Geophysical borehole density logging in the hard rock mining sector, gold, copper, is becoming increasingly important in mine planning and ore body knowledge. Geological resources are commonly modelled as volumes which need to be converted to mass using density values. Poor bulk density data will result in unreliable tonnage estimates and may impact negatively on mine scheduling, design and reconciliation of mineral production against reserves.

Tread carefully, on the types of density measurements:

• Dry bulk density DBD (g/cc) is the density measurement (volume x mass) made from an air dried sample. There is no moisture content factor included. Typically used for mineral processing with the measurement made from core and/or drill cuttings.
• In situ Bulk Density ISBD (g/cc) is the density measurement which includes the moisture content. Essentially water content within the pore spaces. This is the measurement given by geophysical borehole density logging.
• Specific Gravity is a density measurement which can be compared to the in situ bulk density as long as the weighing process involved water. The measurement is said to not reflect porosity or water content.

An observation of the dry bulk density method and specific gravity method is that both are labour intensive leading to lower sample rates for the data set. Geophysical borehole logging for density is a commonly used, precise technique which generates a large dataset for modelling purposes.

Geophysical borehole density logging is a rapid, precise and cost effective method of generating a large bulk density database. Utilising back scattered gamma radiation for a small radioactive source within the geophysical probe, samples every 1cm of logged interval can be generated. Quality control of the density measurements can be achieved by using the caliper data (from the same probe) to identify zones of enlargements or washouts where measurements may be compromised. Dry density values can be generated from a geophysical density data set with knowledge of the porosity and local groundwater density.

The example above is from a South Australian copper mine and demonstrates the high sample population generated and the ease of application of the dry density index formula.

Geophysical Borehole Logging – Geological Intelligence for Ore Body Knowledge.

(Reference: Bulk Density of Industrial Minerals, A. Scogings, Mining Engineering, July 2015)

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“Knowledge of your target’s physical properties will aid in the selection of the most applicable geophysical logging method for your drill program.”

Environmental contamination investigation and monitoring projects are commonplace today as older industrial sites change their land use. It is important for the regualtors and new landowners to know if an appropriate site clean-up has been undertaken to remove any hazardous waste, allowing a trouble free handover. Common industrial waste products are hydrocarbon based.

Environmental contamination investigation projects commonly involve drilling numerous shallow monitoring boreholes to intersect the local groundwater. The monitoring bores typically comprise 50mm PVC casing to the drilled depth, with the bottom few metres having slotted casing to allow groundwater access. Gravel pack, cement and bentonite plugs are used in the casing external annulus to seal off the slotted casing zone, so only groundwater from the slotted interval gets into the casing. Water level monitoring and groundwater sampling is then routine on an ongoing basis in the monitoring bore.

How can geophysical logging help these projects?

Identify the geology (Gamma and Resistivity/Induction-Conductivity)

• If the contaminant is fluid or gaseous, it can only travel with the groundwater or in the vadose zone through the pore space of the rock and structural fractures through faults and/or joints. Equally as important as pore space is the rock permeability or its ability to transmit a fluid. Note that a rock maybe highly porous but have a very low permeability = clay
• The ability to define different rock types intersected by the borehole is important. In a sand/clay sequence, it is the sands which typically have higher porosity and associated permeability and are the likely travel paths for contaminants. Conversely, the clays can act as an aquitard, thus acting as a barrier to contaminant flow.

Simple gamma, resistivity and/or induction-conductivity logging are powerful tools in identifying the local geology in a sand/clay sequence.

The example above shows a variable sand/silt and clay sequence, typically thin sands within an otherwise clay sequence. The induction-conductivity data shows a conductive body below 10m – so a resistive contaminant above (?). Look at the detail or lack thereof in the geologists log!!

Identify the contaminant

• If the contaminant is hydrocarbon based, then it is likely to have a resistive signature. Within a PVC cased monitoring borehole, induction-conductivity logging is the best method for measuring a resistivity profile. The resistivity profile will reflect changes in the geology from sand to clays and changes in the fluid (groundwater & contaminant) within the pore space. If a hydrocarbon contaminant even partially replaces groundwater in the pore space of the sands, there will be a significant change to the resistivity profile.
• Bear in mind that it is this principle (hydrocarbons replacing groundwater) which the oil industry have exploited for their sophisticated analyses in oil exploration and reservoir development.

