1 Issue 4 March 2014 WIRELINE WORKSHOP A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION More on sonic logs, density and rock strength The Acoustic Televiewer log 1. Overview - Rock Strength Sonic and density logs have a range of applications in mining exploration. Previously, their conversion to estimations of formation elasticity was discussed. This geotechnical theme is extended in this issue where conversion to intact rock strength (IRS) parameters will be considered. The laboratory measurement of rock strength in mining is UCS (uniaxial and/or unconfined compressive strength). It is the pressure-induced stress, usually described in megapascals or MPa, at which a sample of rock fails. UCS tests are therefore destructive and can only be performed once on any particular rock sample. In "clean" sandstones and limestones, density and sonic log values are a function of porosity. Clean, in this sense, means that there is an absence of clay. In the case of clean sandstone, there is just the crystalline quartz matrix and pore spaces. Logically, one might think that the greater the fraction of pore spaces, the weaker the rock and, generally speaking, this is true. There is a strong relationship between a sandstone's porosity and its UCS. There are variables within this scenario, such as grain size, type of cementation, alteration, texture and saturation, but porosity is the primary factor. In clean formations, estimation of percentage porosity from sonic and density is straightforward. So an empirically-based estimation of the strength of clean sandstone from logs should be valid. Actually, sonic transit time has proved to offer the better estimate of UCS and, perhaps surprisingly, in rocks that include a significant clay fraction. This is empiricism (it is coincidental). An increase in clay fraction slows the pressure wave and, as it happens, reduces rock strength. It should also be remembered, from issue 1, that the sonic log measures the quickest path through the formation whereas the density log is a volume average. Sonic transit time ignores some fractures, vugs and other irregularities and is therefore a better measure of intact rock strength.
2 There is a rather famous paper called "The prediction of geotechnical rock properties from sonic and neutron logs" (1990) by GH McNally that describes an empirical relationship between sonic logs, captured in the Bowen Basin and Hunter Valley coalfields of Australia, and laboratory-based UCS measurements. The graph on the right shows McNally's sonic data with exponential regression. Attempts to achieve similar results using neutron logs were less consistent and very site specific. When looking at the scatter on the cross-plot, it should be borne in mind that the sonic log is mostly precise. The destructive lab test, by its very nature, cannot be as precise as the sonic log and so some of the scatter will be attributed to methodology. McNally employed a very large data set to mitigate this problem. Despite the wide range of samples used in McNally's study, the formulae he produced are not used universally. They are a guide but, because of the variables listed earlier, it is prudent to describe a site-specific relationship before employing UCS logs. McNally introduced the concept. Most sandstones contain some clay and clay fraction represents a significant variable. At clay fractions above 40% the rock in question is no longer considered as a sandstone, becoming a mudstone or shale, and porosity ceases to be the only parameter in rock strength estimation. A significant correction for lithology (clay fraction) is required (section 4). Having determined the intact strength of a rock mass, the geotechnical engineer can estimate overall rock mass quality by including the additional effect of fractures; their frequency, angle and condition. Wireline logs provide precise estimates of IRS. Fracture count and analysis is available from the acoustic televiewer log (the identification and contribution of fractures is discussed in the next issue). Precision, objectivity and continuity remain the key attributes of geotechnical wireline data. Consider the UCS data, plotted laterally, on the right. The irregularly sampled histogram represents lab measurements on drill core. There is no check on precision because each measurement is a unique event. There are gaps in the data, possibly resulting from core-loss or fractures (tested samples must be intact).
