All about porosity The thermal neutron log Logging through drill pipe

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1 1 Issue 9 January 2015 WIRELINE WORKSHOP A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION All about porosity The thermal neutron log Logging through drill pipe 1. Overview - Porosity In sedimentary rocks, wireline logs of density, sonic, neutron and resistivity tend to describe similar shapes. In the clastic interburden and overburden, these logs are, fundamentally, porosity logs. They are all measures of water fraction rather than formation chemistry. The presence of clay complicates things somewhat but water, or lack of it, is the key influence on the logs. The composite below illustrates the similarity. There are some interesting and important differences between the logs but, overall, they are describing the same thing. All of these logs could be converted to absolute porosity as a percentage of sandstone, limestone and dolomite volume (see section 4). The conversion does not work well in clay-rich formations because moisture in clay volume is not indicative of porosity. Where the clay volume is small, corrections are possible based on an estimation of its contribution using the natural gamma log. In oilfield logging, porosity is the vital measurement and the basis of the science. In mineral logging, absolute porosity is not normally required and, certainly, no effort is made to estimate porosity from the resistivity log (a complex process due to the extra variable of fluid conductivity). From the left: Resistivity, Neutron, Density and Sonic logs captured in a sedimentary formation So why log porosity? Well, for a start, density and sonic logs are often run for other purposes so any derivatives are free of charge. Since the derived porosity logs will only be accurate in clean sandstone, limestone and dolomite, they will only agree (overlay) in those formations. The data provide a useful description of clastic lithology and some log quality assurance in clean sandstone, limestone or dolomite.

2 2 Moisture volume is usually assumed to represent primary porosity, the natural voids created by the sedimentary process. Subsequent fracturing, dissolution (vugginess) and dolomitisation (involving shrinkage of the matrix) also contribute to moisture volume and are referred to as secondary porosity. Because the sonic path takes the fastest route through the formation whereas the density log takes a volumetric average, sonic porosity tends to ignore fractures while density porosity includes them. Separation of the curves in clean formations (sonic porosity lower) indicates the presence of open fractures. Porosity log overlay (left track) in a clean sandstone formation (note the fracture at 165 metres) The porosity logs offer an objective and subtle description of the geology intersected by a borehole. They can describe fractured zones within "clean" (clay-free) formations. So why run the neutron tool? A third log of porosity, based on the neutron tool's sensitivity to the presence of hydrogen (see section 2), is normally plotted with density porosity and sonic porosity, at the same scale. Actually, this log is very sensitive to the presence of hydrogen/water in the formation and a typical clay-rich shale will have an apparent porosity of around 40% on the neutron porosity scale. The neutron porosity log is dominated by water - both free water in pore spaces and bound water in clay. A useful estimate of clay fraction may be created by deducting density porosity from neutron porosity. The new log should be plotted with the natural gamma log (scaled to fit). It is remarkable how well they compare in terms of shape, considering that one is measuring water and the other radioactive elements, but the presence of both is characteristic of clay mineralisation. Migrated uranium peaks feature on the gamma log but not on the porosity-based clay log. A further deduction is, therefore, of some value.

3 3 The log on the right, from a coal prospect in China, proves the point. Coal seams are blocked out and the red curve, derived from the porosity logs (the CPS are scaled to fit over the API scale), is plotted with the natural gamma log. On this occasion, the logs overlay well, as expected...overlay is normally good. The presence of migrated uranium is one exception to the rule. Others might be site-specific phenomena and worth investigating. Natural gamma plotted under a log of clay fraction (red curve) derived from neutron porosity minus density porosity. Casing Normally, density porosity describes the lowest values and neutron porosity the highest. Sonic porosity falls somewhere between the two. The relationships of the three measurements are indicators of rock type. The three porosity logs are plotted together and used for lithological analysis. The graphic below describes the theoretical relationships between three SST porosity logs and gamma ray in the various sedimentary rock types. LST will result in log overlay like that of SST if the relevant conversion is applied. The scales are the same; either SST or LST (or sometimes DOL) percentage porosity. These are meaningless outside clean formations but they offer a fixed method of presentation and log comparison. Coal U Theoretical relationships between three porosity logs for common rock types Note: SST = sandstone, SH = shale, MUDS = mudstone, SILT = siltstone. SPOR = sonic porosity, DPOR = density porosity and NPOR = neutron porosity. In most situations, shale is very distinct from the other rocks due to its relatively high gamma ray counts and high neutron porosity. Mudstone is often seen to be similar in these respects but is differentiated by its lower (faster) sonic values. A log of siltstone porosity is similar to shale but describes markedly lower neutron porosity and lower natural gamma ray counts. The porosity logs offer a simple description of rock type. As said before, sonic and density porosity logs are often free of charge because density and sonic may be run for other purposes. The question is whether to run the neutron porosity tool for the benefit of better lithological analysis. There is the cost in rig time and logging charges as well as the radiation risk factor to consider. In lithological terms, where detailed and subtle analysis is required, the benefit is significant.