Sample the contaminant

• Environmental consultants typically use a small pump in the monitoring bores to provide a surface sample which can be analysed in the laboratory. How certain can you be that the sample is “clean” – not a residue left at the bottom of the bore, and/or is from the appropriate depth in the bore and as such representative of the local conditions?
• The power of geophysical logging is to allow digital sampling “in situ” – from the actual depth of interest. In monitoring bores, this is achieved through water quality logging or an actual fluid sampling probe which can go to any specified depth, capture the sample, then return it intact to the surface.

Geophysical Borehole Logging – Geological Intelligence for Environmental Assessment.

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Knowledge of your target’s physical properties will aid in the selection of the most applicable geophysical logging method for your drill program.

As the project geologist, your desk studies on the most favourable drill targets have been completed, now you have to drill to increase the value of your company’s asset. You will have many competing processes to be undertaken in a timely and cost effective manner to increase your ore body knowledge. Most important will be the cost, keeping the drill rig moving (avoiding rig standby time) and preservation of your samples recovered from drilling (for geological assessment, transportation to the laboratory). Sure your drill program is planned and typically on a pattern, “battleship style”. Imagine what cost savings could be made if you are able to say (DURING THE DRILL PROGRAM):

• The target is not there anymore! Are we wasting drill metres? Do we need to drill deeper?

(Drilling deeper with the drill rig on site is much cheaper than bringing the drill rig back later)

• The target becomes larger and open in another area, we need to adjust our drill pattern in this area?

Let’s look at inter borehole correlation the ability to trace your target laterally away from one borehole to another borehole. You may be tracing the target itself, the property of the target’s host rock or the property of another horizon close to the target, which maintains geological synchronicity with the target, As a geologist you are looking for rock types which would be representative of quiet formational or depositional conditions which would be the same or similar at one given time over a large area. Clays are a good example of such conditions, so is volcanic ash. Both are common in the geological record and are good geological markers with distinctive geophysical log signatures.

• As a geologist, your only geological sample is from the borehole itself and is open to human error between individuals. Sure the geologist is highly trained and today there are good systems in place for sample description but do geologists all see the same thing? One of the more important geological sample descriptors is colour. As humans, do we all see the same colour?

The power of a repeatable, calibrated geophysical logging method is that the response can be distinctive to a geological feature. It is this distinctive geophysical log response and a human’s ability to use the eye to trace the geophysical log response (and any changes related to geological condition change) across a project area which makes geophysical log correlation a powerful tool.

• The example above is from a line of boreholes across a project area. The gamma log peaks are associated with clay horizons. Immediately the geologist can see similar log responses occur at specific depths and can be seen in most if not all boreholes. Differences in the feature elevations in the boreholes suggest geological structure (in this case a syncline)
• Inter borehole correlations can be done manually by the geologist, but increasingly digital computer computations are being used for inter borehole correlations through data crossplots. The key here is calibrated, repeatable, precise geophysical log data.

• There are many different geophysical logs which can be used for inter borehole correlation, gamma is the most common but density, resistivity and induction-conductivity can all be associated with specific responses. In mature mining areas, such as the Pilbara region of Western Australia, a stratigraphy has been established based upon gamma log responses. Today this is used to guide regional geological interpretations, structural assessment and ongoing drill hole planning.

Geophysical Borehole Logging – Geological Intelligence for Ore Body Knowledge.

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There are a number of mineral logging system and probe manufacturers worldwide. All manufacturers offer most main stream logging probes and have strengths in different areas. The issues for the logging contractors is compatibility between logging systems which can impact data quality and comparison between systems and further impact upon operational efficiency.

LOGGING SYSTEM COMPATIBILITY PROBE SPECIFICATIONS:

For most logging probes, there is not a single defined standard for detector type, spacing etc. The specifications are close but not exactly the same.

Take for example the formation density probe:

A collimated density detector housing is common but the collimated material is not.

A single arm caliper arm is standard but the length and strength of the caliper arm is not.

A sodium iodide detector(s) is standard but the size, spacing and number of detectors is not. Most formation density probes would have two density detectors for compensation of the density data, some have three. Most short spacing detectors are between 20cm and 25cm from the source.

For the geologist, when comparing data sets from different probe manufacturers, there can be subtle differences between the data sets as a result of the different probe specifications. However, for many purposes, the subtle differences will not be relevant.

