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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INROD LOGGING AND REGIONAL CORRELATION – PROJECT:

Many mature mining provinces have a record of natural gamma logs from boreholes which have been used to establish a regional stratigraphy. One example is the Pilbara in North-West Australia. The gamma peaks correlate to shales and clay horizons which act as regional markers in the iron bearing formations.

INROD LOGGING AND REGIONAL CORRELATION – AIM:

Most active mining companies use gamma logging inside the drill rods and immediately after the completion of drilling so the site geologist can calculate if the target horizon has been reached or if further drilling is necessary.

INROD LOGGING AND REGIONAL CORRELATION – OPERATIONS:

Gamma logging is undertaken inside the drill rods immediately after the completion of drilling. If the target horizons have not been intersected then further drilling can be undertaken after the completion of the inrod survey.

INROD LOGGING AND REGIONAL CORRELATION – BENEFITS:

Active control of the drill program and the ability for the site geologist to undertake additional drilling whilst the drill rig is set up over the borehole. Thus avoiding costly moves of the drill rig.

Gamma stratigraphy is a powerful tool available to the site geologist.

Gamma peaks from an inrod gamma survey represent shale/clay horizons which are regionally extensive marker horizons.

Borehole collar locations are taken from survey data.

Gamma logs are presented in true vertical depth.

Site geologists can correlate the gamma peaks between boreholes to reveal regional and local geological structures.

This empowers the site geologist to anticipate structure and/or be depth  in the current borehole being drilled.

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The acoustic scanner and 3 or 4 arm dipmeter probes have been used for coal exploration for a long time. It would be fair to say that the acoustic scanner has superseded the 3 or 4 arm dipmeter as being the preferred method for structural or geotechnical analysis. Both methods have their strengths and weaknesses.

Acoustic Scanner

The acoustic scanner is an acoustic echo imaging device which, under the right conditions, provides an orientated (magnetic north or borehole highside) image of the acoustic impedance borehole wall and a multi-fingered caliper profile. From the resultant travel time and amplitude image, structures can be identified, “picked/orientated” for dip and dip direction and classified into fractures, fabric, veins and so on. At the present day it is a routine service provided by mineral logging companies and a routine requirement in any geotechnical investigation or exploration programme now enhanced by the optical scanner which provides high resolution, true colour images of the borehole wall.

3 or 4 Arm Dipmeter

The mineral logging 3 or 4 arm dipmeter is an electrical device, measuring the resistivity of the borehole wall through three or four pad pushed against the borehole wall. A single resistivity curve is produced from each pad. From the arms of each pad a caliper measurement of the borehole diameter is made. Data processing involves a cross correlation technique to identify patterns in the resistivity curves and resultant dip and dip direction of structures. We should note here that the typical mineral logging dipmeter is quite different to the resistivity imaging probes used by the oil industry which have multiple buttons on each pad to record the borehole wall resistivity. The mineral logging dipmeter is a single measurement per pad.

Whilst the acquisition methods are different, the final outputs appear similar and are of great value to the geologist or geotechnical engineer.

Acoustic Scanner: Strength & Weakness

+             greater coverage of the borehole wall.

+             common availability and applications (cased hole and openhole).

+             structural geology, geotechnical and sedimentological applications.

_             Manual “picking” process for structures.

3 arm dipmeter: Strength & Weakness

+             automated processing. Product available at the logging unit.

+             geologists understand the product.

+             a sedimentological tool.

_             limited coverage of the borehole wall.

_             limited availability (Most logging equipment manufacturer’s do not make the system.

Automated structure “picking” for acoustic images is being developed.

It is not unusual for a dipmeter to be required for coal exploration projects today but with education the geologists can become comfortable with the acoustic scanner method.

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Deconvolution is a common process in seismic process where the objective is to “sharpen” the seismic response at depth. The process is necessary as the earth through which the seismic source pulse travels is altered by the rocks which act as a filter to broaden the seismic response at depth.

The rock in a uranium bearing sequence acts in the same way, acting as a filter to broaden the gamma response. Deconvolution of gamma logs aims to strip off the geological effect and realise the in situ distribution of the uranium ores.

Additional to these effects are the probe housings and detector size and electronics, all combining to potential reduce the accuracy of the gamma logs.

At present, deconvolution of gamma logs is considered as specialised processing hence its use has been limited. Furthermore whether the process is actually necessary is sometimes questioned by Uranium geologists.

Recent work by Bruce Dickson has proposed another method for deconvolution of gamma logs using space domain filtering and inputs for detector size and uranium ore zone thickness. Software is being developed to apply Dickson’s work to current gamma data sets.

Gamma logs

Gamma logs are a fundamental part of cost effective uranium exploration. Ideally we would like gamma logs to give exact information on the quantity and distribution of uranium ore with depth along the borehole. In the ideal world, the gamma log intensity would be exactly proportional to the uranium grade and any given depth. However, there are many factors which interfere with the ideal log and distort the shape of the gamma response.

The distortion of the gamma response is related in part to the physical properties of the surrounding rock (known as the geological impulse function) and in part to the features and design of the measuring equipment (known as the system response). It is said the geological and system factors are convolved together with the uranium distribution

The example above is interpreted as a series of very thin uranium bearing zones, yet the gamma log response shows a complex peak anomaly – mixing the response of the all thin zones. the aim of deconvolution is to resolve the gamma data back to the thin uranium bearing zones, free of local geological and logging system effects

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The most common measurement systems involve a scintillation system where gamma photons are converted to light energy enabling digital count rate conversion. The most common crystal used in the scintillation system is sodium iodide. Research has been undertaken into other crystals (such as the Brilliance project) with limited success.

Common Gamma Logging Techniques

The most common gamma logging technique is “total gamma” logging where the whole gamma radiation energy spectrum is measured. The size of the crystal used in the scintillation system of a total gamma probe can vary depending upon the requirement for sensitivity versus potential saturation of the scintillation system, Crystal sizes approaching 25mm diameter and 50mm length are common.
As gamma logging evolved, hybrid gamma probes were developed and made available to investigate specific areas of gamma radiation and/or to redefine the measurement range:

• Spectral gamma logging measures count rates within the scintillation system with certain specific energy windows of the gamma radiation spectrum with the aim of quantifying the amount of the common naturally occurring isotopes of Potassium, Uranium and Thorium. Quite a slow rate of logging due to the large crystal size and low count rates, it has been particularly useful in identifying the presence of Uranium over Thorium of vice versa.
• Filtered gamma logging uses thin layers of a range of metals (typically lead, copper or brass or combinations) around the scintillation detection system as a shield against low energy gamma radiation. The low energy gamma radiation is thought to have a negative effect upon the uranium ore grade measurement particularly where radon gas is produced.
  Calibration of gamma probes is an important part of process of producing a depth based profile of uranium ore grade.

Calibration
The calibration process requires a borehole environment with a range of accurately known grades of Uranium surrounding the borehole. The uranium bearing zone needs to be a minimum thickness and a minimum diameter to allow an infinitesimal response – meaning the probe (instrument) sees only the uranium grade and nothing else.

• Typical dimensions of the calibration environment are 1m vertical thickness and greater than 2m in diameter. This is achieved at the Adelaide Models through models AM1, 2, 3 and 7.
• Instrument factors to calculate during the calibration process are detector deadtime and instrument K factor.
• The calibration unit is eU3O8 where the e prefix indicates derivation from gamma logging.The calibration process requires a borehole environment with a range

Borehole Environmental Corrections.

Environmental corrections need to be calculated at the time of calibration or later in the field. The corrections are required because of any change in logging environment conditions from the calibration to the uranium exploration or mining project. These environmental corrections include:

• Changes in the borehole diameter.
• Changes in the presence of fluid/air in the borehole.
• The presence of drill rods and/or casing at the time of acquisition.
• The density of the rock surrounding the borehole.
• All the above affect the gamma probe response

Data Processing – Deconvolution.

In addition to environmental correction of depth based gamma uranium grade data, some data processing techniques have been developed, the chief of which is the deconvolution of the gamma log. The ideal response of a gamma log is shown below

For a thin uranium bearing zone in a rock sequence otherwise barren, the plot of Uranium concentration becomes a broader based peak due to how we make the measurement and where we take the measurement. These effects can be summarised as:

• Geological impulse response which reflect where the measurement was taken, such as gamma ray energy, formation density, fluid content etc.
• System function response reflects details of the data acquisition system such as crystal size, housing thickness, shielding etc.