4 4 Regardless of purpose, any log of formation porosity is, fundamentally, quantitative in nature. It is therefore essential to use high quality tooling and an effective calibration and borehole compensation system. An analysis of rock type based on three porosity logs plotted using a logarithmic scale and shaded between the curves. The neutron porosity log finds favour because of its sensitivity to clay fraction. The log on the right shows the superiority of the triple porosity plot to, for instance, density alone (plotted on the right of the acoustic image). 2. Measurement Focus A review of one wireline log measurement The Thermal Neutron Log The neutron sonde was designed to provide an estimate of water-filled porosity. It measures, primarily, the abundance of hydrogen atoms in the formation. Because the neutron log does not normally quantify the target resource and because its measurement requires the use of a very expensive radioactive source with a long halflife, it has, to some degree, fallen out of favour in mineral exploration. This, despite the fact that the neutron porosity log is a very useful lithological tool (as discussed in section 1). It should be understood that the neutron log offers several advantages over other log parameters, not least that it may be run through drill pipe or PVC casing. This section describes the principle of measurement as well as some benefits of running the sonde. Fundamentally, the neutron sonde logs the hydrogen index (HI) and should be run in a water-filled borehole in order to estimate saturated porosity. The dual detector version of the sonde works best since the ratio of two logs is mostly immune to absorption effects, particularly from chlorine. That's the general argument, and it is a proven technique, but it's not the whole story. In some circumstances, in hard rocks where hydrogen is a constant (tube-like cored holes) or in dry boreholes where it might be absent, the neutron log should be plotted to overlay density (it does overlay if there is iron in the formation). Where the logs separate, the tool is detecting water in secondary porosity (open fractures). In most situations, in soft or hard rock environments, the neutron log is an excellent correlation tool.

5 5 A thermal neutron log run in dry hole will describe iron ore and so, in some circumstances, might be calibrated empirically in terms of Fe percentage. It can be run through steel or PVC pipe. It has a deeper volume of measurement than the density log (particularly in dry formations) and is less affected by borehole conditions or tool orientation. No caliper or sidewall orientation is necessary. Principle of measurement In mineral logging, a source of neutrons, usually a mixture of Americium and Beryllium (AmBe), is attached to the bottom of the sonde. This chemical source is required to have a relatively high activity, at least 1Ci (37GBq). It has a halflife of years, the half-life of 241 Am. The Americium emits alpha particles, which bombard the Beryllium. The latter absorbs the alpha particles and decays, emitting highenergy neutrons. Some low-energy gamma radiation is also emitted by the Americium but this has no effect on the log measurement and is not normally a safety factor. The detection system employs a helium-filled tube, whose inner surface is the anode, with a cathode wire running longitudinally through its centre. The stable but incomplete helium-3 isotope has a large neutron capture cross section. When a thermalised neutron is absorbed, the He atom disintegrates and its two protons are energised. These positively charged protons ionise tracks through the helium gas allowing current to flow from anode to cathode wire, generating a measurable signal. The energetic neutrons have mass and high velocity so will penetrate deeper into the formation than gamma rays (typically 15-22cm as opposed to 8-15cm) depending mainly on the hydrogen count. They are hardly affected by the presence of steel casing in a borehole. In completely dry formations, some measured neutrons will represent rocks located 30cm away from the borehole wall. The significant interactions between neutrons and atoms, from a logger s perspective, are elastic scattering and thermal absorption. Scattering is the main cause of the neutron s energy reduction. Smaller nuclei take more energy from incident neutrons than large ones do. An analogy is striking a billiard ball with another ball as opposed to striking the side of the table with it. The incident ball is slowed by the target ball which moves away from the point of impact, there is a transfer of energy. In a head-on interaction, the incident ball is left standing still, having transferred all its energy. Little velocity is lost when a ball rebounds from the side of the table. Hydrogen is therefore very effective at slowing neutrons whereas the next smallest nuclei that are likely to be found in any quantity the formation are those of carbon, 12 times larger, oxygen 16, magnesium 24 and so on. Hydrogen dominates the slowing down process and limits the average distance that neutrons will travel away from their source. At a fixed distance from the source, count rates measured by a detector will decrease as the volume of water (H) in pore spaces increases. The smallest nucleus is that of common hydrogen and it is about the same size as a neutron. Hydrogen is far and away the main agent in the slowing down process. If there is no hydrogen, neutrons will be slowed by other elements eventually but, if hydrogen (in water) is present, it dominates completely. It is worth noting that, based on the billiard ball analogy, a neutron could lose all its energy in one collision. Fast neutrons slow down to an epithermal state quite rapidly then to thermal energy (reaching thermal equillibrium with the surrounding atoms and ceasing to lose energy) whereupon they meander around aimlessly (they diffuse), bumping into multiple nuclei, until they are absorbed.

6 6 Absorption rates vary depending on the elements in the formation; some isotopes are more neutron-hungry than others (remember that the neutron carries no charge so is not repelled by a nucleus). This absorptive capacity is described by an isotope's neutron capture cross section, the effective cross sectional area (in "barns") that a particular isotope of an element presents to an approaching neutron. The table on the left lists some elements that might affect the mineral log measurement due to their abundance and absorption cross section. * Chlorine and hydrogen are both significant absorbers and, in wet holes, the chlorine variable is the main reason why single detector systems are considered unreliable. Elements with larger capture cross sections have an effect in some special circumstances but, with the exception of chlorine, the large cross section neutron absorbers do not usually play a big role in neutron log measurement. In dry hole conditions, iron is the most significant absorber because of its relatively large cross section and local abundance. So we have a problem. There are two influences on the count rate at a detector placed at a fixed distance from the neutron source; the slowing down length, dominated by hydrogen, and the diffusion length, dominated in the mineral logging environment, by chlorine and, in dry holes, by iron. Our single-spaced neutron log of hydrogen concentration (porosity) would be perturbed by variations in the absorption rate during diffusion. One solution would be to close out the effect of absorption by measuring epithermal neutrons only. Their population, at a certain distance from the source, would be controlled by the amount of hydrogen in the formation. Unfortunately, count rates would be low and result in the need for very slow logging speeds or cause statistical anomalies in the data. The best option is to measure all neutrons at two detectors placed within the thermal neutron cloud. The size of the diffusing neutron cloud in terms of distance from the source depends on the hydrogen concentration in the formation. This affects the count rates at both detectors but, clearly, if the cloud shrinks towards or grows away from the source, the ratio of long to short detector counts will change. Count rates are governed by the migration length; the slowing down length (controlled by H) plus the much shorter and common (environmental) diffusion length (controlled mainly by Cl). The ratio is dominated by the slowing down length and is insensitive to environmental effects in general. Weatherford Primary calibration of the neutron porosity log is based on empirical trial. The oilfield (API) standard, employing limestone blocks of known porosity, is based at Houston University in Texas. Field calibration/verification is normally performed using a set of nylon jigs. Given the range of fast neutrons, it is important to make sure that the calibrator/detector position is well away from other influences (see picture on the left). This log must be a quantitative measurement! The dual spaced neutron log measures true percentage porosity in clean sandstone, limestone and dolomite. Otherwise, it is an indicator of moisture volume - a log of porosity plus clay fraction. * Referenced from "Well Logging for Earth Scientists" (Darwin/Ellis)

7 7 The thermal neutron log is very useful as a correlation tool. Its main use in sedimentary rocks, for the mineral explorer, is in lithology analysis, discussed in section 1. Beyond that, it has some interesting applications where hydrogen is not the dominant moderator of counts at the detectors. Here are a couple of examples. Log of SSN overlaying iron ore density (centre track) with good coherence above the fluid level The log above describes a magnetite/haematite sequence where the short spaced neutron log clearly describes the orebody above the water table (it overlays density). Given the density log's sensitivity to caving in dry hole conditions, especially in a high density orebody, the deeper measurement of the neutron log in dry rocks (up to 30cm) might be an advantage. The absorptive qualities of iron seem to be dominating the log above the fluid level. Note the lower count range for SSN above the water level. Most of the diffusing cloud might be extending beyond the near detector (comments please!). Neutron ratio log of igneous rocks showing coherence with density in competent zones Iron seems to affect the shape of a neutron ratio log below the water table, where hydrogen/water volume is a constant and significant Cl is absent, as in the lower part of the log above. The picks, on the left, are fractures. Where these contribute to secondary porosity the neutron porosity and density logs separate. There is a general but not perfect agreement between the extent of separation (shaded) and the resistivity log on the right. Note that some fractured zones are completely tight and probably represent a smaller risk to mining operations.

