issue november 2017 wireline workshop - geophysical logging · issue 26 – november 2017 wireline...
TRANSCRIPT
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Issue 26 – November 2017
WWIIRREELLIINNEE WWOORRKKSSHHOOPP A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION
A Review of the Basics (part 3)
Rock Mass Rating ‐ a guest's perspective
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Borehole Images and Structure Logs
We have considered the density log and made pains to point out that, of all the physical property logs, that important measurement is the toughest to get right. It is perturbed by varying borehole conditions and formation chemistry. It also suffers from resolution issues; each 1cm datum describes the average density of a sample of data, depending on source‐detector separation. Boundaries between very different rock densities are not sharp.
Once the various perturbations and resolution issues are understood, the density log may be used very effectively.
Borehole image logs are fundamentally different. Notwithstanding the effects of magnetic formations on its orientation, the image describes the formation reliably, repeatably and at very high resolution.
In the vast majority of cases, the geologist can believe what he is given.
There will always be small errors in image orientation or the borehole navigation data (captured during the same log run as the image) but, in the context of the knowledge requirement, these can be ignored.
Vertical resolution is very high. One can measure the thickness of a coal seam or open fracture with great accuracy...within a millimetres or two in some cases. The optical televiewer image on the right is a good example of measurement resolution. Nothing is missing. Colour, texture and continuity are described perfectly. The angle and orientation of structures may be measured confidently. The log is entirely unambiguous. QA is easy to do.
The explorer is comfortable with image data. He knows enough about his ground to recognise if anything strange or unbelievable is described. His instincts can be trusted, whereas he might sometimes feel technically enfeebled when presented with a page full of wiggly lines. The NMR log, discussed last month, is an example...great technology, important measurement, but, in terms of data fidelity, the geologist depends entirely on his logging contractor who depends very much on his equipment supplier.
There are three types of orientated borehole wall image available to the geologist. These may be grouped into injected electrical resistance, reflected sonic, and various photographic images.
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The electrical option
The borehole micro‐scanner, is a multi‐button dipmeter that images the borehole wall in terms of its resistance to the flow of electric current between an electrode array and the body of the sonde. Guard or bucking currents force the injected current several centimetres into the formation (through mudcake and skin effects).
The advantage here is that, regardless of borehole diameter, the measuring electrodes are close to the borehole wall and so the log is unaffected by signal travel length or borehole fluid density. For that reason, it is preferred by oilfield explorers.
Its cost and size (it is difficult to build a little one), as well as the small diameter boreholes usually encountered by the mineral logger, have generally disqualified the tool from use in mining exploration projects.
There are crossover projects like carbon sequestration, shale gas stratigraphy and Coalbed Methane logging where one can imagine the micro‐scanner being deployed by a modified mineral logging truck. It rather depends on the drilling system employed, the borehole fluid and the diameter of the bore.
In large diameter mud‐filled boreholes the Acoustic Televiewer image will be very poor and the optical version sees nothing. That is when the micro‐scanner is the only option.
One drawback is the lack of total borehole coverage. As the caliper arms open to larger diameters, the pads separate, leaving gaps between them. That is not really a big problem as we can see from the image on the right.
Formation Micro‐Imaging log (from Weatherford)
The log analyst gets used to looking through the bars of his cage in order to see the world beyond. In fact, very little information is lost but, even so, normalisation programs are being developed to automatically interpolate the data and fill the gaps.
It must be said that this technology is not routinely provided by the mineral logger but is included here for completeness.
A moving picture
Perhaps the first mineral imaging tool was the borehole camera. This device offers a continuous real time video recording of its progress down a borehole, mine shaft or other void. It relies on an air‐filled or clean water‐filled environment. For this reason its use is restricted somewhat. It is often employed in groundwater studies and for describing borehole blockages or lost equipment. The borehole camera can include a side view lense to supplement the axial (downward) view and it may, with the addition of a navigation sub or compass, be orientated but it is predominantly a borehole inspection device rather than a geological tool.
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For most mineral exploration and geotechnical studies the borehole televiewer, optical or acoustical, is the main imaging tool for the geologist.
The choice for the geologist is which to use, optical or acoustical televiewer, and whether to run both. There are borehole conditions that prohibit use of one or the other but, even when conditions are favourable to them both, they do not always provide the same knowledge.