As the above effects are convolved (mixed) with the depth based uranium grade measurement made during gamma logging. The aim of deconvolution is to “taken away” these effects and give the cleanest depth based uranium grade measurement.

Disequilibrium

Uranium disequilibrium is a concern for some uranium explorers where the uranium system is within a sedimentary environment with geologically recent groundwater circulation. Under such conditions, gamma logging results can be affected by the movement of uranium with respect to its daughter products. These concern relates to gamma logging responding to the uranium daughter products and not the uranium itself and hence the potential for false positives or false negatives.

• Early techniques to overcome the disequilibrium effect was to compare gamma logging grades to laboratory grades from core and calculate a disequilibrium factor to be applied to the gamma logs.
• Neutron logging probes (PFN) were developed to directly detect the uranium. The early models of the PFN were expensive, had a poor reliability record and poor repeatability at low uranium grades proved problematic and affected the use of the logging technique.
• More advanced neutron activation logging techniques (APFN) are being developed to increase reliability of the method and measure a geochemical profile of the formation.
• Advanced gamma-gamma density logging techniques are being developed to measure the geochemical profile of the formation with a view to directly detecting uranium through photo electric capture.

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Calibrated data, this is often stipulated by a client in a contract but the calibration is commonly misunderstood.

Calibration

A definition of calibration is:
A set of operations which establish under specified conditions, the relationships between values indicated by the measuring instrument and corresponding values of a quantity of a reference standard.

It is important during a calibration the measuring instrument (probe/detector) sees only the reference standard. This can be simple for some detectors such as a caliper arm where each position has been manufactured for a specific diameter. For the radioactive detectors in total gamma, density or neutron porosity probes, the volume of investigation can dictate that very large reference materials are required. For example, the Adelaide Models Uranium reference standards are 1.2 metres in diameter and the mineralised reference zone is at least 1 metre in thickness.

It is not unusual for the calibration to be undertaken at the logging equipment manufacturer’s workshop or logging contractor’s workshop or a specialised facility such as the Adelaide Models.

Normalisation

A definition of normalisation is:
Adjusting one set of instrument values to another set of instrument values.
In mineral geophysical logging, normalisation is achieved through logging a local reference borehole which is located on the client’s property and intersects their mineralisation.

Most of the major iron ore and coal miners have established a series of boreholes for such normalisations. The normalisation does allow some consistency of data between probes and logging contractors and repeat logging over time can be used to indicate precision of the instruments. It is not uncommon for normalisation to be required by the client every 2 weeks of continuous operation.

Verification

A definition of verification is:
An operational check of part or all of a probe’s detector functionality in well defined conditions and on an ongoing basis. In some cases, the verification could use the same apparatus as for a calibration. The caliper jig or ladder is an example. Some verifications require special apparatus in the case of a jig for a total gamma probe in a uranium program. Some verifications can employ the same boreholes used for normalisation.

At the time of a verification, if the results fall outside of an acceptable range, then normalisation or calibration is required. Verification frequency can be daily, before/after logging, fortnightly or monthly.

Each of the above processes are related to each other and allow the ongoing performance of logging equipment to be measured and form part of the data quality assurance program.

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A standard configuration used in today’s coal industry uses a single transmitter and 4 receivers spaced 60cm, 80cm, 100cm and 120cm away from the receivers.

In the photograph below, the fullwave sonic probe is shown hanging vertically with the single transmitter at the base of the probe. Upwards, there is a low velocity compounded section to prevent direct arrivals, followed by the 4 receivers spaced every 20cm. The ray paths of the sound wave pressure pulses are shown in green between the transmitters and receivers

Next to each receiver (in black/white) is a sample of a short recording of the sound wave at each receiver. The millisecond recording allows the compressional, shear and tube wave arrivals to be identified.

Further to the right is a sample output of the fullwave sound wave output at each receiver and over a distance of 20m. This illustrates what the logger can see during data acquisition.

Logging requires a fluid in the borehole to propagate the pressure pulse between the transmitters and receivers and allow the measurements to be made.

Data processing

The most common method of processing fullwave sonic data is called “semblance processing” using softwares such as Wellcad. Essentially semblance processing is a form of signal processing where “in phase” amplitudes when added together increase the signal but “out of phase” amplitudes when added together cancel each other out. The result is a significant decrease in the signal to noise ratio resulting in the compressional, shear and tube wave arrivals to be clearly identified and “picked” to generate a curve.