3 Now consider the addition of a wireline estimation of UCS, fixed by empirical reference to laboratory data from the same site. The two data sets add value when plotted together. Each data set verifies the other. Precision is assured. Accuracy is based on a larger laboratorytested data set. There are no gaps. Missed hazards are now identified (1). A probable discontinuity is identified (2). Boundaries (3 and 4) are clearly described to within 2 or 3 centimetres. The potential for error is reduced. As with the dynamic moduli of elasticity, discussed in the last issue, UCS logs will not correspond perfectly with measurements based on laboratory tests using applied mechanical force. They may be employed within a geotechnical study if empirically corrected using sufficient laboratory data from the same site. The measurements are continuous, objective, precise, in-situ and cost far less than laboratory tests. If an empirical relationship can be described, the wireline logs add value to a drilling project and reduce uncertainty. The sonic-derived UCS estimation has some drawbacks. The sonic log can only be captured in a water-filled borehole, so up-hole operations underground or very shallow boreholes are excluded. The sonic log does not (directly) differentiate between primary (pores) and secondary (fractures) porosity. The highest resolution of the log is about 20cm which means that thin weak layers can be missed or misinterpreted. One solution to this problem could be the acoustic televiewer. 2. Measurement Focus A review of one wireline log measurement The Acoustic Televiewer The ATV log is another sonic measurement. The principal delivered log is a high resolution orientated image of the borehole wall in terms of its hardness. The sonde requires a water-filled borehole and best results are achieved if it is well centralised. Centralisation is important but it can represent a problem. Oilfield loggers use another type of imaging tool that measures electrical resistance at multiple sensors on pads that press against the borehole wall. This is the formation micro-scanner. Oil wells are most often supported by heavy drilling mud so, if a centralised acoustic televiewer is employed, there is some dispersal of the sonic signal during the relatively long journey to and from the borehole wall. The micro-scanner overcomes this problem, albeit at the cost of some data loss between the pads. Until recently, there was no micro-scanner available to the mineral logger
4 but that didn't matter because his boreholes are mostly small in diameter and filled with water, which is less dispersive than drilling mud. The televiewer log has, since its introduction to mineral logging 20 years ago, become a very important measurement, particularly in geotechnical studies. A high frequency (500KHz compared to 20KHz for the standard sonic device) sonic pulse is directed at the borehole wall. Unlike the standard sonic, which measures the transit time of a refracted wave to a distant receiver, the televiewer measures the time and amplitude of a reflected wave at the same transducer that transmitted it. The transducer transmits, then, a few microseconds later, receives the reflected wave within a prescribed time (listening) envelope. The complete transmit - receive cycle takes just a few hundred microseconds. A rotating concave sonic mirror directs the pulse towards the borehole wall. The cycle is continuously repeated as the sonde moves along the borehole thus generating helical logs of reflected time and amplitude which, when truncated, become orientated maps of the borehole wall. Traditionally, borehole images are displayed unwrapped, with the left (and right) edge aligned with magnetic north. There is an option to align with the borehole high-side orientation instead. Measurement resolution is better than 2mm in good borehole conditions. The time and amplitude images may be displayed using any colour palette. This is rather subjective but some palettes clarify the data better than others. In the plot on the left, very simple colour palettes are applied; time in grey scale and amplitude with yellow (hard) to dark brown (soft). One can clearly see bedding or layering overlaid with fractures (mostly drilling-induced). Some fractures have broken out and appear as open events on the time image (darker lines, so further away, on a grey background). These are particularly good images. The sonde was perfectly centralised. Normal ATV sonde configuration with sensor at the bottom. The slightly shaded line passing up the right hand side of the amplitude image is a tool artefact. It is actually quite useful as an indicator of tool rotation. Clearly, this tool was not rotating at all (the artefact is a straight line).
5 The navigation sub, tilt cells and magnetometers, provides tool and image orientation data. A log of borehole trajectory with Northings and Eastings is a standard product, albeit just for that part of the hole that is below the casing. The logs of borehole tilt and azimuth are used to orientate picked structures later in the process. In modern tool designs, the navigation sub calibration is done by the manufacturer and no work in this respect is required of the logger. If the sonic imaging system fails, it's pretty obvious. The images will describe the failure and the logger will usually recognise it and mobilise another sonde. When the navigation sub fails, for whatever reason, the fault is not always obvious. Every ATV sonde design can fail occasionally - there must be a Quality Assurance system in place. The writer has logged and processed data captured with most of the major tool designs available in the marketplace. They are all good reliable instruments but none is immune to occasional problems associated with image orientation. These problems are not always caused by sonde failure. There may be a calibration input error or some outside influence corrupting the navigation data. The televiewer sonde is, actually, remarkably reliable. Failure often seems inconceivable to the logger. Case Study - Canada - Platinum Operations The contractor was required to log diamond-drilled angled boreholes with an acoustic televiewer. He fitted centralisers, to a new sonde design, in the normal way. These particular tool centralisers affect the magnetometers if positioned too close, but he knew where to attach them. He was wrong...this new tool design had its magnetometers located in a different position to the previous version and he placed a centraliser right over them. Unfortunately, in angled bore, a sonde fitted with centralisers does not necessary rotate during transit along the borehole. This sonde did not rotate at all, so the borehole azimuth log was steady and looked correct. Rotation is helpful. If borehole tilt and azimuth remain constant during rotation, the tool is probably OK. The same thing happened in 9 deep boreholes. The logger went home. All his data looked perfect (superb images as usual) but they were all wrongly orientated. The data processor spent two weeks picking structures from the amplitude images and produced a magnificent set of plots, with borehole navigation logs as well. When the client received the data, he scratched his head and commented that borehole directions were completely different to those he had recorded with his multi-shot and, anyway, all the holes were drilled westwards, the logged borehole directions were anywhere, but west. An investigation revealed the cause of the error. The sonde was working perfectly but the centralisers corrupted the navigation logs. To fix the problem, images were reimported with respect to the high-side instead of magnetic north (so no magnetometer data were required). The logs were repicked and the resulting structures rotated to be with respect to true north using the client's multi-shot data. All ended happily on this occasion. If the logger had used a test jig on site, the error would have been spotted immediately. In televiewer logging, data that look perfect are not always what they appear to be and excellent amplitude images are distracting. The logger is so focussed on centralising the tool and recording a high-quality image that he sometimes relies on that achievement as being confirmation of a good data set. It is not.
6 The borehole logger does not normally calibrate the televiewer sonde...but he must always verify the quality of his data. The simple jig on the right works perfectly well. It is made of aluminium. If the slot is taped up and pointed to the west then the jig is filled with water, the logger can place his sonde into it and record a time log. There are two key points: The sonde must be the same device that was run in the borehole (don't take off the centralisers). The sonde must be rotated slowly during the time log recording. So, out of the borehole and into the jig, is the motto. Of course, the jig can be pointed in any know direction using a compass (both devices are based on magnetic north) but it has become convention to refer to the verification as a West- Test and some industry standardisation in this regard would be a very good thing. The recorded log is presented similarly to the example on the right. The slot artefact is located at 270 degrees and the tilt and azimuth logs (far right) are steady and reflect the angle and direction of the jig, regardless of rotation. Quality is assured. A better jig design would have a more stable base and, perhaps, a compass container could be permanently fitted, so that it is not lost or forgotten. The sonde must be centralised at the bottom of the tube. The ATV sonde can cope with a range of hole conditions. The acoustic televiewer requires a water-filled environment and will struggle to produce clear images in large diameter boreholes, particularly if they are full of heavy drilling mud. A micro-scanner is a better option if this condition prevails. Surprisingly, perhaps, the televiewer can capture good logs through PVC casing. The processor ignores the casing arrival and discriminates the next one, which is the formation. The amplitude image log is hardly affected at all by mud-cake or other deposits on the borehole wall. The sonde can produce a perfectly good log in very magnetic formations but (critically) the geologist must make sure that he drills an angled borehole (greater than 5 degrees from vertical). The log processor would import the log with its images being with respect to the borehole high-side. He can then use gyro deviation data or even an estimate of borehole trajectory to reorientate the picked structures. Note that the high-side orientation is the same as borehole azimuth.