The Optical Measurement
The optical televiewer provides a 360 degree orientated photographic image of the borehole wall. It may not be run effectively in opaque borehole fluid. It requires a caliper log to describe the borehole wall and its diameter.
A typical OTV presentation with picked structures, navigation data and a caliper log on the far right
Introducing clean water into a borehole should not be seen as a problem. It is usually straightforward and inexpensive in the context of the drilling programme. In old boreholes, the fines have usually settled out.
Geologists are often disappointed with the acoustic televiewer log produced in a large diameter rough‐walled borehole. The sonic signal is dispersed by the long journey and by the rough surface from which it is reflected. The optical televiewer will produce a better log...if the borehole fluid is clean.
Of course, if the borehole is dry, there is no choice. The acoustic televiewer will not work in a dry borehole.
Some optical images captured in dry boreholes are fantastic...see page 1 and 4.
The big advantage, as with all wireline logs, is that the representation is complete, no gaps, no uncertainty.
In hard rocks, the optical image will describe bedding but for the acoustic device bedding and layering is often invisible (see page 6).
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The optical image on the right was originally rather dark and much of the detail was hidden. It is possible to brighten the original RGB image or, as in this case, convert it into a borehole image with presentation and palette options that allow some improvement. Natural colour is lost but can be approximated.
This is typical optical image quality in a dry borehole after some enhancement for picking
purposes. The logger chose less than maximum resolution, because logging speed/time was an
issue, but the result is fit for purpose.
This version was eventually plotted alongside the original true colour image. Some minor
vertically aligned artefacts are caused by dust on the lense cover.
Classification of picked artefacts is an important acquired skill. In this case, natural flat lying open fractures and bedding dominate. The sub vertical
events appear to be drilling‐induced.
The best optical images, captured in clean water or air‐filled boreholes provide the geologist with a great deal of assurance that nothing is missed. It is an excellent descriptor of texture and bedding and offers very accurate measurement of fracture aperture and bed thickness. Image resolution is higher than that of the acoustical measurement and, frankly, it is a waste of time plotting the OTV log on paper except, perhaps, as part of the final structure log with polar plots etc...the user should interrogate the data on a computer because viewing scales of even 1:1 are now valid.
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The Acoustical Measurement
Let's first try to deal with the semantics. Most loggers refer to the "Acoustic" Televiewer and (we) will probably continue to do so. Strictly speaking, that is incorrect...a bit like referring to the Optic Televiewer. The term "Acoustical" Televiewer is grammatically correct but it sounds rather quaint and dated. Hang on though, one can use the term acoustic transducer because sound is being "transduced". Is sound being televiewed? No, the borehole is being televiewed by means of sound. It's a very fine line because, according to the text book, one can say acoustic signal but not acoustic measurement....Really? It's so fine that the Loggers can be forgiven and, anyway, time and repetition modify language. Whose ever heard of an acoustical guitar?
Sticklers respond please.
The ... ATV generates images of the transit time and amplitude of a high frequency sonic pulse that is reflected directly off the borehole wall. The transit time is governed by fluid characteristics and borehole radius (the tool is centralised).
Acoustic time and amplitude images orientated to the high side of a borehole
drilled at 30 degrees from vertical through a magnetic formation.
The amplitude is governed also, to some degree, by the borehole fluid and the diameter but more importantly by the hardness of the reflecting surface...the borehole wall. The relative time and amplitude values across the two images describe borehole radius (and so deduced diameter) and formation hardness.
The log presentation above shows the time image TIMH (grey palette) describing parallel open or washed out joints that can be seen as well on the amplitude image AMPH. Note that, although both OTV and ATV tools navigate and orientate using magnetometers, they can log magnetic formations by orientating their images with respect to the high‐side of the borehole (using accelerometers from the tilt cells). Structures are picked with respect to the same reference and the structure log is later rotated to be with respect to horizontal and true north using navigation data from a gyro sonde.
If the Geologist wants to log magnetic formations, he should drill an angled borehole...anything over 5 degrees from vertical will be fine.
There are a few limitations to the Acoustic Televiewer.
It requires a fluid‐filled borehole (it cannot log above the fluid level).
Its image becomes fuzzy and difficult to interpret in drilling mud...water is best.
Large diameter and/or rugged boreholes also yield poorly defined image data.
Tool centralisation is important for best results.
The amplitude image does not describe bedding well in very hard rock (see the log on page 6)
Given a good quality data set, the log analyst can describe formation texture, bedding dip and direction, fracture orientation and aperture, stress‐related breakout, fracture frequency, depth and thickness of target layers such as coal seams and he gets an acoustic caliper and borehole navigation log as a bonus.
