“Geophysical logging & Borehole Imaging – understanding the measurement.”
AIG Conference: pushing the boundaries: new frontiers in technology,
Macedon Ranges Hotel, Victoria, 7th October 2016.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Origin:
Most techniques derived from the oil/gas wireline industry.
• Usage:
Mining, groundwater, geotechnical, environmental.
Surface & underground.
• Instrument physical range:
30mm-80mm diameter.
Up to 4m in length.
• Instrument environmental limits:
80°C/20 MPa.
2000m depth.
• Single operator.
• Service areas.
Inrod logging occupies the drill rig
Geophysical logging: The logging service.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
A geophysical log (downhole log, wireline log, well log):
• Remote measurement(s) made by an electronic instrument in the
borehole. (probe, tool, logging tool, stack).
• Multiple sensors.
• Regularly sampled profile.
• Data transferred to a surface unit via electrical conductors.
• Data viewed at the surface in real time.
• Probe/stack move during the measurement process.
• Response strongly dependent upon physical properties of rock.
Benefits
• Objective and repeatable data.
• Measurements made “in situ”.
• Typically, measurements are influenced by a zone significantly greater
than the borehole.
• acquired and processed at the drill site.
• Calibrated, depth verified data available at the drill site.
• Location, borehole, drill data linked to the data set.
Geophysical logging: Definitions.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
Geophysical Logging: Measurement scale.
• Link between geology and surface geophysics.
Samples more than the borehole volume.• Borehole imaging approaches geological scale.
GEOLOGIST GEOPHYSICIST
borehole
¼ core sampleinstrument
sample volume
Limited Coverage Limited Resolution
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
Geophysical Logging: Measurement Precision.
• Precision: repeatability of the measurement.
confidence in data.
Poor repeatability.
60% - 70% outside
acceptable range.
Good repeatability.
All close to true value.
• Geophysical logs are a good source of precise data.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• What effects the response:
the rock grains:
mineralogy.
the binding agent:
cement.
the space between grains:
amount of porosity
type of porosity.
what fills the pore space.
Geophysical logging: Geological component of the log response.
Coal
Shale - mineralogy change
Gold - disseminated, unseen
unlikely log response
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Precise, accurate calibrated data allow further analysis:
Use of crossplots to define
rock type.
mineralisation.
grade.
Geophysical logging: Crossplot Analysis & Automatic lithology generation.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
Borehole imaging involves high resolution vertical & radial sampling allowing an
image to be presented of the borehole wall
Typical image vertical resolution is 1mm-5mm.
Presented in “unwrapped format”.
Images are orientated by internal magnetometers/accelerometers.
Primary method of “insitu” structure orientation.
Magnetic North/ Borehole Highside orientation.
Data can be used to calculate deviation survey.
Gamma data commonly available.
The major borehole imaging methods available in slimhole market are:
• Acoustic - Ultrasonic:
Travel time and amplitude (signal strength) of reflected ultrasonic pulse.
False colour image of acoustic impedance contrast.
• Optical:
High resolution, true colour digital image.
Air or fluid filled boreholes (must be able to see = clear fluid).
• Resistivity/Induction:
Pad device.
Favoured technology in the oil industry.
Only just becoming available to the slimhole minerals market.
Borehole Imaging: Borehole Imaging Methods.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Borehole acoustic imaging probe.
multiple “echoes” around borehole measuring acoustic impedance.
• Multi-detector probe.
acoustic head: transmitter-receiver/rotating mirror
images: travel time & amplitude
image orientation: 3 axis magnetometers & accelerometers
• Logging/Sampling/Resolution.
90, 120, 180, 360 rotational sample selection.
> 3mm vertical feature resolution
probe needs to be centrallised in the borehole.
requires fluid filled borehole.
Borehole Imaging: Acoustic Borehole Imaging.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Travel Time.
two way: transducer-borehole-wall-transducer.
“multi-fingered caliper”
borehole volume, borehole shape
• Amplitude.
acoustic impedance contrast (false colour).
high amplitude: hard rock
low amplitude: soft rock, fluid
• Best Results.
centrallised probe.
smooth borehole wall (core).
clean borehole fluid.
• Reduced image quality due to.
Poor probe centrallisation.
Mud/debris on the borehole wall.
Thick borehole fluid.
Gas in the borehole fluid.
Borehole Imaging: Acoustic Borehole Imaging.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Non cored drilling types.
RAB, Reverse circulation.
Borehole imaging is the only method to obtain structure orientation data.
• Logging through PVC casing.
Probe located inside PVC casing.
Able to record image of openhole outside of casing.
Using appropriate timing gates & digital filtering.
PVC casing must be centrallised.
Borehole Imaging: Non Cored Boreholes - Acoustic Borehole Imaging.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Borehole optical imaging probe.
high resolution CMOS sensor.
• Multi-detector probe.
optical head: CMOS sensor focused through prism for 360 slice recording
images: true colour
Logging/Sampling/Resolution.
720, 1440 pixel resolution selection.
> 1mm vertical feature resolution
probe needs to be centrallised in the borehole.
Borehole Imaging: Optical Borehole Imaging.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Optical Image.
True colour image
high image resolution
• Best Results.
centrallised probe.
clean borehole wall (no mudcake)
clean borehole fluid.
• Reduced image quality due to.
Poor probe centrallisation.
Mud/debris on the borehole wall.
dirty, opaque borehole fluid.
Borehole Imaging: Optical Borehole Imaging.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Data Processing.
involves interactive “picking” of structure(s) identified in an image.
dipping structures form sine curves.
from sine curve, apparent dip/dip direction is calculated
further correction for true dip/dip direction
.
Borehole Imaging: Data Processing - Borehole Images.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Data Processing – Acoustic Images.
High resolution, multi-fingered caliper.
Ovalisation, in-situ stress directions.
casing inspection.
Borehole Imaging: Data Processing - Acoustic Images.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Structure picking.
Ability to automate and control structure picking.
Apply confidence value to the “pick”
Controlled processing work flow
Borehole Imaging: Developments - Borehole Images.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Cluster Analysis of Structures.
Common comment of image products is “too much data”.
Cluster analysis based on depth, dip, dip direction and confidence
More objective results
Borehole Imaging: Developments - