1 Issue 10 – March 2015

WWIIRREELLIINNEE WWOORRKKSSHHOOPP A BIMONTHLY BULLETIN FOR WIRELINE LOGGERS AND GEOSCIENTISTS ENGAGED IN MINING AND MINERAL EXPLORATION

• Logging - make a plan

• The magnetic susceptibility log

• Cross-plots

11.. OOvveerrvviieeww -- BBeesstt PPrraaccttiiccee The time to discuss a wireline logging programme is early on in a project, not as an afterthought.

The first question is; what knowledge do we want to acquire? The next question is; how are we going to acquire it?

That requires a review of geological and geophysical options that might apply - including wireline logging. If a particular log represents critical knowledge, its inclusion might affect the drilling methodology. Most often, the logger is expected to cope with the borehole that he is given.

It is necessary for an experienced logger to be at the table during these discussions so that incorrect assumptions about what the logger can achieve are avoided. He will hope for a user-friendly borehole but that is not always possible and this criterion depends on which measurements are required.

Dirty water is fine for acoustic televiewers but not for optical ones. Is there clean water available near the site? Can a water tanker be made available? How big is it? Does it have a pump? What volume of water is required for each borehole? Is the added knowledge worth the extra cost? What is the alternative? What will the alternative log look like?

Optical and acoustic images of a steeply dipping fracture in

dolomite, logged in clean water

During round table discussions on how to get the most knowledge from a drilling project, it is easy to overlook problems that might be predicted by the logger or at least referred to on a check-list.

The appendix to this issue comprises a list of topics and questions that should be addressed at the conceptual stage of a drilling programme.

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What can go wrong? A lot can...and sometimes does! Cap it! The logger drove for 9 hours to the operational area and stayed in a motel near to the site. He brought with him large tool suite that included caliper, density, resistivity, sonic and neutron systems. The truck was full of sondes and sources. It had been a long drive. Next day, the logger arrived on site and set up on the first borehole. It was blocked at 6 metres. He went to a second borehole...'same thing happened. The geologist arrived and immediately noticed that most of the unsampled core was missing from each site. It became clear

that little boys from the local village had returned the core (in the wrong order) to where it had come from. The boreholes should have been securely capped and labelled before the driller left site.

Hang it! On a base metal job in Africa, the logger was required to log multiple dry angled boreholes on a mountainous site. The rocks intersected were friable, so it had been decided to log through PVC pipe.

A mountain site with gyro sonde

The important density log looked really good, describing layers of varying density, high and low density beds. The geologist was bemused because the boreholes did not intersect bedding; the formation had folded almost perpendicular to the Earth's surface. The holes were drilled parallel to the beds. After a lot of thought it was

realised that the PVC pipe had fallen downwards and the sidewall density sonde was being forced away from the borehole wall as the pipe spiralled. In retrospect, it would have been better to log through suspended steel drill rods rather than through spiralled PVC casing.

Check it! The logger had captured a superb televiewer image. He was really pleased with the log...nearly 500 metres of perfectly centralised image captured in difficult caving conditions. After removing the sonde from the borehole, the logger, quickly placed it into the west-test jig (it was getting dark). A short time-based log with

tool rotation was recorded and the logger packed away his gear and headed home. Sometime later, after the log had been delivered to the client, local geologists questioned the log analysis. Fracture directions did not match their geological model of the area.

After a period of uncertainty, somebody looked at the log from the jig test. The vertical artefact was not located to the west but wandered all over the page...the orientation was incorrect. Later investigation found that the sonde had been transported next to a CCL tool, with magnet. Sliding the sonde in and out of the tool rack had magnetised the televiewer's pressure tube.

This is certainly a very rare case and rather unlucky for the logger but it illustrates the need for multiple layers of QA. Perhaps the geologist should demand a test jig log be presented as part of the deliverables.

He should check it...in case the logger forgets to.

3 Review it! In deep boreholes drilled through Witwatersrand meta-sediments, the dipmeter log has proved to be an invaluable assist to the geologist. The rock is so hard that acoustic televiewer images describe fractures but not bedding...there is no contrast in terms of rock hardness. Surprisingly perhaps, as long as the borehole fluid is fairly fresh, the dipmeter describes plenty of contrast and bed definition in terms of electrical resistance. On one occasion though, a borehole would not yield a good log, the pad traces were all over the place...no correlation.