8 8 3. The logger on site Sometimes open-hole logging is too risky but the geologist still wants the data Logging through drill pipe and casing There are many occasions in mineral exploration where borehole stability is so poor that it is considered unsafe and unwise to run an expensive or radioactive sonde. In those circumstances, the explorer has various options, one of which is to abandon the logging job altogether. Other, more positive, options involve supporting the borehole wall using chemicals or heavy mud or logging the borehole from within some form of conduit, either drill pipe or casing. The easiest and cheapest option in deep boreholes, is to log through steel drill-pipe. In most cases, and in terms of detailed lithology measurements, only natural gamma ray, density and neutron logs may be captured through steel pipe or casing. Actually, these logs represent a significant amount of knowledge if their measurements are reliable. Caving behind steel conduit will seriously affect the density log and, to a slightly lesser extent, the neutron log but caved zones can be flagged as such if identified using dual density overlay comparison. The trick is to make the drill pipe part of the sonde and characterise log responses through it. This will be very much an empirical correction based on logs run through pipe and in (safe) open-hole conditions in a test well. It is an effective technique if done correctly. The ideal scenario for density measurement is a slightly angled borehole into which drill-pipe, with no internal upsets, is lowered then suspended at about 1 metre above the bottom of the hole. The object here is to log only the low side of the bore. If the pipe is allowed to stand up on the bottom of the borehole, it will spiral downwards under its own weight and leave the low side. This introduces variations in tool position with respect to the borehole wall and results in a poor density log. Constant tool orientation is important. A sidewall density sonde always logs the low-side - caliper pointing upwards. Logs run in suspended rods will be usable if caving is not too severe. The higher resolution density measurement will suffer most from caving and swelling at rod joints. It should be possible to recalibrate the logs so that BRD overlays LSD in good hole conditions. A sidewall density sonde in suspended PVC pipe, logging the low side - this is the ideal situation Constant orientation and proximity The diagram on the left shows a sidewall density sonde in suspended PVC pipe. PVC is a useful casing medium because it is inexpensive and may be discarded (left in the borehole). It also allows extra measurements such as full waveform sonic, magnetic susceptibility, inductive conductivity and acoustic televiewer. Suspension is problematic however, due to the pipe being weak at the joints (usually glued and riveted). In dry holes deeper than 50 metres the PVC casing should be allowed to rest upon the bottom of the borehole.

9 9 In that case, it will always collapse downwards (especially in air-filled boreholes). The density log will be severely compromised (other logs less so) because of variable proximity to the borehole wall. Three sondes with pipe pressed (from the left) to the low-side, high side and high side again (collimated mandrel) If the pipe, PVC or steel drill rods, is allowed to spiral down into the borehole, it will be pressed against the sides of the bore in a helical shape, sometimes forced against the low side and sometimes against the high side - all orientations will occur. The diagram above illustrates the effect on sonde position and measurement volume. Sometimes, in the case of PVC, spiralling is unavoidable because the pipe is not strong enough to be suspended. What is required is a constant orientation with respect to the borehole wall. In the far right example in the diagram, the sidewall density sonde will be logging an empty void (air or water) in the centre of the bore for much of the time. In that case it is better to go backwards in time and employ a simple uncollimated density sonde centralised in the pipe. Its location, in terms of distance from the borehole wall, will be constant. In the two situations on the left, it is clear how even a 360 degree measurement is affected if the sonde is not centralised in the pipe. Every few metres, the sonde moves towards and away from the formation by a few centimetres as the pipe spirals. When logging through PVC or steel conduit, the only uncontrolled variable (apart from the measurement itself) should be borehole caving. Density and neutron logs are best captured through suspended drill rods. It is necessary to labour this point because preparation for thru-pipe logging is usually done incorrectly.

