local knowledge. If someone, knowing a company geologist is in town, approaches you with an interesting rock specimen he has found, treat him (it’s always a him) with respect. Listen to his story and make sure you protect his interests if you or your company follows up on any new knowledge or insight he has brought you.

In 1962, Lang Hancock, a cattle station owner in the Hamersley Ranges in the remote northwest of Western Australia, brought the iron ore potential of the area to the attention to Rio Tinto. He was rewarded with a generous ongoing royalty from Rio’s subsequent, hugely profitable iron ore mines. Hancock died in 1992, but the royalty stream continued. Hancock’s only child Gina Reinhart, who had grown up on her father’s Mulga Downs station, leveraged her small inherited fortune into a much greater one to become today Australia’s richest person with over $46 billion (US$32 billion) in assets.

Lang Hancock and his daughter Gina, circa 1980. Photo by the Sydney Morning Herald.

It seems to me likely that the well-publicized story of Lang Hancock’s new-found wealth prompted a Nanango resident called Nobby to approach a Rio Tinto geologist in a pub in late ’67 or early ’68.

Bill Johnstone’s decision to follow up on his local knowledge was the key to the discovery of the Tarong coal deposit.

Epilogue

After I left Queensland, Bill’s career and mine took different paths and we never met again.  Much later, I was told by a mutual acquaintance that when Rio lodged a Mining Lease over the Tarong coal deposit, Nobby was given a similar royalty deal to that of Lang Hancock on any future coal sales by Rio Tinto in the South Burnett Region. I was also told that Bill subsequently married one of Nobby’s daughters. Although I have no independent verification for any of that, if it is true, good luck to them both. They deserve it.

The post The Power of Local Knowledge: the discovery of the Tarong coal deposit: appeared first on Roger Marjoribanks.

In 1997, the mining world was rocked by the dramatic exposure of a $6 billion gold salting scandal. From 1994, a crew of corrupt Filipino geologists, working in a remote jungle location in eastern Borneo for a small Canadian mining company called Bre-X, had undertaken industrial-scale salting of a gold prospect called Busang. This was at such a scale, and continued over such a time, as to convince hardheaded global mining companies as well as Mum and Dad Canadian investors that Bre-X had discovered the largest gold mine in the world. Some analysts, working from Bre-X stock exchange reports, estimated well over 200 t of economically recoverable gold.  The resulting 3-year frenzy of greed caused the penny stock Bre-X soar to over $285, valuing the company at over $6 billion. It also exposed corruption at the highest level in Indonesia and lack of oversight by regulators in Canada. When qualified external assessors finally got access to the remote location the fraudsters knew the game was up. Bre-X Chief Geologist Michael De Guzman, on his way back to site to face the music, fell (or did he jump, or was he pushed, or was he even there?) from a helicopter. His body (or at any rate, someone’s body) was recovered on the jungle floor four days later, partially eaten by pigs. The corpse was identified by one of his colleagues from the Busang site before he, and the remaining Filipino members of the crew, flew back to the anonymity of Manila.

Apart from De Guzman, no one was ever named, and no one convicted of crime either in Indonesia or Canada. The Bre-X stock price collapsed overnight, and the company quickly went into receivership. David Walsh, the President and Chief Executive of Bre-X, and his Vice-President of Exploration John Felderhof, retreated to their respective villas on the Cayman Islands.

The whole story revolves around the actions of De Guzman. Bizarrely, although the podcast describes in gruesome detail what De Guzman’s (supposed) corpse (what was left of it) looked like, none of the interviewees describe the living man in any detail, and none give his background or evaluate his character. He remains a shadowy figure. What the witnesses give is a cardboard villain, a toothless, bespectacled, gold-bedecked conman drinking and whoring his way through the fleshpots of Jakarta, Samarinda and Toronto. But De Guzman must have had some impressive qualities to fool so many for so long.

I listened to the podcast in one marathon session and thought it brilliant. The depth of research by the team of journalists working for the BBC and CBC, and their ability, after a 25-year gap, to ferret out and interview key witnesses, was impressive. I have no doubt that this podcast will become the definitive account of the Bre-X scandal. New revelations are unlikely, but who knows? There is still time for an aged de Guzman to reappear from the shadows and tell his side of the tale.

There are numerous unresolved questions, but three major ones stand out:

1. The involvement of Indonesian President Suharto’s family and close associates in the bidding war to gain control of the promised riches of the Busang mine and the actions they may have taken when the extent of the fraud first became apparent.

2. The true involvement of the pilot, the only person in the helicopter besides De Guzman at the time of the incident. He was/is an Indonesian Army Airforce officer and provided the only account of the incident, then was made unavailable for further comment by Government authorities. And what was in the large “box” that witnesses saw being loaded into his Squirrel helicopter at Samarinda airport in Kalimantan immediately prior to the fatal flight? And what credence should we give to the unconfirmed report that a corpse from the Samarinda morgue mysteriously went missing at around that time?

3.  The extent to which David Walsh and John Felderhof were involved in the fraud. Walsh, the President of Bre-X, was an entrepreneur with little previous mining background who would have relied on his Vice President of Exploration, Dutch Canadian geologist Felderhof, for all technical advice.  At the height of the Bre-X boom both Walsh and Felderhof made tens of millions of dollars selling Bre-X stock. After the scandal broke, Felderhof was charged with insider trading by the Toronto Stock Exchange. He pleaded that he was just another innocent, albeit naive, dupe of the fraudsters and, after more than ten years in litigation, was acquitted of the charge. Both Walsh and Felderhof are now dead.

As a geologist, I did much work in Indonesia throughout the 1990’s and have a personal view on many of these matters (see HERE). In my opinion, Felderhof (whom I met and worked with in the field) was a good geologist who, at a critical juncture, allowed desire for fame and riches to overwhelm his professional judgement.

Contract Filipino geologists were much favored for field work throughout SE Asia at that time. All those I worked with were honest, dependable and great companions in the field. They could blend with the local population, were acclimatized to tropical conditions, technically competent and hardworking. But from the point of view of Western mining companies they were, above all, cheap. They were in fact being exploited, and they knew it.

With hindsight, I think it foolish of Bre-X to have offered their site crew extra remuneration in the way of stock options. This was meant to buy loyalty, but once the low-paid field geologists realised that adding $10 worth of locally purchased alluvial gold to a single drill sample could add $100,000 to their potential net worth, the temptation to do just that must have been great. By succumbing, they unwittingly stepped onto an ever faster moving treadmill that could only result in dramatic consequences for all when it finally stopped. It was a classic Ponzi scheme.

The post The Busang $6 Billion Gold scam – A Review of the BBC-CBC Podcast appeared first on Roger Marjoribanks.

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

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 is the same technology as used to orient the screen on your smart phone or tablet). The data is stored in a memory against a time stamp. After the barrel with its contained core has been pulled to surface, the barrel orientation 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 gripped by the core lifter.

The 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. The orientation of the core barrel can be used as a proxy for the orientation of the core. 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 core trays, and delivered to the geologist. By matching broken surfaces, the geologist or geology technician then re-assembles the run on a separate channel. This enables the driller’s end-of-core mark to be transferred as a continuous Bottom Of Hole line along the length of the run.

However, the end piece of core may have broken free from the ground by the turning drill bit and rotated by some unknown amount prior to its being gripped by the core lifter. This can happen when the core contains fissile surfaces such as bedding or cleavage. Soft, incompetent surfaces such as mudstone or siltstone horizons, or gouge-filled faults, may fail through ductile shear. Surfaces at a high angle to the drill hole are more likely to fail than those at a low angle.