LOGGING SYSTEM COMPATIBILITY LOGGING SYSTEMS:

Surface logging systems are different between the main logging manufacturers, it is their point of difference. The logging systems do not “talk” to the logging probes are other manufacturers (although some claim they do). Typically for a logging manufacturer’s probe to run on a different logging manufacture’s system there has to be some release/exchange of electronic copyright and an electronic upgrade.

                                                      It is not unusual for some logging contractors to have multiple logging systems for various probes in their logging unit to cover the requirements of the mining company. This means two sets of surface logging systems in the logging unit. More equipment, more storage, more chance of electronic or mechanical failure, and more runs in the borehole. Less reliability for the mining company and more chances of delay during a drilling program.

There are benefits to having a single logging system in your units:

No duplication of surface logging equipment.

More stable, ergonomic and efficient workspace.

Greater integration of logging probes across the fleet and a lower spare stocking required.

Less probes (risk) going into the borehole, quicker logging time.

Client is always getting the same specifications in their logs from the same probes

Lower manual handling requirements and the system are setup and secure in the logging unit. No need to be continuously unpacking and repacking transit cases at each borehole site.

A single and reliable logging equipment supplier leads to many efficiencies across the logging contractor and importantly all data is sourced from the one type of probe (i.e. all density comes from a same probe type), not least efficient operations which is what the mining company want

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Cost pressures on the mining industry means contractors are always under pressure to come up with innovative solutions to reduce costs. Geophysical logging is a valued service for mining producing accurate and cost effective data for ore body knowledge and mining solutions. Still the multiple logging runs required for a complete data set, especially when a drill rig is tied up by the service, means questions will always be asked, “Can we do this smarter?”

LOGGING PROBE STACKS STACKING SYSTEM:

A logging stack refers to a series of probes screwed together to form a single instrument with multiple sensors. This means a single pass in the borehole. The stack is assembled at the borehole prior to logging operations by a single operator.

LOGGING PROBE STACKS OPERATIONS:

A typical coal logging dataset could require up to 4 or more logging runs and a total of 8 hours to complete with a conventional logging system. Using the logging stack system, the number of runs can be reduced to 1 or 2 with logging completed in less than half the time. A time saving of 50% which means costs are reduced. “Time is Money”.

                  As an actual example, it is not unusual for a conventional mineral logging operation in coal exploration to have to make an additional logging run for magnetic deviation and potentially another extra logging run for temperature. Using a stacking system both magnetic deviation and temperature can be Incorporated into a single stacks, resulting in only 3 logging runs in a borehole to complete the required dataset.

In theory the arrangement of the logging probes in a stack is up to the user, however there are a few restrictions. For example, typical mineral radioactive probes have the source at the base of the probe, so typically probes cannot be stacked below this point. To overcome this restriction, side loading sources have been designed and are available today.

The logging winch remote system is an aid when the logging stack becomes too long for manual handling by a single operator. This option becomes available when logging off a drill rig or where high reach logging booms are available.

Another plus for the stack logging system is that when a probe fails, only that part of the stack needs to be replaced/repaired. In a conventional logging system if a detector fails then the whole probe has to be replaced/repaired.

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In the slimhole logging market, there are a wide range of vehicle based logging unit designs aiming to provide an efficient, safe and cost effective service. The design needs to cover a number of factors which can occur during everyday operations and hamper the efficient operations.

LOGGING UNIT DESIGN VEHICLE:

Due to the environment logging operations are undertaken (remote) the logging vehicle needs to have 4WD capability as it is very rare to have sealed bitumen access to the borehole. However, in today’s market, there are many vehicles with 4WD drive capability.

The vehicle needs a certain weight capability. Logging equipment, including the winch and wireline, is heavy. We have to comply with safe, legal vehicle limits. This rules out many “ute” based 4WD drive models.

Efficient access to remote borehole locations. To reduce access preparation costs and site clean-ups post drilling, drill pad size is kept to a minimum. Sometimes the site itself (on a hill side) can restrict the drill pad size. Therefore there is an upper size to the logging unit. It cannot be articulated in any form and is typically restricted to less than a small flatbed vehicle.

At Borehole Wireline, we decided on the Toyota Troopcarrier which ticks the box for the three factors above but many other factors contribute to the Troopcarrier being the vehicle of choice, such as reliability, regional availability of spare parts and back up maintenance.