It is important to recognise drilling‐induced events on the images. Drilling‐induced fractures normally propagate parallel to the path of the bore and are usually orientated roughly in line with the prevailing stress tensor. Including these DIFs in a fracture count would be misleading.
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Sometimes the geologist is best advised to run both types of televiewer.
The log on the right, captured recently in Mozambique as part of a dam wall foundation study, illustrates the differences in information provided by the two types of borehole image.
The geologist needs to be aware that the two sonde types do not always measure the same thing, or he will fail to design his logging programme correctly.
From the left; depth, ATV acoustic travel time image, ATV reflected
acoustic amplitude image, OTV optical image and three‐arm caliper.
The acoustical images are far better at describing fractures but, in these hard rocks, they miss most of the bedding.
The optical image offers fine geological detail despite the borehole fluid, which
was not perfectly clean.
In this case, both types of image are orientated with the left edge being aligned to magnetic north. The log analyst laid one structure log over both images in turn in order to measure and classify the various artefacts. Running both logs provided a high degree of quality assurance in terms of structure orientation.
Having said at the outset that image logs are fundamentally different to density logs or, for instance, spectral gamma logs because the image is an accurate representation of the borehole, we have to confess that image analysis, picking and classification, introduces a very large subjective element to the process.
The acoustic image makes life easy in terms of identifying fractures, particularly in hard rocks, but classifying them and counting them all for fracture frequency and RQD calculation is not an exact science.
The logging contractor might offer the finest equipment but lack an experienced analyst. Drilling‐induced fractures might be classified as natural fractures.
As with density calibration and QA, it is incumbent on the geologist to invest some time in understanding the analysis of image data.
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A major fault system with bedding drag and block rotation
In summary, the major imaging tools in mining, mining exploration and civil engineering are the optical and acoustic televiewers. Each has advantages and sometimes it is prudent to run both. The latest designs offer very high resolution of the borehole wall. Results are not usually ambiguous but analysis might introduce a subjective element to the data. Overall, because of water clarity issues, most metres are logged with the acoustic tool. The acoustic image is usually easier to interpret, particularly in geotechnical applications.
22.. LLeeggaall MMaatttteerrss
The logging contractor will normally protect himself contractually when engaged by a client and the length and content of logging contracts varies quite a bit. The biggest threat has always been loss of equipment downhole but there is also the data fidelity issue to consider. Here is a typical indemnity statement found on a log header:
In making interpretations of wireline logs, Wireline Workshop and its employees will give the customer the benefit of their best judgement always, but since all interpretations are opinions based on inferences from electric or other measurements, Wireline Workshop cannot and does not guarantee the accuracy or correctness of any interpretation made. Wireline Workshop shall not be liable or responsible for any losses, costs, damages, proceedings or expenses incurred by the customer resulting from any interpretation made by Wireline Workshop and its employees.
The logger is letting his client know, clearly and not hidden in the small print, that if the client sinks a mine‐shaft based on the dipmeter log provided and the log is 180 degrees out, the logger will not pay for a new shaft or, indeed, for anything but he should certainly offer to relog the borehole.
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It is a very good idea to place such an indemnity on a log header. It is not an admission of a probable failure on his part...it just acknowledges the difference in scale of the mining operation to the logging job and the need to avoid making decisions based on one source of knowledge. That is one of the benefits of wireline logging...extra knowledge for the decision maker.
Perhaps the bigger issue is tool entrapment within the borehole and the need to recover a radioactive source. Here is an example from a client advisory.
Awareness of the environment in which we live is very much a feature of our lives these days. Many industrial processes have been highlighted as potentially dangerous to some part of the biosphere in which we exist, and the use of radioactive material poses many questions in this regard.
This information is intended to let you, our client, know about the radioactive sources that we use, and to give you a clear picture of the potential dangers inherent in their use. It is also to clarify the responsibility that we ask you to assume when you ask us to log your borehole.
When invited by you, our client, to run logging tools in your borehole, Wireline Workshop does so on the explicit understanding that the client has responsibility for our equipment once it enters the borehole. Borehole conditions are better known to you than to Wireline Workshop and we assume you would not allow us to lower equipment into your borehole if conditions were unsafe for the nature of the operation to be carried out. After all, you have spent a lot of money in drilling this borehole and you are aware of the risks involved when anything is introduced into it.