Where this has occurred, the driller’s orientation mark is meaningless and 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. Accuracies of less than 1 degree are claimed. But the orientation of the barrel is not necessarily the same as the orientation of the core inside the barrel.

Figure 1: The Reflex ACT electronic core barrel orientation tool.  The tool 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.

The second method, and in my opinion a much more reliable one, is to orient the core stub before it is drilled. The core stub is the fresh 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 similar to that used in core-barrel orientation tools.

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 generally gave 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.

Figure 2: Operation of the simple down-hole spear core-stub orientation tool. It is lowered on the wire line after a barrel of core has been extracted to make a mark on the lower edge of the core stub. (Reproduced from Marjoribanks 2016, fig B2, p185).  Down-hole Spears were usually manufactured by drilling companies in their own workshops.

More sophisticated core-stub tools operate on the same basic principle as the spear 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.

Figure 3: Operation principle of the Craelius simple template core-stub orientation tool. The procedure required a separate operation between each drilled run. This tool is no longer available, but the figure serves to illustrate the basic idea behind core-stub orientation. (Reproduced from Marjoribanks 2016, fig B3 p186).

Figure 4: The pointy end of the EzyMark core-stub orientation tool. This tool fits inside the bottom of the core barrel. The pins and wax pencil record the shape of the core stub. Lockable level bubbles inside the tool record the gravity vector at moment of contact. This all-mechanical tool is no longer available, but a close copy is available from Well Force International (see below). The red plastic block (called an OriBlock) containing the pin template and down-gravity mark is extractable from the tool and is intended to be left in the core tray as a permanent orientation record and depth marker – as seen in photo. The OriBlock system (although they don’t use that term) is also available on the Reflex VertiOri and Well Force Front end tools (see below).

The image was accessed in 2013 from the 2icAustralia company website. 

Figure 5: The Reflex Verti-Ori core-stub template tool. It fits within the bottom of the core barrel. Steel pins and wax pencil record the shape of the core stub.  The gravity vector is determined by an inbuilt accelerometer and recovered by means of an electronic user interface.  

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.

Available Core Orientation tools (as of October 2023)

The core-barrel orientation systems that I am aware of are:

BALLMARK was an all-mechanical system, first introduced in the late 1990s but no longer available.  The gravity vector was measured and recorded by the pressing of a steel ball into a soft aluminium disk at the moment of core barrel extraction. This tool (made in Orange, NSW) is the earliest core barrel orientation system that I am aware of. The company that made it may well have been the originator of the core barrel orientation as proxy for actual drill core idea.

REFLEX (their ACT™ systems LINK). Reflex is a subsidiary of IMDEX Corporation

DEVICO (their DeviCore BCT™ and DeviHead™ systems) LINK. Devico is a subsidiary of IMDEX Corporation.

BOART-LONGYEAR (their Tru-Core™ system) LINK.

AXIS MINING TECHNOLOGY (their Champ-Ori™ system) LINK. Axis is a subsidiary of the ORICA Group.

 There may be other available systems that I am unaware of. As far as I can tell from website description, in their basic method of operation, all the above tools are essentially clones of each other that offer slightly different electronic user interfaces.

The core-stub orientation systems that I am aware of are the Spear (figure 2) the Craelius (figure 3), the EzyMark^TM   (figure 4), the Reflex VertiOri (figure 5) and the “front-end orientation tool” made by WELL FORCE INTERNATIONAL Ltd.

The down-hole Spear system has fallen out of fashion for the reasons given above, although any drilling company could easily resurrect it.

The Craelius system is no longer available.

EzyMark  was an all-mechanical system, taken over in 2014 by the IMDEX subsidiary Reflex and then marketed the EzyMark as their Reflex Auditor^TM System. The Auditor tool has now (as far as I can tell) been discontinued and replaced by the Reflex Verti-Ori™ system.