7 One damaging but hard-to-see effect on the amplitude image in particular is pixel-stepping. Sometimes, due to a combination of borehole angle, wall condition and centraliser set-up, a sonde will not run smoothly along the borehole but move in a series of steps. The depth wheel moves, for a few millimetres (sometimes up to 6 pixels) but the sonde does not (same values recorded). The sonde is sticking to the borehole wall. This is damaging to the image when it is displayed at structure-picking scale and small fractures become difficult to analyse. The log on the right shows the stepping problem on the AMPM image then the relog to its right. At a normal logging display depth scale, in this case the logger used 20:1, the problem is not visible. It only came to light during data processing. The logger returned to site and relogged the borehole using slightly weaker centralisation and a faster logging speed. It is worth checking amplitude image quality at expanded scales during logging. One confusing phenomenon is the dampening effect of rough borehole surfaces on the amplitude image. A common manifestation of this is the low amplitude signal reflected from coarse sandstone. The log on the right illustrates this. There is a coal seam near the top, clearly described by gamma and density, then, further down, some dark layers that might look like coal but are, in fact, coarse sandstone. The angular borehole surface is dispersing the sonic signal on reflection. For this reason, the use of image-derived amplitude logs as qualitative measures of rock hardness is not always valid. More on televiewer logging and image data processing in the next issue. 3. The Logger on Site Take a few minutes to set up safely During an oil-shale exploration project in Morocco, the logger was completing the P-wave sonic run (the sonde was about 30 metres down the borehole) when the geologist client arrived in his truck. The geologist drove right next to the borehole and leapt out, enthusiastically asking how things were going. At that exact moment, the sonic sonde became stuck and splashing could be heard down the borehole. The geologist had driven too close to the borehole collar and caused a cave-in. The sonde was fished out but destroyed in the process. On another occasion, during the logging of a deep gold borehole in South Africa, the driller visited the logging site on his motorbike. He failed to spot the wireline cable, between truck and borehole, and it knocked him off his bike which continued onwards unmanned. The logging cable did not break but came off the bottom pulley which resulted in a depth error. Otherwise, no serious damage was done...luckily.
8 Non-operational personnel, even geologists, should be kept outside a cordon during logging and only allowed to cross it if invited by the logger. The cordon can comprise barrier tape or just a few traffic cones but the logger's space must be defined. Members of the public need to be warned of the dangers associated with the logging site. No vehicle access should be possible. Reliance on the logger keeping his eyes open is no longer acceptable Very often, it is not really practical to lay out a cordon. This might apply on very shallow single-run jobs where the time spent laying out effective barriers is disproportionate. A logging boom requires a smaller work area anyway. In that case, clear signage on the logging truck and a warning light is usually adequate. A logging company policy in this regard is recommended. 4. Guest Article A detailed discussion on rock strength issues by Peter Hatherly (Coalbed Geoscience, Australia) Geotechnical Evaluation based on Wireline Logging Quantitative versus qualitative analysis In previous issues of Wireline Workshop, various roles for geophysical logs have been discussed. One role for which the quality of the geophysical logs is not a major concern is to facilitate hole to hole correlation and determination of depths and formation thicknesses. However, when the role is expanded to provide accurate measurements of parameters such as density, the quality of the tools employed, their calibration, the borehole conditions and the methods of analysis all come into play. To assist geologists understand these issues, explanations of the physical basis of the individual log measurements and the underlying assumptions are being provided by Wireline Workshop. These issues also lead to matters relating to the role of quantitative analysis versus qualitative or empirical assessment. This is an interesting philosophical point which extends into sampling theory. After all, what is the point in knowing the exact properties of a core sample when we may need to extrapolate 10s of metres between samples within a borehole and 100s of metres between boreholes? A judicious mix of quantitative measurement and empirical analysis provided by a skilled geological engineer usually provides the most appropriate results. Sonic velocity and modulus For sonic logging, the exact relationships between the wave velocities (compressional and shear) and the elastic moduli (bulk and shear modulus) and the density were given in Issue 3 of Wireline Workshop. As explained, the