In some circumstances, Wireline Workshop will offer its client special insurance for the risk of equipment loss or damage but not for the recovery operation or any environmental impact that might result from the recovery process.
In the rare event of a logging tool becoming lodged in a borehole, please remember that you have the responsibility to recover it and that Wireline Workshop can only act in an advisory capacity during recovery operations. If the logging tool is carrying a radioactive source, then additional precautions must be taken not to damage the source holder and to avoid dispersal of the radioactive material into the borehole environment and subsequently on to the surface through the drill fluid circulation process.
This is a very useful explanation of the division of responsibilities and should leave the logger's client in no doubt as to where he stands if his borehole collapses onto his logging contractor's sonde. The Logger's terms and conditions will usually include something more formal like:
If the Customer requests the Contractor to perform logging operations using the Contractor's own equipment, the Customer recognises that he has superior knowledge of the borehole, formations and conditions existing in the borehole, and also recognises that the Contractor’s equipment can be seriously damaged by severe conditions which are not normally encountered in boreholes. The Customer shall, therefore, notify the Contractor in advance of severe or hazardous conditions existing in the borehole, of which he is aware, in particular, high temperature and pressure, gas or chemicals, deviated holes and obstacles in the bore.
In the event that any of the Contractor’s instruments or equipment become lodged in the borehole, the Customer shall make every attempt to recover the instruments or equipment without cost or risk to the Contractor. During such recovery or fishing operations the Customer assumes the entire responsibility and risk for such operations but the Contractor will, if so desired, without any responsibility on the Contractor’s part, act in an advisory capacity for the recovery of the equipment and instruments.
The Customer recognises that the radioactive sources used in the Contractor’s instruments are potentially dangerous and should such a radioactive source be lost in the borehole, special precautions must be taken
to avoid breaking or damaging the source container. If a radioactive source is not recovered, or if the container is broken, the container or radioactive material must be isolated by cementing it in place or by other appropriate means by the Customer in accordance with local statutory regulations.
If any of the contractor’s equipment is lost, destroyed, or damaged in the well, at the well site, or whilst being transported by or on behalf of the customer, by transportation arranged by the customer, or whilst in the customer’s custody, then the customer shall reimburse the contractor for the repair of such equipment, if repairable, or the replacement cost of such equipment, if destroyed or not recovered. Damaged or lost equipment subsequently recovered shall be returned to the contractor. All rights to such equipment shall at all times remain with the contractor.
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Sometimes, a geologist will solicit borehole logging services without availing himself of the logger's terms. He might not have been given a simple advisory that clarifies his responsibility. He will be aware of the usual standard terms and conditions, peppered with words like governing law and force majeure, but he should recognise that borehole logging introduces unusual risks and that these should be understood. The wording examples above should assist him as they are fairly standard in mineral logging.
All this should not give the geologist any sense of trepidation. Stuck sondes are a rare event and ninety percent of stuck sondes are fished out of a borehole quickly and without serious damage. Radioactive equipment loss and the need to cement it in place is extremely rare.
33.. GGuueesstt AArrttiiccllee By Neil Andersen and Julian Luyt
The Use of Wireline Geophysics in Rock Mass Classification
1 Introduction
During the design phases for the extension of an existing open cast or underground mine there is often very little detailed information available on the rock mass characteristics, in situ stress, rock strengths and hydrological characteristics for the evaluation of the project. Geotechnical core boreholes would now be drilled and geotechnically logged to determine the rock mass parameters required. Multi‐parameter Rock Mass Classification schemes such as those developed by Bieniawski (1989), Laubscher, (1977), Barton et al., ( 2002), Andersen (2015) and others can then be applied.
However, if there are existing mineral exploration boreholes, which are still accessible, these can be used to provide geotechnical information, by running a suite of wireline tools and using the geophysical parameters as a proxy for geotechnical attributes. This provides an alternative means of determining the down‐hole geology and structural attributes without the need for coring and core logging. These structural and petro‐physical properties are exploited by the authors to devise a pseudo Barton Q‐Factor rock rating based on an understanding of the geophysical properties alone.