A pretty good dipmeter log

This was disappointing for the geologist who asked for a review. The logger suspected that there must be a conductive fluid ingress at about 1600 metres because he had noticed that, when plotted at 2000:1 depth scale, the pad traces curved like a chevron, pointing to a particular depth, where the caliper log described a fracture.

It was decided to bring in a tanker of fresh water to replace the borehole fluid, top up from it while withdrawing the rods and relog quickly. The result was a perfect dipmeter plot. Fluid conductivity had been the cause of poor pad traces in very resistive formations. At least one logging contractor offers a high-contrast dipmeter system that is able to cope in resistive metamorphosed sediments.

Prep it! Given a good borehole, properly cased and capped, the logger normally captures the required data. Good holes are easy to log. Most problems are related to fluid level, borehole access or casing issues. The driller is an important member of the team because he prepares the borehole conduit for the logging contractor.

In Australia, the logger sat up with concern when his televiewer sonde suddenly stopped moving upwards. The borehole was blocked, which was strange because, according to the navigation log (the three magnetometers), the sonde was inside the steel casing.

The sonde would not budge and, eventually, the cable broke and a fishing job commenced. After several days, the driller reported that there was no sonde in the borehole, it had vanished.

It was later determined that the sonde had passed behind split casing...cable inside and sonde outside. The sonde was lost and the logger's client faced a hefty bill for its replacement. Poor quality or substitute casing is the enemy!

The original good quality casing should always be left in the borehole, not replaced by cheaper casing or PVC pipe when the hole is completed and the driller moves to another site.

The writer's only experience of permanently losing a radioactive source occurred when original casing was replaced by black plastic water pipe in an attempt to save on costs. A lost sonde, a lost source, two week's fishing and the convoluted process of reporting to the Department of Health, grouting in the source and marking the location with a warning sign cost a king's ransom in the end.

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22.. MMeeaassuurreemmeenntt FFooccuuss A review of one wireline log measurement

The Magnetic Susceptibility Log

The magnetic susceptibility log measures the effect of an induced magnetic field on the formation in terms of its magnetisation. It is the ratio of the magnetism of the formation versus the applied magnetic field. The magnetic susceptibility sonde generates a local magnetic field using an electrified copper coil enclosed in a non-metallic housing (see below).

All rocks are magnetically susceptible. They will become magnetised with the application of an external magnetic field. The strength and character of the resultant magnetic field in the formation depends on its mineralogy. Only a few minerals, mainly those containing iron, sustain their own permanent magnetic field without any inducement. These are known as ferromagnetic minerals.

Permanently magnetic rocks will normally retain the magnetic field orientation (that induced by the Earth’s magnetic field) that prevailed at an earlier geological time – when a volcanic rock last cooled through its Curie temperature or when a sedimentary rock was deposited. In both cases, the locked-in magnetic field, known as remanence, is unaffected by later changes in the Earth’s ambient magnetic field unless the mineral is reheated back through its (unique) Curie temperature.

It is important to recognise that the magnetic susceptibility log is mainly a measure of the strength of ferromagnetism, which is mostly a function of iron content and oxidation. It does not differentiate magnetic remanence or measure its orientation.

A word or two on magnetism

The nature of magnetism, at the quantum scale, is extremely complex and well beyond the scope of this practical guide or the knowledge of its author. However, it is important for the user of magnetic susceptibility logs to understand, in a general sense, what is being measured by the sonde.

A magnetic susceptibility sonde with calibration

block in place

Magnetism is a force that either attracts or repels another magnet. Generally, both forces occur at the same time but one will dominate. Surprisingly perhaps, all materials, and therefore all minerals, are magnets; which means that they may be magnetised, to some degree, by another magnet.

Most minerals are not permanent magnets. They become magnetic only when placed near a magnetic field (produced perhaps by a magnetic susceptibility sonde). This applied magnetic field causes two effects; either repulsion or attraction.

5 Repulsion; at the subatomic scale, an applied magnetic field causes electrons to change the orientation of their spin axes, which results in a very weak negative magnetic moment that opposes the applied magnetism. This is known as diamagnetism.

Attraction; an applied magnetic field aligns (orders) an existing but disparately orientated (disordered) magnetism caused mainly by the movement (angular momentum or “spin”) of electrons. Most often, the microscopic magnetic field of one electron is cancelled out by the opposite field of a second “paired” electron within an atom. Only where at least one electron is unpaired (uncompensated) does the atom, as a whole, exert a force (a magnetic moment). This force contributes to a larger magnetic field when some of the positive magnetic moments are ordered by an external magnetic field. It is known as paramagnetism.

Mag Susc plotted with caliper to describe far field open fractures

In practice, iron content is the key to the magnetic properties of most minerals. Minerals that contain even tiny amounts of iron will be paramagnetic and will exert a weak attraction when exposed to a magnetic field. Minerals that contain no iron, such as pure quartz, exert a very weak repulsion. This repulsive force is always dominated by a paramagnetic force if the latter exists. Therefore, if there is any iron in a mineral, it will normally exhibit some attraction to a magnetic field.

If there is a large amount of iron in a mineral, the greater number of unpaired electrons can, sometimes, result in a spontaneous alignment of neighbouring magnetic fields. These neighbourhoods are called domains. If the magnetic fields of the domains in an iron bar are aligned by an applied magnetic field, the resulting field can be much stronger than the applied one. Some magnetism (alignment) is retained when the applied field is removed and a permanent magnet, with north and south poles, is created. This is called ferromagnetism. Remanent magnetism can only exist in ferromagnetic minerals.

Iron, cobalt and nickel, together on the periodic table with atomic numbers 26, 27 and 28 respectively, are the major ferromagnetic elements. In mineral exploration, iron dominates due to its universal presence. It is not normally found in its pure state but, most often, as an oxide, such as haematite (Fe2O3) or magnetite (Fe3O4).

The term ferromagnetism is used loosely in this context. It includes true ferromagnets, iron, cobalt, nickel and some of their alloys as well as ferrimagnets (opposing moments generally unequal, often very magnetic) and antiferromagnets (opposing moments ordered, mostly cancelling each other out, so not very magnetic).

The naturally occurring iron oxides have different crystal (molecular) structures that determine the level of interaction (cooperation) between the outer electron shells. True ferromagnets rely on the partially filled outer electron shells with unpaired

6 electrons combining with those of neighbouring atoms and order their magnetic fields to produce an overall constant field. Oxygen is important because, if it is present in the crystalline structure of the mineral, direct interaction between iron atoms cannot occur. It occurs in a variety of indirect interactions that characterise ferrimagnets and antiferromagnets.

The two common iron oxides Haematite and Magnetite are differentiated by the presence of one extra oxygen atom in the latter but these have quite different crystal structures and magnetic properties. Haematite is normally antiferromagnetic. Magnetite is ferrimagnetic.

Mag Susc (purple curve) scaled to overlay density in a tight ultramafic formation

The mag susc sonde is used to differentiate haematite and magnetite in dense orebodies. It will detect other minerals that contain iron such as the paramagnetic titanium-iron oxide, ilmenite, in heavy sand deposits. Iron-manganese carbonate, siderite, is weakly paramagnetic as is the iron sulphide, pyrite. If iron is present in sedimentary rocks, those originating from mafic rocks for instance, the mag susc log will describe it. In non-porous rocks containing grains of magnetite (igneous rocks often contain as much as 2% magnetite in their mineralogy), there is often a strong relationship between a formation's density and its magnetic susceptibility.

Principle of measurement

A flowing electric current generates an associated magnetic field. Conversely, a magnetic field will affect a flowing electric current. The sonde generates an oscillating magnetic field which produces current flow in a doughnut-shaped region in the formation. This in turn generates a secondary magnetic field which is detected by the receiver coil in the sonde. Formations containing ferromagnetic minerals will be magnetised. The strength of the measured field compared to the applied field is the level of susceptibility. Sample size is small and the log required significant correction for borehole diameter or stand-off.

Two (shaded) logs of magnetic susceptibility in an HQ borehole. For the darker log the sonde is

centralised.

The tool manufacturer will provide compensation factors for various borehole diameters and these can be applied dynamically if a caliper log is available.

7 The correction is not linear. Supplied corrections do not normally extend beyond 100mm so creation of a cross-plot and extended polynomial regression curve may be necessary for large diameter boreholes. The curve is rather flat beyond 100mm diameter anyway.

The magnetic susceptibility log is subject to baseline drift. This is mostly temperature induced and will be significant in deep boreholes. There may be ways to employ a temperature log to fix the baseline but if none is available, one imperfect but practical option is to describe the drift manually (at a large depth scale) and deduct it from the raw log. It is, anyway, important to refer to the temperature log, if it is available, in order to allow for elbows in the curve. Clearly, relative drift will be much greater where the range of measurement values is small.