Products

The typical products of a fullwave sonic include digital arrival curves for the compressional, shear and tube wave arrivals. From the compressional and shear wave arrivals, Poisson’s Ratio can be calculated. If density data is available the range of calculable rock elastic properties increase to Young’s modulus, shear modulus, bulk modulus etc.

Sonic Transit Time Probes

The sonic transit time only probes are older versions and only record the compressional wave arrival time. No fullwave data is recorded. Multichannel curves are commonly generated from this probe (and also generated by the fullwave sonic) representing a range of receiver pairs and their respective distances – covering 20cm, 40cm and 60cm.

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• Acoustic imaging: requires a fluid filled borehole
• Optical imaging: need to be able to see the borehole wall (air or clear fluid)

Both methods are commonly requested on the same borehole due to water level being mid borehole and/or the water/fluid in the borehole is opaque and dirty. An upper optical section above the fluid level and a lower acoustic section in the borehole fluid.

Applications

Common applications are:

• Geotechnical site investigations where slope stability, rock support are key requirements.
• Tunnelling
• Structural geology.
• Casing integrity.

Borehole imaging techniques.

The imaging techniques provide an orientated image of the borehole wall (by orientation we mean a Magnetic North reference or a reference to the high side of the borehole).

Both data outputs can be used to “pick” structures for classification, orientation:

• Optical images are commonly much higher vertical resolution compared to the acoustic method, additionally the optical images are true colour and can show great detail on the rock’s character.
• Acoustic images are false colour and represent acoustic impedance contrasts of the rock. Whilst lower vertical resolution to the optical images, the acoustic data can provide high resolution caliper data to be used in subsurface stress studies and casing inspection.

The images below show the different strengths of the optical and acoustic borehole imaging methods. The optical image is to the left and the amplitude image from the acoustic scanner is to the right (both images are orientated to Magnetic North)

• Much higher vertical resolution in the optical image. See the gravel band.
• True colour of the rock in the optical image
• The sedimentary laminations can be identified in both the acoustic and optical images.
• The bright, high amplitude signature of the acoustic image indicates a hard, competent rock.

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The optical scanner uses a high resolution digital camera together with magnetometers and accelerometers to provide an orientated true colour image of the borehole wall.

Principal features

• Identifies and orientates (measures the dip angle and dip direction) of fractures, joints or other rock breaks.
• Measure the frequency of fractures/joints with depth

Applications

• Geotechnical site investigations where slope stability, rock support are key requirements.
• Tunnelling
• Structural geology

Technique

A single image is produced through a digital video camera which can capture image data at a rate of 60 frames per second. Full pixel by pixel colour calibration occurs to ensure a perfect colour balance.

The camera is focused through a prism allowing 360° slices of the full borehole wall to be recorded.
A ring of LED lights provides light for the digital camera

Orientation of the image data

Orientation of the image (by orientation we mean a Magnetic North reference or a reference to the high side of the borehole) is achieved using a three axis magnetometer and a three axis accelerometer inside the probe.

Borehole and Probe Requirements

The following conditions in the borehole are required:

• The probe needs to “see” the side of the borehole wall
• Air filled boreholes.
• Clear fluid filled boreholes.
• The probe needs to be centrallised – achieved by using two spring centrallisers located towards the top and bottom of the probe.
• Borehole diameter range:
  – Minimum diameter of 50mm.
  – Maximum diameter up to 400mm depending viewing conditions.

Data acquisition settings

• Pixel resolution settings range from 720 dpi up to 1440 dpi.
• Optical image resolution below 1mm is possible. The greater the image resolution, the lower the logging speed.
• User control over light, frame rate and exposure settings covers most borehole conditions.
• Optical scanner data files can be very large.
• Data transfer speeds of the logging system control the logging speed during acquisition. A logging speed of around 4 m/min is not uncommon.
• Newer logging systems allow a higher data transfer speed, hence a greater logging speed for the same image resolution.

Data Processing

Once the image data has been acquired, the power of the borehole imaging technique is that structures which cut across the borehole can be identified and have the dip angle and dip direction individually measured.

Remember the “unwrapped image”? In this format, any dipping structure forms a sine curve from which the structure’s dip angle and dip direction (initially with respect to the dip and dip direction of the borehole) can be calculated.

Whilst the technique could be likened to structure orientation from drill core, the acoustic scanner data is more diverse, more accurate and much more time effective.