The Verti-Ori system is a core-stub orientation tool with built in accelerometer and magnetometer that records the gravity vector and magnetic lines of force across the tool (LINK). By providing a magnetic vector, the tool can orient core from very steep-angled to vertical holes where gravity alone is unable to provide accurate orientation data (although EzyMark claimed accuracy for their tool in holes with inclinations of up to 88 degrees). I am informed that there are current availability problems with the Verti-Ori tool.

Well Force International’s  “Front End Mechanical Orientation Tool” (LINK) is a core stub orientation tool which appears (from their website description) to be a very close copy of the now unavailable EzyMark tool of 2icaustralia.

I am indebted to Sarah Sulway (Imdex) and Olivier Cȏté-Mantha (Agnico Eagle Gold Mines) for some of the details in this post. All expressed opinions are of course mine.

You can read my previous (2013) post on this subject HERE

Reference:

Marjoribanks R W 2010. Geological Methods in Mineral Exploration and Mining. Springer pp238. ISBN 978-3-540-74370-5.

The post Drill Core Orientation Tools – A Review appeared first on Roger Marjoribanks.

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.

Core Frame Instruction Booklet

The post Marjex Core Frame Instructions pdf appeared first on Roger Marjoribanks.

Speak to exploration geologists and you will find two opposing views about what a geologist should do when observing outcrop or drill core in the field.  Some seek merely to be unbiased objective recorders of what they see.  Others observe the rock in the wider context of their theories about region or prospect geology and ask questions of the exposure to help choose between them. I characterize these two approaches with the metaphors of  geologist-as-camera, and geologist-as-interrogator.

Geologist as camera – 1

I arrive at the exploration site to find a team of three young geologists engaged in making a geological map of their large property. Using GPS, they walk predetermined traverses spaced 400m apart. They each aim to average 8km in a day and, between them, by taking alternate lines, they cover a large swathe of country before their next field break. Observations on each geological feature along the line of march are logged into a field-hardened tablet computers by going through a series of pull-down menus and checking boxes on pre-determined questions. The geologists have only the vaguest ideas about the geology of the property they are mapping. Two of them have never thought much about this: the brightest of the three specifically rejects the notion of understanding her observations: she sees herself as an unprejudiced, objective observer – confirmation bias is not for her. Eventually all this data is computer-plotted to 2D images as a linear sequences of point observation.  Another, more senior geologist, might eventually produce a geological map by joining the dots. Probably there is a software program that can do much of this task for him (or her) and thus obviate the need for too much thought or hard work.

Geologist as Camera – 2

I travel to an exploration site on the other side of the country. Here, a much larger company is at the final stage of drill testing a substantial metal deposit. Four diamond drill rigs are going 24/7 and have reached hole number 300 and something. Stacked on pallets, kilometers of core are waiting to be logged. Four geologists are hard at work logging core laid out on long racks in a big shed. As geologists come and go on field break, or come and go through resignations and new hires, it seldom happens that any one hole is logged in its entirety by the same person. They log on to analytical spread sheets in laptop computers using pre-determined menu options – there are more than 50 columns on their spreadsheet. The same log form has been used since DDH 001. The geologists have never seen a drill section of the prospect (there are none). There are no detailed maps or level plans either. The geological model, such as it is, was produced a year before by an outside consultant who spent ten days on site. Back in the head office a specialist Ore Reserve Geologist ignores most of the vast data base of geological observations and calculates a resource based on assay numbers, virtual reality 3D string models and geostatistical techniques. For the geologists at point, slaving in the core yard, their job as 100-meter-a-day, core-logging automata is mind-numbingly boring. They count the days till their next field break, or until they have earned enough money to find some other more intellectually stimulating employment.

After a day data logging, Susan contemplates a career change. Picture by author.