1100 metres of drifting mag susc log, manually drawn baseline and final (deducted) log

Careful correction of the baseline allows the lateral scale to be expanded in order to describe small variations in the susceptibility of weakly magnetic formations, sometimes even in coalfield logs. The mag susc log below shows remarkable variation in susceptibility throughout, although the high values below 812 metres are probably caused by proximity to the bottom of the borehole and an inadequate correction of the baseline drift.

Coalfield sediments blocked using a much expanded and filtered mag susc log (centre)

The magnetic susceptibility tool captures a log of formation chemistry and should form part of any detailed sedimentological typing suite, with, perhaps, PE density, spectral gamma and formation resistivity. It can

8 provide an estimate of the proportion of iron-bearing minerals in the sediments but, since ferrimagnetic magnetite is 1000 times more magnetic than antiferromagnetic or paramagnetic minerals, some circumspection and further analysis in this respect is advisable. The log will not identify a particular magnetic mineral unless the measurement is, from the scale of the log deflection, obviously dominated by magnetite.

Magnetic minerals are usually heavier than clay or quartz and may be eroded, sorted and altered during the sedimentological process. Patterns of higher and lower susceptibility values are, in some circumstances, indicative of climatic changes during the time of deposition.

The Mag Susc log is an excellent cross-hole correlation tool. The main use of magnetic susceptibility is in iron ore logging. In the log below it clearly describes a change in chemistry within the dense orebody. This seems to correspond to a fracture close to the fluid level.

Gamma, density, mag susc and caliper in iron ore

Instrument calibration

It should be understood that, despite there being just one calibrator, this is not like a natural gamma job, it is not a single point calibration. With natural gamma, it is assumed (correctly) that zero API equals zero CPS. With the magnetic susceptibility tool, zero CGS (or SI) does not equate to zero CPS...There is a count rate for zero (sonde held in the air well away from anything that might be magnetically susceptible) and a higher count rate for the sample jig (sonde on a stand with jig placed in the centre of the coil section).

This might seem obvious to some but the writer has witnessed the wrong assumption being made on more than one occasion. The response curve is described as linear.

It makes sense to warm up the sonde for ten minutes and make sure that the sonde stand raises the tool well away from the ground.

For the commonly used Bartington BSS02B coil section, the calibration sample is placed (centred) midpoint between the two upper joints on the non-magnetic enclosure. Whichever tool design is used, it is wise to check on calibrator placement. The jig shown on page 4 (calibration witnessed on site) seems to be slightly high and the sonde too close to the ground.

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33.. TThhee llooggggeerr oonn ssiittee Mineral logging tools are slimline, but they can be made to work in fat boreholes

Logging big diameter boreholes

Most mineral exploration boreholes are slimline. That is to say, less than 150mm in diameter. There are occasions, however, when a client drills a big diameter borehole and wants to acquire some

reasonably accurate wireline data from it.

In kimberlite diamond assay boreholes, for instance, there is a requirement to at least capture an accurate caliper log for volume estimation. These boreholes are usually well over 300mm in diameter.

Generally speaking, problems are caused by the need to centralise or sidewall sondes with relatively weak centraliser or single caliper mechanisms. Angled holes result in easier side-walling but harder centralisation, particularly of heavy sondes. The density sonde, with long caliper arm, will normally require the help of a double bow-spring to sidewall properly in a big diameter vertical borehole. The caliper snakes up the far side of the bore causing the sonde to look slightly left and right rather than consistently into the formation. The top pulley wheel should be positioned eccentrically over the collar so that it does not pull the sonde way from the borehole wall. One can learn a lot by looking down a dry hole, with a torch if necessary, as the density sonde travels upwards.

A double bow-spring

Some measurements will be poor whatever the logger does. The acoustic televiewer, even if properly centralised, will generate a very fuzzy image due to dispersal of its acoustic signal as well as its reflection off a (normally) rugged surface. In very large diameters there is almost no usable reflection. As a last resort, the logger can run the tool with small centralisers in order to log a snapshot of one side of the hole. Orientations will be difficult to pick but a log of fracture aperture and frequency offers some added value.

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Where the optical televiewer is used in dry boreholes, some older designs require extra lighting to illuminate the borehole wall. Battery-powered lights may be fitted to the centralisers for this purpose. The most recent designs offer much more powerful lighting and can cope with larger borehole diameters.

