By Nick Jervis Bardy

The WIREBmr^TM or Borehole Magnetic Resonance is a downhole geophysics tool that measures water contained in subsurface environment safely and accurately. WIREBmr^TM is specifically tuned to sense fluids within pore network, enabling precise determination of rock’s total porosity, mobile fluid content (specific yield), bound fluid content (specific retention) and permeability (convertible to hydraulic conductivity). The technology differentiates between moveable and non-moveable water fractions and reliably predicts how easily groundwater flows through an ore body eliminating the need for other traditional methods that require radioactive sources and produces a lithology independent result in contrast to traditional methods such as density and neutron logging.

Theory

Nuclear magnetic resonance (NMR) is a phenomenon that results from the interaction of these nuclei with external magnetic fields. In a volume of water, or other hydrogen-containing fluids, the magnetic fields of the various hydrogen nuclei in the different molecules will be randomly oriented. If an external magnetic field is introduced, these nuclei will align themselves with the external magnetic field.

The application of a second with appropriate frequency for an appropriate time interval will tip the nuclei by 90 generating an oscillating electromagnetic field that can be detected. A series of subsequent secondary magnetic field pulses, called a CPMG pulse sequence, to refocus the nuclei and generate a measurable decay.

In the subsurface the rate of this decay is controlled, primarily, by interactions between the hydrogen nuclei being stimulated and the pore walls. This interaction directly reflects the pore size distribution of the rock. The measured decay can be processed into a T2 distribution that reflects total water measured in the rock as well as the pore size distribution.

Application

NMR has been used in the oil and gas industry to measure pore fluid and for fluid typing for 30+ years, WIREBmr^TM brings this technology to other markets with a smaller size and lower price point than its oil and gas equivalents. has been used for broad application across. It has been applied across hard rock mining, in-situ recovery mining, groundwater and geotechnical spaces including:

• Mapping total porosity, specific yield and dry bulk density in iron ore deposits to determine blend for feed stock to the crusher, quantify resource and pick open/closed fractures (in combination with ATV/OTV).
• Developing dewatering strategies in and around underground and open pit mines.
• Mapping aquifer hydrogeology to guide development of comprehensive groundwater management strategies.
• Mapping brine hydrogeology to determine the economic viability and shaping development strategy of brine mining operations.
• Map coal seam gas content and permeability distribution for optimization of gas production for both coal and CSG.

Data Processing

Data processing involves QAQC of raw data to properly mark data that has been affected by hole conditions (washouts, magnetics, conductivity) as well as the generation of a T2 fluid distribution, fluid volume estimates and permeability / hydraulic conductivity estimates that using multiple models.

Further Reading

Contact Orica Orebody intelligence at www.orica.com/wirebmr or Orica’s APAC Regional Manager, Calvin Lau, directly at: calvin.lau@orica.com for more information as well as processing, interpretation, and training services.

Service Agreement

The WireBMR service is run by Borehole Wireline through a 3rd party agreement with Orica Digital Solutions.

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Application:

The optical scanner is a high resolution CMOS camera borehole imaging probe which is commonly run in combination with an acoustic 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.

Method:

The optical scanner is a borehole imaging probe which is capable of a high resolution, true colour image thus creating an image of the full 360 degree borehole wall. The high resolution CMS camera system is focussed through a prism to all 360° slices of the borehole wall to be recorded during a survey.

A single, true colour image is produced from the survey.

The optical scanner operates in air filled and/or clear fluid filled conditions. Ideally the probe should be centrallised in the borehole to avoid unwanted shadow effects.

Calibration:

Of importance for the optical 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.

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.

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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Application:

Resistivity logging involves the measurement of electrical properties of the rock. Changes in the rock’s electrical properties can be due to the clay mineral content, water content and porosity, temperature and water conductivity.

Method:

Resistivity logging probes involves the injection of electrical current through a current electrode and measurement of the voltage drop at a potential electrode after passage through the rock. The dual focussed laterolog is an electrical probe with a short and long electrode measuring different rock volumes away from the borehole. Focussing of the electrical injection current is achieved through auxiliary electrodes whose job is to create a sheet like injection. The focussing allows excellent true formation resistivity measurement and good vertical resolution of thin beds.

Calibration:

A calibration box with a range of resistors capable of setting values from 1 through a 100 ohm-metres is used for calibration and verification.

Data Processing:

Commonly used for lithological identification and correlation, advanced data processing can involve derivation of the Formation Factor and groundwater salinity. For hydrocarbon exploration, resistivity data has been used for water saturation and by inference hydrocarbon volumes through Archie’s Law.

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Project:

Rehabilitation of old industrial land is common place today for future land re-use. Shallow drilling and geophysical borehole logging is common practise to map the location of subsurface contaminants from the old industrial sites. This project in an urban setting involved the logging of 40 shallow boreholes.

Aim:

Induction-conductivity logging was used to map potential contaminants throughout the project area. Common industrial contaminants can have distinctive resistive and/or conductive signature.

Operations:

The induction-conductivity logging was undertaken in 40 boreholes across the project area after the completion of all drilling and within shallow 50mm PVC cased boreholes.

Benefits:

High resolution identification, correlation and mapping of sub surface contaminants.

Example #1:

Induction conductivity data from a single investigation borehole. See the significant increase in conductivity values at depth related to an industrial contaminant.

Example #2:

Borehole collar locations are taken from survey data.

Correlation section using multiple borehole conductivity data to map the distribution of a saline industrial contaminant across the project area.

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Application:

Induction-conductivity logging involves the measurement of induced electromagnetic fields in the rock and resulting eddy currents. Changes in the rock’s electrical properties can be due to the clay mineral content, water content and porosity, temperature and water conductivity. Induction-conductivity logging can be performed in an air and/or water filled borehole and with the probe inside PVC. It is a common tool for the shallow environmental contamination market.

Method:

Induction-conductivity logging creates a magnetic field around the probe’s transmitter which in turn creates eddy currents based upon the formation’s electrical conductivity. The conductivity response is based upon factors similar to the resistivity probe such as moisture content, water salinity, clay minerals and porosity.

Calibration:

Calibration rings, each with an assigned conductivity, are used for calibration and verification. An electrically quiet area is essential for calibration to avoid unwanted electrical interference.

Data Processing:

Commonly used for lithological identification and correlation, advanced data processing can involve derivation of the Formation Factor and groundwater salinity. For hydrocarbon exploration, the induction-conductivity data has been used for water saturation and by inference hydrocarbon volumes through Archie’s Law.

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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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