LOGGING UNIT DESIGN CAB DESIGN:

Surface logging equipment is secured and mounted in a manner which allows safe and efficient access by the logger as well as safe carriage between boreholes during transit. Minimising the damage to essential electronic equipment during transit means less breakdowns, less equipment replacement, less freight and more uptime and efficiency. As a logger the last thing I want to do at every borehole is unloaded and unpack equipment from carriage boxes – as soon as I see this in the field, I think it is a scientific/academic exercise, or maybe just a one off remote hole to be logged, certainly not production logging..

            At Borehole Wireline, we have developed and built a drawer system to carry our probes safely with minimal impact on the electronics and making the ergonomics for the operator easy..

Distance to the model facilities commonly means a series of field calibration boreholes are made available, where core density values are known, to act as a calibration normalisation facility.

LOGGING UNIT DESIGN LOGGING OPERATIONS:

Logging operations need to be conducted in a safe, efficient manner. Tripods over the borehole are very in efficient, less than ideal from a manual handling viewpoint. Logging booms associated with remote winch operational capability, trestles for stack preparation, wireline washing capabilities to keep the wireline free of dirt/mud and large, high logging boom distance – all allow safe manual handling in the logging operation.

LOGGING UNIT DESIGN CUSTOM DESIGNS:

For small winches and remote locations, there are a large range of portable unit designs. All tend to be designed around the size of the winch.

Logging units can be designed for a specific purpose. We have designed and used logging units which can be operated from the driver’s seat. For short, large number of boreholes in a blasthole logging environment, our innovative forward facing logging boom and probe holster allowed operation from the driver’s seat after an initial setup on the 1^st borehole. This enabled a production number averaging in excess of 100 boreholes logged in a single 12 hour shift.

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The acoustic scanner is an ultrasonic borehole imaging probe which is commonly run in combination with an optical scanner for detailed, high resolution images of the borehole through 360°. Ideal and commonly used for structural studies and geotechnical investigations but has applications in casing inspection and insitu stress analysis.

BOREHOLE IMAGING: ACOUSTIC SCANNER METHOD:

The acoustic scanner is a borehole imaging probe which is capable of high resolution ultrasonic images of travel time and amplitude thus creating an image of the full 360 degree borehole wall. A single transducer in the acoustic head acts as both a transmitter and receiver of the ultrasonic pulses using sequential timing of the rotating mirror. It is the rotating mirror above the transducer with focuses the acoustic beam and provides 360° coverage of the borehole wall in the direction of logging creating a helical spiral of data.

Two images are produced:

Travel time of the ultrasonic signal from the probe to the wall and back, acting as a high resolution caliper.

Amplitude of the ultrasonic signal, or signal strength, relating to rock hardness.

The acoustic scanner requires a fluid in the borehole to allow transmission of the ultrasonic signal, together with a central position in the borehole to allow the ultrasonic signal to be perpendicular to the borehole wall. The technique works best in boreholes with a smooth wall, namely cored boreholes, where the acoustic signal is perpendicular to the borehole leading to little dispersion of the return signal. However much of the value of the technique is the ability to generate good images from non-cored boreholes.

BOREHOLE IMAGING: ACOUSTIC SCANNER CALIBRATION:

Of importance for the acoustic scanner is the marker position which is established during manufacture and/or servicing. The marker position acts as the reference point for all subsequent image orientation. Internal magnetometers and accelerometers, used to probe and image orientation are factory calibrated.

BOREHOLE IMAGING: ACOUSTIC SCANNER VERIFICATION:

Verification, performed using jigs or project boreholes where a known reference or structure orientation is available, focus on confirmation of the marker position and the functionality of the internal magnetometers and accelerometers.

BOREHOLE IMAGING: ACOUSTIC SCANNER DATA PROCESSING:

There are two main areas of image data processing:

Orientating, filtering and de-spiking the images, as well depth validation.

Picking and classifying or structures through to true structure dip and dip direction generation.

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Geophysical borehole density logging is a rapid, cost effective and accurate method to obtain bulk density values for ore modelling. In many mineralisation styles, the density value can be related to the ore grade. Outside of ore grade the bulk density measurements can be used for overburden stripping tonnage estimation or as a porosity measurement.