The sonic tool would normally be centralised but practice has shown that it works perfectly well when trolled up one side of a very large diameter borehole using just 1 centimetre stand-offs. Actually, centralisation would probably result in the direct fluid wave being the first arrival at the shorter spaced receivers in slow formations.

Density, neutron and magnetic susceptibility will require significant borehole diameter compensation, very often based on rule of thumb estimations. The mag susc will require a very large correction but it is also very important to retain sidewall contact. A bow-spring is recommended in vertical boreholes of large diameter. Swinging across the centre of the bore will have a dramatic effect on the magnetic susceptibility measurement.

Gamma ray counts will be lower than if run in a smaller hole (particularly in water-filled holes) but, since the log is most often qualitative in nature, that does not normally matter much. If it does matter some correction should be applied. Water-filled sections of borehole require correction due to the shielding effect of the fluid.

44.. WWiirreelliinnee ddaattaa pprroocceessssiinngg aanndd aannaallyyssiiss How to get the best from the logs

Some thoughts on cross-plotting coal densities

There has been some discussion, in a previous issue (issue 8, November 2014) of this bulletin, on density log resolution and resource evaluation. It was suggested that depth control and the precision of the wireline log were its main attributes. A precise data set, comprising many boreholes, may be adjusted retrospectively if indicated by laboratory analysis or some reliable measure of true in-situ density. The weakness of the density measurement was deemed to be its poor resolution of thin beds and the "shoulder effect".

If we look at a set of laboratory measurements of coal relative density (rd) compared to (an imaginary) true density, the plot is scattered. The laboratory measurement is not precise because it comprises multiple individual events, none of which will be perfectly accurate.

A cross-plot of lab relative density versus true density

The resolution of both true and laboratory measurements is considered the same (same samples). The scatter is caused by errors...a lack of precision. There is also a systematic error and it is caused by an imperfect estimation of in-situ porosity. The lab relative density, the ratio of the mass of a volume of finely ground coal to the mass of the same volume of water, is converted to a measure of in-situ density by the inclusion of an estimate of in-situ porosity (moisture) volume.

11 The borehole log may be compared to (imaginary) true in-situ density. Again there is scatter. In this case, the log is vertically precise (it is one event). Some minor scatter would result from variations in formation chemistry but the majority of scatter is caused by lack of resolution at coal seam boundaries and partings...the shoulder effect.

A cross-plot of logged density versus true density

Any density datum logged at a boundary, where the measurement is not fully resolved, would be an outlier on the plot. Actually, the lab data would have a similar shoulder effect if samples were selected at regular depths. Instead, the samples are selected visually to avoid boundaries and describe homogenous lamina of a coal seam.

The overall error or bias in the wireline data set is caused by equipment calibration and borehole compensation variables. Each logging equipment type would produce a slightly different log, some better than others, most precise, but none perfectly accurate. Variations in chemistry, mostly water fraction, should normally affect the measurement by less than 2% of logged density.

A cross-plot of block-averaged density versus laboratory RD

The strength of the wireline log is its precision. So, given one set of logging equipment, we capture a precise log of in-situ density. The precision of the laboratory data might be tested by cross-plotting both data sets and, using a regression derived formula, shifting the wireline data to match the overall laboratory data set and plotting the two logs on the same track. This will not produce an accurate log but it will test the confidence placed in the laboratory process. A cross-plot of block-averaged wireline data and lab data usually results in less scatter on the plot, but block averaging can be confusing - more next time.

NNeexxtt IIssssuuee:: A review of fluid logs and the impeller flowmeter

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GGeenneerraall ssaaffeettyy -- iissssuueess tthhaatt sshhoouulldd bbee ddiissccuusssseedd wwiitthh tthhee llooggggeerr

# Subject Points for Discussion

1 Driving Driving is statistically the biggest threat. Some companies have a policy of not driving at night. The logger should be made aware if this rule applies to him. The client should make it clear that he is willing to pay for a hotel to avoid excessive driving hours. Discuss the rules.

2 Safety meeting Best practice includes a tailgate safety meeting for non-routine operations and, perhaps, a weekly meeting on or near site for regular work. The driller, if on site, is always included.

3 The site It should be agreed that there is a handover of the rig to the logger before logging operations. The driller does not then initiate work on the rig without consulting the logger. The logger should close off access to site between logging truck and drilling rig.