10 10 If pipe is used, particularly steel drill rods, there is a need to cope with rod joints and compensation for the pipe offset and density. The damaging effect of internal upsets at rod joints on the density logs run through a coal sequence. The truncated caliper log on the right clearly describes the joints. It is possible to characterise the rod joint effect using a test well. Density logs, based on original calibrations, run in flush jointed rods through coal measures. LSD is the blue curve and BRD is the black curve. The BRD log is shifted further to the right but retains its high resolution. Recalibrated density logs run through suspended drill pipe and showing reasonably good agreement throughout the range of densities. Magnetic susceptibility logs run OH (shaded) and through spiralled PVC pipe (red curve) in the same angled borehole. The variable difference in count rate is caused mainly by changes in proximity to the borehole wall rather than PVC chemistry. To allow an accurate empirically-based correction, the sonde should be centralised in the pipe.

11 11 The full waveform sonic tool will operate effectively through PVC casing (below the fluid level). Both P and S wave logs are usually measurable in good borehole conditions and if the shear wave front is present. FWS log through PVC pipe With permission - Weatherford In the Full Waveform Sonic log example above, a very weathered (and slow) formation is cased off down to the intersection of a very hard (and fast) formation, at about 82 metres. The P-wave front is weak in the hard rock and invisible on the semblance image on the right. The slow rocks above the intersection do not support a shear front so the dominant feature behind the casing is the P-wave front. This sort of transition can be confusing. Another sonic based device, the Acoustic Televiewer will also work well through PVC pipe. The logger must set up the software to discriminate the second formation reflection rather than the first one. Amplitude images set to describe, on the left, slotted PVC casing and, on the right, the formation beyond the casing. This thru-pipe method does not work well in steel drill rods or casing because most high frequency sonic energy is reflected before it reaches the formation. It will work to some degree in Full Waveform Sonic logging through steel casing as evidenced in CBL logging where the formation is described by the long-spaced receiver if there is a good bond between injected grout and the formation. Digital Surveying It does not work in loose casing so, as an exploration logging technique, this is generally not a practical option. Logging through drill pipe or PVC casing can be very successful but log quality is improved if the conduit is suspended - not allowed to collapse downwards.

12 12 4. Wireline data processing and analysis How to get the best from the logs Porosity logs In mineral log processing, porosity, as a percentage of volume, may be calculated where the matrix (the skeleton) of the rock mass comprises one mineral, usually quartz, calcite or dolomite, and the pore spaces are filled with water. If the log analyst is provided with just a density log or a sonic log, he can make the conversion to porosity by applying simple formulae. In the case of density to sandstone percentage porosity: DPOR = (( {LSD}) / ( ))*100 Where LSD is the available calibrated density log, 1.0 represents the density of the fluid in the pore spaces and 2.65 is the density in gm/cc of the quartz matrix. Mathematical syntax is replaced by straight line formulae with log mnemonics enclosed by {}. For limestone formations the matrix density is changed to 2.71gm/cc and for dolomite to 2.87gm/cc. Sonic log conversion is usually based on a time average equation proposed by MRJ Wyllie and others in SPOR = (({PWAV} - 167) / ( ))*100 PWAV is the sonic transit time log in microseconds per metre, 167us/m is the transit time of the quartz matrix and 620us/m is the transit time of fresh water. For limestone, the matrix transit time is changed to 156us/m and for dolomite, to 143us/m. The log analyst relies on the logging contractor to supply the neutron porosity log. Given just a long spaced neutron log in SNU for instance, it would be difficult to convert to reliable percentage porosity. The complex ratio calculation is performed on calibrated short and long spaced measurements with a correction for borehole diameter. If necessary, one can make the neutron log align with DPOR and SPOR in clean sandstone and with 40% porosity in clay-rich shale, thus generating a usable approximation. A log of apparent halite porosity in an evaporite sequence The author has used the descriptive powers of the three porosity logs in an unconventional way by applying them to potash exploration. In the log on the right, halite (rock salt) is given a (best fit) neutron porosity value by employing limestone ratio porosity +2 percent. DPOR uses a matrix density of 2.17gm/cc (from the literature) and SPOR a matrix transit time of 244. The logs overlay in clean halite and allow some interesting descriptions in other formations.