9 moduli determined in these expressions are called dynamic moduli and the values are greater than corresponding static moduli obtained in laboratory testing. These differences are attributed to the differences in the strain (rock particle displacements) between the very small strains caused by the propagation of seismic waves (order of microstrains) and the much larger strains imparted on a rock during laboratory testing and during mining (order of millistrains). Presumably rocks appear stiffer under the small strains imparted by the propagation of sonic waves because micro-cracks and pores do not experience as much closure. Sonic velocity and UCS As discussed in this issue of Wireline Workshop, empirical relationships between P-wave velocity and UCS are frequently sought and employed to estimate rock strength. These relationships allow extrapolation of rock strength data between test samples within a borehole and estimation of compressive strength in holes where geotechnical testing has not been conducted. When considering roof support requirements in underground coal mining, this information provides a qualitative assessment of the strength of various rock units within the roof. For longwall caving, it is also possible to locate potentially problematic units which may lead to excessive weighting on the supports, hang-ups behind the face and the development of cavities ahead of the supports. For more quantitative work, a number of authors and geotechnical engineers have used sonic UCS values to identify rock units and their strength as input into geotechnical rating schemes such as the Coal Mine Roof Rating (CMRR) developed by Chris Mark and Greg Molinda from NIOSH 1. However, this scheme is itself empirical and defined in terms of the rock properties that a geotechnical engineer can determine from core logging and testing. Given that wireline logs provide an alternative means of determining the down-hole geology without the need for coring, the question arises as to whether it is possible to devise a rock rating scheme based on an understanding of the rock properties provided by geophysical data alone. Geophysical Strata Rating To answer this question, we need to know which rock properties can be revealed by geophysical logging. In general terms there are three: 1. The porosity as determined from density logs 2. The clay content which can be determined from natural gamma logs and also from neutron and resistivity logs (provided the porosity is also known) 3. The modulus as revealed by sonic logs From a geotechnical perspective, these are important properties. Porous rocks and rocks with low clay contents are more likely to be sandstones. If their porosity is high, they are likely to be poorly cemented. Clay-rich rocks, on the other hand, tend to be siltstones and claystones. For these types of rocks, those containing clay rich laminations usually have lower porosities. Siltstones are more porous. So by knowing the porosity and clay content, we know something about the rock type and therefore, its potential geotechnical behaviour. Also, the sonic velocity provides us with information about the modulus. This understanding has led geotechnical engineer, Terry Medhurst, working with the author to propose an empirical geotechnical classification scheme based solely on the analysis of geophysical logs. Called the Geophysical Strata Rating (GSR) 2, this scheme provides a value for the rock quality between about 15 and 100, whereby rock quality improves with increasing value of GSR. In this sense, the GSR scheme has a similar range to the CMRR and the Rock Mass Rating (RMR) 3 which is mainly used in hard rock mining. The GSR was first described in 2004 and since then, the author has worked with Terry Medhurst to provide GSR assessments at numerous coal mines in Australia, mainly in the Bowen Basin. There has also been research into applications into open cut mines and a relationship has been developed between GSR and long-wall caving behaviour, mine layout and support capacity. It makes intuitive sense that sonic velocity (or modulus) should also reflect changes in rock type caused by variations in clay content and porosity. For years, the Rock Physics Laboratory at Standford University has been investigating the connections between geophysical properties and intrinsic rock properties, mainly in the context of petroleum exploration and recovery. One empirical equation relating velocity, porosity, clay content