Peter Hatherly et al., (2005) proposed a scheme for a rock mass rating of clastic sediments based on wireline geophysical measurements, which allow an approximation of rock composition. They developed a Geophysical Strata Rating (GSR), which is based on (i) the porosity determined from density logs, (ii) the clay content derived from the natural gamma (also neutron and resistivity) and (iii) rock moduli determined by sonic logs. The scheme provides a value of rock quality between about 15 and 100, whereby rock quality improves with increasing value of GSR.
In this paper, the authors use the Acoustic Televiewer (ATV), Full Waveform Sonic (FWS), P‐Wave Sonic (VL2F) and Focused Electric Resistivity (Res), to simulate the three quotients used by Barton et al. (1974) to calculate the Q‐Factor. In addition the Sidewall Stability Index (SSI, Andersen, 2015) is calculated. The wireline surveys used here were conducted in a vertical borehole drilled in the Main Zone, (anorthosite and norite) of the Bushveld Igneous Complex.
2 Calculating the Q Factor
Barton et al., (1974), chose 6 parameters to describe rock mass quality and combined them in the following way:
Q = (RQD/Jn) . (Jr/Ja) . (Jw/SRF) where:
RQD Rock quality designation (Deere, 1963)
Jn Joint set number
Jr Joint roughness number
Ja Joint alteration number
Jw Joint water reduction factor
SFR Stress reduction factor
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The Barton parameters would normally be derived from geotechnical logging of the borehole core, but in this note, the authors have used geophysical wireline values to simulate the three quotients. The range of possible Q values, extends from approximately 0.001 to 1000 and encompasses a spectrum of rock mass qualities from exceptionally poor to exceptionally good.
2.1 Definition of Geotechnical Zones
The first process is to pick the ATV image classifying all the structures present. Using only the medium to well developed joints/fractures, a fracture frequency per metre is calculated, and geotechnical zones are then defined. The geophysical data is then average and interpreted within each zone to define the Barton (2002) parameters described below.
2.2 The Block Factor
The first quotient, RQD/Jn is also described as the block factor as it indicates the blockiness of the formations. The RQD is derived directly from the fracture frequency per meter (λ) as picked from the ATV imagery. In this calculation, only the fractures, with a fully developed sine wave as seen on the ATV image are used. The following equation is used to calculate the RQD.
RQD = 100 (0.1 λ+1)e‐0.1 λ where λ is the fracture frequency, Vali and Arpa, (2013).
The number of joint sets Jn is derived directly from a clustering of poles on a stereographic plot of the fractures and joints developed over the geotechnical zone being examined, not by a single pole.
2.3 Relative Frictional Strength
The second quotient Jr/Ja is the relative frictional strength of the least favourable joint set or discontinuity within a geotechnical zone. Jr is the rating for the roughness and Ja the rating for the degree of alteration or clay filling. A proxy for Jr is derived from the P‐Wave Velocity of the geotechnical zone and a proxy for Ja from the resistivity for the same zone. In this case the 20cm P‐Wave velocity (limited to 7 000m/s) has been used (VL2F).
The mean value, plus two standard deviations is assigned to a Jr value of 4, indicating discontinuous or irregular joints, which would be tight and have a high velocity.
Joints with values of the mean –1, ‐2, ‐3 standard deviations are assigned to progressively smoother and more planar joints which would impede the P‐wave to a greater extent.
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A proxy for Ja is derived from the focused electric resistivity values. The more altered and clay and water filled a joint set or fracture zone are, the more conductive it will become.
By drawing a histogram of the resistivity values of the entire borehole, the most resistive zone would indicate unaltered joint walls or no structures, with a Ja value of 1. Conversely, the most conductive zones would have joints/fractures with the most clay with a Ja value of 4. The rock type also needs to be taken into consideration when doing this evaluation.
2.4 The Active Stress
The third quotient Jw/SRF is defined as the active stress. Jw is the rating for water inflow or pressure that could cause outwash of alteration products in a jointed or fractured zone, and SRF is the rating for the degree of competency of the sheared or jointed zones. In a borehole environment, Jw is difficult to define unless pump‐
out tests have been done, or else there is a driller’s record of water losses incurred while drilling. A physical geotechnical log of the core could indicate zones of oxidation, related to fracture zones, which would indicate water movement.
In the absence of this data, fracture width, fluid conductivity and differential temperature can be used as a proxy to water movement.
In this case, fracture width measured using the televiewer imagery, was used to assign a Jw of 1, to zones such as that from 100 – 136m below, or else medium flow, a Jw of 0.66, to the open fractures and joints such as that from 136 – 144m.