13 13 5. Time for a quick review It's a New Year. Where are we now? In his world of winches and wireline technology, the mineral logger vainly searches for direction and certainty. It can be a tough business to be in. There is always the lack of a constant current in exploration. Yes, the oilfield guys suffer the same short intense peaks and interminable troughs characteristic of the upstream end of mining and energy but they have a greater sense of their worth. They know for certain that nobody is going to drill an oil well without logging it. It's not just best practice, it's the only practice, chiselled onto limestone tablets by the API and the AAPG...the logger is indispensible...thou Shalt Log! In terms of resource evaluation, the oilfield geologist trusts the logs. The mineral geologist does not. Why? It's really all about focus and risk. In the oil business, there is one type of target in one type of formation - oil & gas in porous sediments (OK, there's fracking now, but it hasn't changed the culture). The technique has been honed and standardised by repeated use. Once oil is intersected, there is no need to hang around. The log is partially analysed before the "combo" is winched back to the surface. Logging is more immediate than core analysis and the decision to produce, deflect or abandon can be made quickly. The well is often producing oil within days. On land, the risk associated with using a particular source of data is small because the well is already drilled and there are plenty of others already drilled and logged with which to compare results. The mining geologist looks a lot harder at all available data before he sinks his one lonely billion dollar mine shaft...and he has plenty of time. For the mineral logger, who has a range of targets, a large proportion of boreholes are not logged at all. Only one commodity comes close to the oilfield scenario, in terms of logging applications...coal. One can describe the resource. Certainly, most coal boreholes are wireline logged and for this reason coal logging is the backbone of the industry. In most cases, but not all, logging is used mainly for correlation and depth & thickness rather than for coal grade estimation or resource measurement. A lot of effort is expended by the logger in calibrating his expensive sidewalled collimated dual-spaced compensated density sonde...in many cases, he might as well use a trisonde (omnidirectional probe with no caliper or collimation). Logging in a Colombian coal field Greater use of properly calibrated and quality assured density data in coal resource evaluation is routinely championed by this bulletin. Recently, there has been significant growth in iron ore logging, based, again, on the density measurement and the ability to directly describe the ore body. In terms of ore quality, the log is not completely trusted and the geologist has a point, it is difficult to produce accurate logs at high density if borehole conditions are not close to perfect. Some efforts are being made to meet this challenge.

14 14 One area of real improvement is geotechnical logging. Safety and disaster avoidance are big issues and the advent of the acoustic televiewer as a fracture and fault logger has been pivotal. The geologist has embraced this sonde completely. He will usually buy a full wave sonic log as well. If asked what he intends to do with the full waveform data he is most often uncertain but laudably keen to get as much geotechnical data as possible. In mineral logging there is not one universal way of doing things and the geologist is not trained to use the logs to their full potential. This is understandable because a deep-mine gold geologist has quite different technical challenges to an opencast coal geologist. There is no standard-setting organisation for mineral logging - perhaps because of the diverse range of mineral targets or because the geologist has not yet demanded one. There is plenty of time to study surface geology and geophysics, core data, wireline data etc because a mine shaft or opencast pit development is not immediate...nobody turns a sod for years. There's loads of data...merge them all together into a mine plan and borehole geophysics will always be subordinate to but, hopefully, underpin the geological model. The use of porosity logs, the thermal neutron measurement and logging through pipe described earlier are peripheral but useful techniques. Not all geologists will have employed them. There are plenty of other techniques available from the logger that the geologist might be unaware of. Logging equipment manufacturers are continually developing their tool designs and extending their range of measurements. They are a force for improvement. Logging contractors use the equipment and computerisation to increase data quality and service reliability. There are lots of players in the marketplace and that should result in an overall uplift in service quality. What is required is more (informed) technical planning in the early stages of a project that sets standards, predicts outcomes and maximises value. As alluded to earlier, there are no industry guidelines, no standard way of logging a diamond pipe or a copper porphyry, for instance. That might be asking too much, but some basic rules, supported by case studies, would be helpful. Next Issue: A set of basic wireline logging guidelines for the geologist. The Mag Susc sonde. Any suggestions, corrections, criticisms or helpful comments ed prior to that publication will be gratefully received and acknowledged. All the very best for 2015! Marcus Chatfield January 2015 Copyrights apply (see wirelineworkshop.com) Acting editor/contact: Wilna (wireline.workshop@telkomsa.net) For back copies, go to and click on "Previous Issues". If you want your colleagues to receive the bulletin, tell them to go to and click on "Signup".