4 Electrical Lives have been lost due to the logger touching live overhead wires with a sonde.

5 The winch The winch is the most dangerous piece of equipment in the mineral logging truck. Make sure that the drive chains have guards fitted and access to the moving winch spool is restricted.

Normally, the logger will generate a journey management plan, a risk assessment and a record of the safety meeting.

RRaaddiiaattiioonn ssaaffeettyy -- iissssuueess tthhaatt sshhoouulldd bbee ddiissccuusssseedd wwiitthh tthhee llooggggeerr

# Subject Points for Discussion

1 Signage Proper signage on logging vehicles is required to warn the public. Consider if the vehicle is involved in an accident. Would the public/police recognise the danger? Does the signage include emergency telephone numbers?

2 Source transport A source should be transported within a locked shield, stored in a locked cage, in a locked vehicle. Internal signage should be in place. Chains are not best practice.

3 Source on site The source(s) should be placed in a demarcated location in line of site from the logging truck. If the site is not secure, they should be chained together or to something that is immovable.

4 Radiation meter The contractor should have available at least one calibrated radiation meter, preferably one for each logging truck (to check that the source is in its shield before leaving site). It is important that this be available to monitor fluid returns during fishing jobs.

5 Source store For long term jobs, there should be a source store on site. It should be a lockable bunker or container (two padlocks) within a fence with a lockable gate. Appropriate warning signage inside the container, on its door and on the fence, facing in all directions, is recommended.

SSoommee ffuunnddaammeennttaall qquueessttiioonnss

# Subject Points for Discussion

1 Measurements There are usually one or two measurements that form the core of the operation and are indispensible. What are these? There should be sufficient back-up available.

2 Quality assurance What QA is in place? Does the logger require a test well? Does he require a calibration area with water tank? The geologist should demand evidence of quality assured.

3 Added value If the logger is on site running the key measurements, the cost of an extra tool or two will be relatively low. Consider, from the list below ("Which Measurements"), if more value can be extracted from the operation.

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WWhhiicchh BBoorreehhoollee??

# Criterion Comments on Best Practice

1 Borehole diameter

Usually dictated by core requirement but, generally, for mineral logging tools, HQ (96mm) is the best diameter. Most contractors can log down to 60mm diameter bore. Very large diameter boreholes, over 200mm, are not well suited to mineral tools but most measurements can be captured effectively, perhaps requiring some correction.

2 Drill depth It is necessary to drill beyond the target depth in order to allow for the length of the sonde as well as some caving and settling of fines in the borehole fluid. 5 metres is normally sufficient but 10 metres is recommended for deeper boreholes.

3 Angle from vertical

If the geologist requires orientated televiewer data in magnetite deposits, he should drill an angled borehole...more than 5 degrees from vertical. The logger will orientate his images with respect to the high side of the borehole and correct the structure log using edited navigation data, a gyro log or an estimation of borehole trajectory.

If the logger is to rely on gravity to deliver his sonde, the borehole angle should not exceed 50 degrees from vertical. In non-cored holes (with rough wall conditions), that should be reduced to 45 degrees.

If an open hole log is required from a hole drilled through unstable ground, a vertical hole will stand up better than an angled hole and a dry hole will stand up far longer than a wet one.

4 Casing

It is necessary to case off the unconsolidated layer in order to make the borehole safe. Caving is the cause of most equipment losses. It is important to make sure that the original casing is left in place. Sometimes, the driller removes the (costly) original casing and replaces it with an old piece of casing or PVC pipe. This is asking for trouble and has resulted in equipment losses, including radioactive losses, in the past. The use of PVC casing is therefore not best practice unless it is very substantial and set in place properly.

5 Conduit

If intersected ground is considered unstable, where it is unsafe to lower expensive or radioactive equipment, data may be captured through the drill rods or through inserted PVC pipe. In this case, it is important to drill an angled hole. Suspended rods or pipe will lie on the low side of the borehole throughout its length. Varying conduit location is not best practice. See "Which Measurement", below.

6 Fluid level

Most sondes work best in a water-filled environment. Many will not work at all in a dry hole. The removal of drill rods from a tight borehole will result in a water-level draw-down. It is safer to top up the fluid level during removal of the rods than afterwards. The fluid level may be raised during logging but this practice can sometimes result in caving above the sonde. Sometimes, a borehole can be topped up to the surface and held at that level during logging.