These are anonymized projects, extreme examples perhaps, but based on my observations of many actual projects, with many companies in different countries over many years. Not all exploration projects are conducted like this, but I am sure that readers will recognize the method of doing geology that I describe. The geologists are not to blame. In many cases it is their first job, they are on short-term contracts, and they are doing what their employer asks and expects of them. They probably wonder what the point was of much of the time they spent learning at university and have come to believe that the essence of their job as exploration geologists is to convert light signals from their eyes into digital computer feed according to pre-set formulae. A kind of biological digital camera.  Without a pre-existing context in their brain, each observation they make has equal importance, each pixel equal weight.

There is a better way.

The geologist as interrogator

All observation is made in a particular context. The context which should guide the exploration geologist is a matrix of different competing theories about the true nature of the geology being observed. This context provides the questions that must be asked of each outcrop or each piece of drill core. It defines the strategy to be followed in the search for those critical observations that allow selection between multiple working hypotheses (1), (2). The idea of multiple working hypotheses was first propounded by 19th Century US Geological Survey geologist Thomas Crowther Chamberlin. It is a methodology now used in all fields of science research, but geologists can be proud that the first clear statement of the idea came from their profession.

Critical observations are those that fit into a pre-existing context or, just as importantly, those that do not. These are prioritized and not lost in a sea of trivial observation. The method does not guarantee the you will arrive at the correct solution. Your evidence may be less than ideal, the expression in the rocks of geological events may be atypical.  Nature can be chaotic and unpredictable: the true hypothesis you should be testing  may be a Black Swan – the one you have never thought of. In Geology – indeed in all science – neat, clean results where all the data slots in exactly are rare and should be always regarded with some skepticism. But in spite of all that, the method of multiple working hypotheses is the best known procedure for approaching the truth.

After a morning of fieldwork, Robert interrogates his lunchtime sandwich using the method of multiple working hypotheses. Picture by author.

Collecting data to test hypotheses in this way is biased observation. All geologists have cognitive biases which were imprinted when they were taught to be geologists at University, and reinforced by early-career mentoring and  reading geological literature relevant to the job.  But this bias is a good thing where it is up-front, acknowledged and constantly revised and re-focused in the face of evidence. Without bias, there is no way of separating signal from noise – not even by the most sophisticated of statistical procedures. This is a quite different kind of bias from the one that uncritically accepts evidence that confirms a single pre-existing theory, or a theory you have fallen in love with,  and ignores, or explains away, evidence which does not. That is Confirmation Bias and is rightly condemned.

All scientists have a point of view and all views must have come from somewhere. Anyone who claims to make unbiased observations merely lacks self-awareness. Their biases are unconscious or lie in the biases of whoever drew up the detailed procedure manual which they follow.

The metaphor I use for this approach is the geologist as an interrogator of each rock exposure or core piece, asking a series of relevant questions that come from his or her views as to what might be happening, with each question determined by the answer to the previous question.

RM ’14

In a previous post I explore how using the technique of multiple working hypotheses can be used when constructing a geological map (LINK)

The wider context

The true nature of scientific investigation is focused observation guided by emergent theory. This is a long established and respectable idea and contrasts with seeking to find your theory in your data after you have completed your observations.

Collecting data should not be a fishing expedition that hopes to fortuitously entangle a new understanding or ideas on its line.  Its purpose should be to provide evidence to choose between a range of pre-existing hypotheses.  It therefore follows that the hypotheses must come first.

Seeking your hypothesis in your results is such a widespread and acknowledged error in many scientific disciplines that it has been given its own acronym - HARKing[3] (Hypothesizing After Results are Known). In statistics-based research the same error is known as p-hacking [4].  Both techniques have been, and undoubtedly still are, widely used in so called science [6].

The parable of the Texas Sharpshooter provides a good example of the HARKing technique at work [5] (6):

“The Texan directs sixty shots at a barn door. He then selects the best grouping of the random impacts and draws a target around them. A carefully cropped photograph is then used to establish his sharpshooting credentials.”

The Texas Sharpshooter – Picture by author. 