10 and depth (pressure) which has a direct bearing on the discussion here was proposed by Eberhart-Phillips and others 5. It states that: V p = 5.77 6.94φ 1.73 V shale V p is compressional wave velocity, ø is the porosity, V shale is the clay content and p e is the confining pressure. V p is measured in km/s, ø and V shale are fractional values between 0 and 1, p e is measured in kbar. The equation was established through laboratory measurements of sonic velocity over a range of confining pressures on 64 different sandstones with porosities between 0.02 and 0.30 and clay contents between 0.00 and 0.50. Consideration of this equation and the relationship between geotechnical properties and rock type provides insights into why sonic velocity can be empirically related to UCS. GSR is also largely based on sonic velocity but through its formulation, direct consideration is made of porosity and clay content. As shown by the figure, GSR analysis allows a continuous representation of rock quality in a geological context. + 0.446 16.7 p ( p e e ) e To return to the discussion on quantitative versus qualitative analysis, GSR is not a quantitative measure of a fundamental rock property. However, it is a meaningful qualitative rating which exploits the numerous advantages offered by geophysical wireline logging for many of the empirical activities within geotechnical engineering. Peter Hatherly Email: geophys@bigpond.net.au Terry Medhurst Email: TMedhurst@pdrengineers.com.au References 1 Mark, C. and Molinda, G.M., 2005. The Coal Mine Roof Rating (CMRR) a decade of experience. International Journal of Coal Geology 64:85-103. 2 Medhurst, T., Hatherly, P. and Zhou, B., 2010. 3D geotechnical models for coal and clastic rocks based on the GSR. 10 th Underground Coal Operators Conference, University of Wollongong. Available on-line at http://ro.uow.edu.au/cgi/viewcontent.cgi?article=1962&context=coal. 3 Bieniawski, Z. T. (1989).Engineering rock mass classifications: a complete manual for engineers and geologists in mining, civil, and petroleum engineering. Wiley-Interscience. pp. 40 47.See also article on rock mass rating at https://www.rocscience.com/hoek/corner/3_rock_mass_classification.pdf. 4 Barton, N., 2002. Some new Q-value correlations to assist in site characterisation and tunnel design. International Journal Rock Mechanics and Mining Sciences39(2):185 216. 5 Eberhart-Phillips, D., Han, D.-H. and Zoback, M.D., 1989. Empirical relationships among seismic velocity, effective pressure, porosity and clay content in sandstone. Geophysics54:82 89. 5. Wireline data processing and analysis How to get the best from the logs The Verticality Log The acoustic televiewer produces orientated time and amplitude images but also a log of borehole tilt and azimuth in non-magnetic formations. It is always worth comparing this log with the driller's multi-shot or gyro deviation data. That's just another (prudent) layer of quality assurance. It is also well worth plotting tilt and azimuth logs two-dimensionally for detailed interrogation.
11 Subjective analysis of log quality is made easier if the log is truncated. In the example above, the depth scale is 5000:1. Elbows in the logs often occur at faults or stratigraphic boundaries. Tool rotation allows the analyst to check on calibration and sensor phasing. In this case, there is either some lack of alignment between tilt and magnetometer sets or centre-frequency drift, but the problem is not serious. It is manifested as a sine wave effect as the sonde rotates and is normally most apparent at very low deviations from vertical. In this example, the log analyst has manually drawn logs through the centre of the data ranges rather than apply a heavy filter. Developing a theme Quality assured density and sonic logs have many uses in mining and exploration. Both logs have an application in the important geotechnical field. Precise logs of dynamic elastic moduli may be derived but these require correction using laboratory data. Logs of intact rock strength are applicable to mining. Empirical analysis is valid. The acoustic televiewer is a key tool for the geotechnical engineer. The next issue is all about fractures. Marcus Chatfield March 2014 Copyrights apply (see wirelineworkshop.com) Editor/contact: Eugenie Joubert (wireline.workshop@telkomsa.net) For back copies, go to www.wirelineworkshop.com/bulletin and click on "Previous Issues". If you want your colleagues to receive the bulletin, tell them to go to www.wirelineworkshop.com/bulletin and click on "Signup".