7 Fluid type

A borehole filled with clean fresh water usually results in a good data set. It is essential for the optical televiewer. Saline fluids are problematic for the dipmeter log. If heavy mud is employed, some correction to the density log is necessary and, in hole diameter of over 100mm, the acoustic televiewer log will suffer from signal dispersal (the image will be fuzzy). Mud cake will usually form on the borehole wall in permeable zones. Gas bubbles (methane or radon) in the fluid will be very damaging to sonic and acoustic televiewer logs.

The time to discuss borehole conditions is well before drilling commences. All too often, boreholes are drilled and the logger is expected to cope with whatever he is

given. The objective for the team must be top quality data!

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WWhhiicchh MMeeaassuurreemmeenntt??

# Common measurements Description Wet hole

Dry hole

Thru-drill rods

Thru PVC

casing

1 Dummy sonde Low risk check on borehole accessibility * * * *

2 Borehole caliper 1, 3 or 4 arm log of borehole diameter * * * *

3 Borehole deviation (mag) Tilt and azimuth of bore (magnetometers) * * * 4 Borehole deviation (gyro) Tilt and azimuth of bore, in rods (gyroscope) * * * * 5 Natural gamma ray Total gamma radiation emitted by formation * * * * 6 Spectral gamma ray K, U and Th contribution to total gamma ray * * * * 7 Density (gamma-gamma) Electron density to bulk density and porosity * * * * 8 Photo-electric density PE effect, based on atomic number, lithology * * * * 9 Sonic transit time Refracted P and S wave, formation porosity * *

10 Acoustic televiewer Orientated sonic image of fractures and beds * * 11 Optical televiewer Orientated photographic image of b'hole wall * * * 12 Thermal neutron Hydrogen index and inferred porosity * * 13 Prompt fission neutron Direct measure of uranium-235 * * * * 14 Electrical resistivity Resistance to electric current * 15 Spontaneous potential Natural voltages caused by presence of bore * 16 Dipmeter (micro-res) Dip and direction of bedding * 17 Inductive conductivity Electrical conductivity of formation * * * 18 Magnetic susceptibility Magnetizability in applied magnetic field * * * 19 Induced polarisation Chargeability, disseminated metallic minerals * 20 Fluid temperature Temperature of borehole fluid * * * * 21 Fluid conductivity Electrical conductivity of borehole fluid * 22 Fluid/gas flowmeter Natural or induced fluid flows, up and down * * 23 Fluid/gas sampler Discrete sample of borehole fluid at depth * 24 Water quality Fluid temp, conductivity, PH, oxygen in-situ * 25 Cement bond log Bond between casing, grout and formation * 26 Casing collar locator Depth of steel casing joints * * * 27 Seismic reference Calibration of integrated transit time from sonic * * * 29 Vertical seismic profile Image of reflected events below weathering * * * Note: * means possible but not usually applicable

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SSoommee mmeeaassuurreemmeennttss iinn ddeettaaiill

# Log Description

2 Borehole caliper Three-arm tool for check on borehole condition and estimation of volume. Four-arm tool for ovality, better volume estimation and breakout. Tools usually require support in angled boreholes. Selection of correct arm length is important.

5 Natural gamma ray Total gamma ray - lithological profile based on clay content. Useful for cross-hole correlation in all rock types. Will log U grade if deposit is near to equillibrium. Used for logging potash grade. Will log through PVC pipe and steel casing.

6 Spectral gamma ray K, U and Th contributions to total gamma with ratios diagnostic of clay type and source rocks. Mostly used for lithological analysis. Requires locking source. Without locking source, it will log uranium and thorium daughters.

7 Density Definition of target orebody or coal seam. Will measure coal and iron ore grade. Generates porosity log and lithological profile with tonnages. Calibration and borehole compensation are critical and should be backed up by test well for detailed study.

9 Sonic transit time P wave, S wave and tube wave logs. Will not log shear wave in slow formations. Compression wave used for porosity, seismic reference and correlation. Derived moduli of elasticity and UCS logs can be fixed by reference to laboratory analysis.

10 Acoustic televiewer Bedding, fractures and faults described and orientated in wet hole. Data processing is time consuming. Boundary resolution is excellent. Used mainly as a geotechnical tool. Will not work well in heavy mud. Will log through PVC pipe. Provides borehole navigation log.

11 Optical televiewer Bedding, lithology, fractures and faults described and orientated in dry hole and in clean water. Results damaged by deposits on borehole wall. Image is very high resolution but might require extra illumination in big diameters. Provides borehole navigation log.