This idea that theories should precede observation is the exact opposite of what is popularly believed. Sherlock Holmes, for example, is often quoted approvingly:

“It is a capital mistake to theorize before one has data”.

But Sherlock’s patronizing throw-away line to Watson could not be more wrong. Contrast it with these quotes from real scientists – as opposed to a fictional one.

In Biology:

“How odd it is that anyone should not see that all observations must be for or against some view if it is to be of any service.”  Charles Darwin.

In Fundamental Physics:

“We never draw inferences from observations alone, but observations can become significant when they reveal deficiencies in some of the contending explanations.” David Deutsch, The Fabric of Reality, 1998.

In the Philosophy of Science:

“The facts that we measure or perceive never just speak for themselves but must be interpreted through the coloured lens of ideas…. We can no more separate our theories and concepts from our data than we can find a true Archimedean viewpoint – a God’s eye view – of ourselves and the world”. Michael Schermer, Scientific American, 2007

In Medical Research:

“…you cannot find your hypothesis in your results. Before you go to your data…you have to have a specific hypothesis to check. If your hypothesis comes from analysing your data, then there is no sense in analysing the same data again to confirm it.”  Ben Goldacre, Bad Medicine, 2008.

In Psychology

“I am more and more convinced that the only way to obtain clear answers from Nature is to ask her clear questions.” Eric-Jan Wagenmakers, Professor of Neuro-Cognitive Modelling, University of Amsterdam, 2014. 

 Final words

But in spite of this, it is my observation that the geologist-as-camera view is becoming increasingly common in our profession.  This slows down the acquisition of geological knowledge and can be disastrous for understanding.

Data is not information

Information is not knowledge

Knowledge is not understanding

Understanding is not wisdom (8)

Without geological understanding, drill holes are put in the wrong place, or ten are drilled where one would have sufficed. Without good geological models that reflect reality, the certainty required to convert an Ore Resource into a bankable Proven Reserve cannot be easily or cheaply achieved.

This is a modified and updated version of an essay first posted in 2014

The post The Camera and the Interrogator appeared first on Roger Marjoribanks.

Diamond drilling is always undertaken for the needs of the moment. But fresh-cut rock, in most cases, will last hundreds, if not thousands, of years. Drill core is a resource which can keep on giving as we go back to it again and again with fresh ideas, questions, techniques or agendas. This could be by you or your company, or by other geologists and other companies as the years go by. However, core can only provide this continuing value if indexing and storage systems match the longevity of the core itself.

Roy Woodall, former Exploration Director for Western Mining Corporation said in 1995: “We have re-logged the core from Kambalda (West Australian Ni/Au Camp) three times, and each time we do it, we find a new ore body”. 

Drill core is an expensive product providing direct physical sample of the subsurface and will always be an invaluable and irreplaceable resource. As a condition of granting a Mining Lease or Licence to Explore the State should should specify strict enforceable requirements for permanent indexing and storage of core. That core will have cost between $100 – $500 per metre to acquire, so it is not too much to ask that a company spend around $1-$5 per metre extra to properly index and store their product. That company or that geologist might one day themselves be the beneficiaries of similar forethought.

You could be one of them.

I certainly was.

In the mid-1970s, as part of a structural re-assessment of the giant Broken Hill  Pb/Zn/Ag mining camp in New South Wales, I had to re-log a number of holes that had been drilled in the 1920s and 1930s. This core was the product of one of the first major diamond drilling campaigns be undertaken in Australia. It was stored in the long-abandoned Consolidated Zinc core yard – an acre of ground on the line-of-lode, sandwiched between old head frames, abandoned machinery and infrastructure, mullock and tailings heaps.  The core had been stored in wooden trays laid out in long lines on the ground. Tens of kilometres of core carpeting the entire yard. At the head of each core run a steel stake had been driven into the ground with the  hole ID traced on it with solder. Even more importantly, the drillers’ original wooden depth markers had been replaced by small rectangular zinc tags on each of which the hole number and down-hole depth had been stamped by a steel punch.