The final product

Once the image data has been acquired, the power of the borehole imaging technique is that structures which cut across the borehole can be identified and have the dip angle and dip direction individually measured.

• Structures are orientated and classified, then ready for presentation as a list and/or in a stereographic projection.
• Combined with other geophysical or geological data from the borehole
• 3D borehole image representation

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The acoustic scanner, also known as the acoustic televiewer, uses high frequency sound wave echoes together with magnetometers and accelerometers to provide an orientated travel time and amplitude image of the borehole wall.

Principal features

• Identifies and orientates (measures the dip angle and dip direction) of fractures, joints or other rock breaks.
• Measures  the frequency of fractures/joints with depth
• High resolution caliper tool

Applications

• Geotechnical site investigations where slope stability, rock support are key requirements.
• Tunnelling
• Structural geology

The technique.

Two images are produced which are presented as “unwrapped” images (imagine pealing a layer of skin from the borehole wall and laying it on a flat surface):

• A Two Way Travel Time (TWTT) image which represents the time taken for the acoustic pulse to travel from the acoustic source in the probe through the borehole fluid to the borehole wall and back through the borehole fluid to the probe. It represents a detailed, high resolution caliper curve.
• An amplitude (C AMP) image which represents the “strength” of the recorded acoustic pulse in the probe after reflection back from the borehole wall. The image is a false coloured image of acoustic impedance contrast. Essentially bright colours represent high amplitude signal, hard rock and dark colours represent low amplitude signal, soft rock or borehole fluid.

How the image data is sampled?

The 360 degree image coverage of the borehole wall is obtained through a rotating, angled mirror below an acoustic transducer which through clever timing synchronisation acts as a transmitter and receiver of the ultrasonic echoes. Another feature of the probe which increases the vertical resolution of the image data is focusing of the acoustic echoes through a curved and angled mirror.

Orientation of the image data

Orientation of the image (by orientation we mean a Magnetic North reference or a reference to the high side of the borehole) is achieved using a three axis magnetometer and a three axis accelerometer inside the probe.

Borehole and Probe Requirements

The following conditions in the borehole are required

• A fluid in the borehole:

– An ultrasonic signal cannot be sustained in air.
– The fluid can be water (of any salinity) or drilling mud.
– Heavy drilling muds can affect the signal quality.

• The probe needs to be centrallised:

– Achieved by using two spring centrallisers located towards the top and bottom of the probe.
– Allows the acoustic echoes to be reflected perpendicular to the borehole wall.

• Borehole diameter range:

– Minimum diameter of 50mm.
– Maximum diameter up to 400mm depending upon borehole fluid properties.

Data acquisition settings

• The number of data samples per revolution of the acoustic mirror varies between acquisition systems.
• Variations include every 90 (72), 180 (144), 270 and 360 (288) samples per revolution.
• Lower rotational sample rates are common for smaller diameter boreholes (180 (144) common for 96mm diameter)
• Larger diameter boreholes demand greater rotational sample rates and a slower revolution of the acoustic mirror.
• Most acoustic scanner logging systems require a minimum and maximum setting for the expected acoustic echo. This is typically borehole diameter and fluid dependent. Some logging systems have an “automatic mode” where digital capture settings are based upon the received echo features.
• Acoustic scanner data files are large.
• Data transfer speeds of the logging system control the logging speed during acquisition. A logging speed of around 2 m/min is not uncommon.
• Newer logging systems allow a higher data transfer speed, hence a greater logging speed for the same image resolution.

Data Processing

Once the image data has been acquired, the power of the borehole imaging technique is that structures which cut across the borehole can be identified and have the dip angle and dip direction individually measured.

Remember the “unwrapped image”? In this format, any dipping structure forms a sine curve from which the structure’s dip angle and dip direction (initially with respect to the dip and dip direction of the borehole) can be calculated.

Whilst the technique could be likened to structure orientation from drill core, the acoustic scanner data is more diverse, more accurate and much more time effective.

The final product

Once the image data has been acquired, the power of the borehole imaging technique is that structures which cut across the borehole can be identified and have the dip angle and dip direction individually measured.

• Structures are orientated and classified, then ready for presentation as a list and/or in a stereographic projection.
• Combined with other geophysical or geological data from the borehole
• 3D borehole image representation

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