12 Thermal neutron Porosity based on hydrogen index. Will log through PVC and steel casing/rods. Excellent lithology analysis and correlation tool. Will describe coal seams but not evaluate them. Some applications in geotechnical logging in hard rocks. Will describe iron ore in dry holes.

14 Electrical resistivity Sensitive to conductive fluids in pore spaces and clay as well as conductive matrix in hard rock applications. Will define coal seams but mainly used as a lithological analysis tool. Very sensitive to gradational changes in sediments and fractures in hard rocks.

16 Dipmeter (micro-res) Real time log of dip and direction of sedimentary bedding, both recent and meta-sediments. Less expensive and quicker than televiewers but does not log fractures well. Provides borehole navigation log and high resolution logs of formation resistance.

17 Inductive conductivity Will provide formation conductivity in wet or dry boreholes. Not good in even moderately resistive formations but preferred in conductive Cu deposits. Logging dry hole is confusing due to moisture (wet, damp, saturated etc) variable. Will log through PVC.

18 Mag susceptibility Logs magnetic or paramagnetic formations. Often used in iron oxides to differentiate magnetite/haematite (extent of weathering). Useful in hard rock logging of paramagnetic orebodies. Will log wet and dry holes and through PVC pipe.

19 Induced polarisation Time domain formation chargeability. Used to log disseminated ore, sulphides, metallic minerals, Cu deposits in porphorytic environment. Requires fluid filled borehole and will not log through PVC. A log of electrical resistance at low values is also generated.

20 Fluid temperature Temperature gradient and fluid flows logged immediately after withdrawal of rods then ambient temperature logged after 48 hours. Note fluid temperature assumed to represent far field formation temperature. Not always valid even after 48 hours.

21 Fluid conductivity Conductivity of borehole fluid affected by inflow after withdrawal of drill rods. Used to flag aquifers at inflow depths and compare with differential temperature log. Should be run soon after rods are withdrawn. Used in groundwater pollution studies.

22 Fluid flowmeter Impeller type used where flow is induced for geotechnical applications. Inflow rate in litres/hour at known pressure imbalance may be determined. Used in groundwater studies but small natural flows require tracer flowmeter (heat-pulse or electromagnetic).

16

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# Sonde Discussion

1 Gyro deviation survey Through the drill rods

Consider now whether to run gamma ray and density through the drill rods if borehole condition is expected to be very poor. Remember to hang the rods (no more than 1 metre above TD).

2 Dummy sonde Run in old holes or in prospects where very unstable hole conditions prevail.

3 Temperature, conductivity Run as soon as rods are withdrawn if logs needed for aquifer identification.

4 Three-arm caliper Normal first run tool in all but very stable well-known ground.

5 Sonic, Res, Mag susc, induction If required, these inexpensive non-radioactive tools are run after caliper.

6 Density-gamma-caliper An expensive tool, run mid programme due to the radiation issue.

7 Thermal neutron This sonde employs a very expensive source with a long half-life.

8 Televiewer, Dipmeter Very expensive and/or rather fragile tools are run late in the programme.

9 Temperature Run as late as possible for ambient temperature profile.

These are not hard and fast rules, just worth considering. If a very slow tool, such as spectral gamma, is required, that can be run last, so the logger has plenty of time to pack up and do his paperwork and plotting during its ascent. Quite often in coal logging, especially in a well-known prospect, only the density combination is run without caliper or dummy runs to check the borehole. The logger is responsible for making the correct decision in this regard.

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# Subject Points for Discussion

1 Printer Does the logger have a printer in the truck? Providing the geologist a rough plot of density, for instance, on site can be very useful. Sometimes, the driller will appreciate a caliper log immediately after logging ...if he has problems with the borehole or wants to set a wedge.

2 Paper plots Print quantity and depth and parameter scales should be agreed on. Quite often, individual geologists prefer different depth scales, which causes confusion on site.

3 Digital data Some protocol for naming raw and processed data files is advantageous. Final deliverables usually include the raw data and some column format files (LAS or CSV).

4 Image data Image data may be converted to LIS or LAS image files but, most often, are delivered in specialised log analysis formats such as WellCAD. Evidence of image quality (2:1 depth scale) and a plot of the jig test is recommended. 'Better to find the problem before the logger leaves site.

5 Backup Digital data can be corrupted or misplaced. Computers can be stolen. Backups held on a memory device (not in the same bag as the PC) and a CD handed to the client on site represent prudent best practice.

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