…in the sea of sand the steel stakes still stood proud…

During a heavy downpour in the 1940s, a nearby fines dump had collapsed and tailings had flowed outwards to cover the yard in fine white sand to depths of up to 50 cm. But in the sea of sand the steel stakes still stood proud, and although not a trace of the original wooden trays remained, with some excavation (you could call it industrial archaeology: the Mining company provided the labour) the core was soon exposed in its original neat parallel rows, as fresh and easy-to-be examined as it had been fifty years before. I mentally thanked the foresight of the long-forgotten project geologist, and remembered the first occasion, seven years before, when I had been given the task of re-logging legacy core.

Rum Jungle, 1967.  A mine operated by a subsidiary of Rio Tinto in Australia’s Northern Territory. They were engaged in mining and processing uranium and copper ores from a series of open cuts in the surrounding district . Re-logging old drill core was deemed a suitable task for a rookie geologist. The core had been drilled 5 to 6 years previously and stored in wooden trays stacked over 10 high at the back of an old shed behind the mill. Around 3 kilometres of it. When I pulled back the doors of the shed to let the light in, I found a slope-sided mountain of broken and disorganized core pieces out of which a few bits of rotten, termite-gnawed wood still projected.  We organised a front-end loader and quickly transferred the lot to the nearest mullock heap.

…a slope-sided mountain of broken and disoriented core pieces out of which a few bits of rotten, termite-gnawed wood projected…

A few years later, during the nickel exploration boom in Western Australia (1968-71), I frequently scouted old exploration properties abandoned by their former owners. Diamond drill core was left abandoned in the bush, sometimes in their original stacks and sometimes tipped into the nearest gully. The galvanized metal core trays then used were immune to termite attack, but even when stacked, the core was almost as useless as the Northern Territory core described above. The trays were typically in random order, and neither geologist nor driller seemed to have realised (or more likely did not care) that felt or fibre tipped ink marking pens, or painted lines, are not permanently legible, despite what the labeling on the product might say. Exposed to baking heat, rain, UV light and the passage of time, numbers fade and become illegible in a period that can be as short as a few months.

…exposed to baking heat, rain, UV light and the passage of time, numbers quickly fade and become illegible…

Nobody seemed to care that this expensively-acquired product would soon become virtually useless.  Or perhaps they did, but reasoned that once they had extracted all the value they could from their one-off pass, no one else – not even themselves -  should be given another opportunity. A true dog-in-the-manger attitude.

How not to store drill core. In fairness to Rio Tinto, their Hidden Valley project was heroic exploration in an extreme environment near the crest of the Owen Stanley Range. The only access was by helicopter in the brief intervals when the clouds parted and the rain stopped. In the 1990s, after a series of extraordinary force majeur events (Bougainville, Wafi, Mt Kare), Rio took the decision to walk away from all their Papua Nugini operations.

Best practice core storage and labeling. This is core was acquired by the Homestake company from their Trilogy Pb/Zn/Au project in Western Australia. Trays are permanently labelled with riveted metal markers and placed in racks that allow easy individual tray extraction.

I have emphasized how durable diamond drill core is. But sometimes that is not the case..

In the West Java province of Banten, the epithermal gold prospect of Cibaliung was explored with a major diamond drilling program by a junior Australian company during the late 1990s. Many of the rocks on the footwall of the ore body consisted of crumbly acidic clays which swelled to two or three times their volume when exposed to air and moisture. Needless to say, core recovery through these units was poor.  I undertook a major re-logging exercise on drill core that was up to 3 years old in order to resolve structure. The core had been stored, according to then best practice, in steel racks under cover, but in a tropical environment it is very difficult to exclude moisture. In the critical section below the ore body, core had expanded to a spongy mass – a gross acidic efflorescence that dissolved the base of the galvanized iron trays cementing them together into an inchoate mass. Pity the poor geologist who had to try to make sense of this stuff (and pity the poor miners who were proposing to develop an underground access decline through it).