These are Barton’s (2002) parameters and the geotechnical zones evaluated are shown in the diagram on the left.
The SRF is Barton’s (op. cit.) ranking for the degree of competency of the zone and ism ranked from 10, for multiple weak zones down to 2.5 for a single shear zone in competent rock. In this example the SRF was ranked on what could be seen on both the ATV and FWS images. The igneous rocks logged are competent so the zones from 100 – 136m were assigned a value of 2.5. The shear zone from 136 – 142m was judged to be a multiple shear zone in competent rock was assigned a value of 7.5.
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3 Sidewall Stability Index (SSI)
This is a parameter that was developed by Andersen Geological Consulting in order to quantify the stability of the sidewall of a raise bore. It is essentially an early warning system to alert the operators as to where sidewall conditions have deteriorated and sidewall collapse may occur. The index is not an absolute value, but is a probabilistic determination between good and very poor.
This index relates to the stability of the sidewall so dips were ranked from 60º to 90º, with a dip number of 2 being allocated to the category 60º to 65º, and that of 12 for the category 85º to 90º. Any dip less that 60º was given the value of 1. It was considered that dips of less than 60º were less likely to cause sidewall problems during raise boring than steeper dips. The SSI was initially designed for assessing the stability of shafts, but can be equally applicable to the hanging wall of stopes. The SSI was calculated and is plotted on the rock mass classification below.
4 Rock Mass Classification using Wireline Geophysics
The chart below shows the upper 250m of a borehole drilled in the Bushveld Igneous Complex.
The Fracture Frequency, shown as a bar graph in the fourth column was used to determine the geotechnical zones used for the calculation of the “Geophysical” Q‐Factor, and the Sidewall Stability Index (SSI).
The Q‐Factor Rock Mass Quality ratings are based on those developed by Barton et al. (1974, 2002), purple being very good and blue being fair.
The SSI shows two zones where sidewall conditions will be poor (red) should tunnels be excavated in that vicinity, due to the presence of steeply dipping fractures which have possible clay filling.
Column 3 is a bar graph of the focused electrical resistivity values. There is a clear relationship between high resistivity values (shown in red) and low fracture frequency.
Column 4 shows the relationship between high fracture frequency and the FWS response.
Neil Andersen MSc, Pr. Sci. Nat., FGSSA (Life Fellow)
Julian Luyt BSc (Hons), MGSSA, Pr. Sci. Nat.
13 References: Andersen, N.J.B., (2015). Pre‐Sink Shaft Safety Analysis using Wireline Geophysics. SAIMM Vol. 15, May, 2015. Barton, N.,
Lien, R. and Lunde, J. (1974). Engineering Classification of Rock Masses for the Design of Tunnel Support. Rock Mechanics 6, 189‐236. Springer‐Verlag. Barton, N., (2002). Some new Q‐value correlations to assist in site characterization and tunnel design. International Journal of Rock Mechanics & Mining Sciences 39 (2002) 185‐216. Bieniawski, Z.T., (1989). Engineering rock mass classification. New York: Wiley. Hatherly, P., Medhurst, T.P. and McGregor, S.A., (2005). A Rock Mass Rating Scheme for Clastic Sediments based on Geophysical Logs. Proceedings of the International Workshop on Rock Mass Classification. CDC Stacks. Laubscher, D.H., (1977). Geomechanics classification of jointed rock masses – mining applications. Trans. Instn. Min. Merall. 93, A70 – A82. McCracken, A. and Stacey, T.R., (1989). Geotechnical rock assessment for large‐diameter raise‐bored shafts. Trans. Instn. M. Metall. (Sect. A: Min. industry), 98, Sept‐Dec, 1989. Vali, B. and Arpa, G., (2013). Finding the Relationship between RQD and Fracture Frequency in the different OkTedi lithologies. Procedia Earth and Planetary Science 6 (2013) 403 ‐ 410
44.. FFoooottnnoottee That Caliper job in Brazzaville
On page 6 of the July 2017 issue, the author mentioned a caliper job which was due to commence quite soon. It involved extra long arms designed to describe very large bored pile diameters before the reinforcing cage was to be lowered and the concrete poured.
Well, after a few mishaps and adventures, everything went rather well. The aluminium arms stood up to the test. Over fifty piles were logged.
Next Time...
Radiation and logging ‐ the right mindset
MMaarrccuuss CChhaattffiieelldd –– NNoovveemmbbeerr 22001177
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