Borehole Density Measurements – Geophysical Logging METHOD COMPARISON:                

Density values can be calculated from core and/or bulk trenching, as well as geophysical borehole logging. Some mining operations prefer the former methods at the expense of geophysical borehole logging. Both core measurements and bulk sampling have downsides when compared to geophysical borehole density measurements.

Density measurements from a core sample (commonly the core volume has been reduced from the original volume) sample volume is the sample itself. The geophysical density measurements, as with many other geophysical borehole measurements, is influenced by a volume of the rock far greater than the borehole itself. Other factors which influence the core measurements are the handling of the core from the barrel to the laboratory which can have a significant influence on the measurement. The laboratory measurement is as good as the sample received.

Density measurements from bulk sampling through trenches would have the largest sample volume but sample handling and expense of the methods must be questioned

Borehole Density Measurements – Geophysical Logging  COMPARING DATA SETS:

Understanding the different density measurement methods and their data format is important when comparing the method values. It is not uncommon for core density measurements to be presented every 1m, every 2m up to every 10m. There is great potential for averaging errors. Geophysical borehole density measurements are much more consistent and high resolution compared to the core data. Look at the example below, where thin high density intervals are highlighted in the ore zone. The averaging of the core values has completely eliminated these intervals.

Geophysical density measurements are easily presented through re-sampling in the same data sampling format as for core measurements – see red curve below.

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BLASTHOLE LOGGING IN COAL PROJECT & GOALS:

Identification of coal seam parameters (density, top boundary, bottom boundary and partings) within pre-split blastholes prior to blasting.

BLASTHOLE LOGGING IN COAL DATA SETS:

Gamma, dual spaced density, borehole diameter and magnetic deviation data are acquired, rapidly and cost effectively in blastholes

Data in entered into the mine modelling package before blasting so the blast programme can be modified with the updated geological data.

BLASTHOLE LOGGING IN COAL DATA COMMENTS:

The geophysical density data provides accurate data on the coal seam parameters and distribution. In this example the geophysical data has been displayed as a section showing proximity of an intrusion which has changed the coal seam physical properties. The resulting burnt coal is not a mineable commodity and the blast programme was adjusted accordingly.

BENEFITS:

Integrating operational blasthole data into a mine modelling package significantly improves geological modelling which leads to improved coal predictions, improved structural interpretations including fault prediction, detailed mapping of heat affected areas and better positioning of the coal roof location.

REFERENCE:

“Integrating blastholes into exploration data to improve coal recovery and modelling”

Helgi Stedman 2014, 9^th International Mining Geology Conference, Adelaide, South Australia.

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DENSITY LOGGING APPLICATION:

Measurement of the formation involves a small radioactive source (CS¹³⁷ or CO⁶⁰) acting as a gamma radiation source to induce back scattering of gamma radiation in the formation. The back scattered gamma energies are detected at a range of scintillation detectors (typically two but can be three detectors) within a collimated shield. The level of back scattered radiation measured is inversely proportional to the electron density of the rock surrounding the borehole. More back scattering occurs in dense rock which results is less gamma radiation being detected. Over a density range of 1.5 g/cc to 3.0 g/cc where the atomic mass to atomic number ratio approaches 0.5, the measured electron density is said to equate to bulk density. Outside of this density range some adjustment factors are required.

DENSITY LOGGING METHOD:

Openhole formation density logging probes are designed to minimise borehole effects (diameter, rugosity) but housing the detectors in a dense tungsten shield with a thin vertical window for measurement. A single caliper eccentrallises the vertical measuring strip against the side of the borehole.

DENSITY LOGGING CALIBRATION:

Master calibration typically occurs in models such as the Adelaide Models, AM11 and AM8. Equally a series of large defined blocks can be used.

Distance to the model facilities commonly means a series of field calibration boreholes are made available, where core density values are known, to act as a calibration normalisation facility.

DENSITY LOGGING VERIFICATION:

A site verification borehole is normally established within the project area which is routinely logged weekly or fortnightly. If the measured density values fall out of a specified range (typically +/- 2%) the probe would be re-normalised before any further logging.

DENSITY LOGGING DATA PROCESSING:

With the multiple detectors in a modern formation density probe, additional data processing to compensate the detector measurement for borehole effect is common. Compensated density data can be further filtered or re-sampled to fit the same measurement style as other density measurements such as bulk sampling where a density value can refer to 1m or more blocks.

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