…a gross efflorescence had cemented many of the trays together into an inchoate mass…

But you might say, what could anyone do to recover and store drill core from refractory rocks such as these, especially in the heat and humidity of a tropical environment.

Well, there is a way…

2007, Central Thailand. I consulted for the Australian mining company Oxiana (now OZ Minerals) at their Wang Pong epithermal gold exploration project. Oxiana was (and, for all I know, may still be) one of the most technically competent explorers that I have come across .

…one of the most technically competent explorers that I have come across…

Parts of the prospect consisted of swelling acidic clays similar to the problematic rocks that the Indonesian explorers had to deal with. To drill this successfully, Oxiana employed the following strategies:

• Large diameter core (HQ and PQ)
• Triple tube drilling
• Split inner tube for core extraction.
• Wrapping core in clear plastic (industrial strength cling film) to exclude air (see photo)
• Use of plastic core trays (now thankfully becoming an industry standard)
• Long term storage of severely affected sections in sealed shipping containers with nitrogen atmosphere

Note: core wrapped in clear plastic to exclude air and moisture. Twelve metre long racks allow re-assembly of broken core pieces from up to 2 core runs – this facilitates accurate depth measurement and placement of  long-core orientation lines

The moral to be taken from the examples above is should be sufficiently obvious.

The post Look after your Drill Core: it’s a resource that keeps on giving 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.

Within or adjacent to a fault zone, various minor structures can be present that enable the sense of movement across the fault to be determined.  These structures are often called kinematic indicators.

In Part 1 of this series of posts, I classified kinematic indicators as T (tension), S (compressive), R (simple shear) and C (laminar flow). The part described how and why these structures formed and explained their use as kinematic indicators.  Part 2 consisted largely of photographs of actual structures in outcrop and drill core. In this final post, simple tabulated rules are provided that enable the various classes of structure to be identified in outcrop.

Most faults, especially large ones, have poor outcrop so when you are lucky enough to find an exposure of a fault it is worthwhile spending time examining it in detail to find out what it can tell you.  The type of exposure that is most likely to provide useful kinematic indicator structures is a section that cuts at right angles (orthogonally) across the full width of the zone. A stream section offers the best chance of finding such an exposure.   Although relatively rare in nature, orthogonal or near orthogonal sections across fault zones are frequently exposed in man-made outcrop such as road cuttings, trenches, the walls of open cuts, underground openings or in drill core. Next to an orthogonal section, the next most useful fault exposure for the structural geologist to work with is an exposure of the face of a fault – the plane of movement.  This is probably the most common type of natural fault exposure.

Don’t expect every exposure of a fault to contain sense of movement indicators that can be reliably identified and interpreted.  In fact, most will not. The minor structures within fault zones can be chaotic, apparently contradictory and difficult to assess in three dimensions.  If you are not certain how to identify the minor fault structures that you see it is better to walk away and try for a better exposure elsewhere. 

Being able to distinguish between different sense of movement structures is vital. Confusing S surfaces with T surfaces, for example, leads to radically-different interpretations of the direction of fault movement. The Table below sets out the criteria that can be used to identify structures.   Note that no single criteria is definitive and many of the fields overlap. The more criteria that a particular structure meets, the more certain is its identification.  

It is worth bearing in mind that identifying one single structure, or even a number of structures from a single outcrop, does not give 100% certainty as to the overall fault movement.  You have good evidence, but the important point is that a credible working hypothesis about the nature of the fault can now be constructed.  From this hypothesis predictions can be made which can tested against the evidence from future exposures of the structure.  This is the Scientific Method and from this process knowledge is gained.

The post Sense of movement structures in fault zones Part 3: Identification Criteria appeared first on Roger Marjoribanks.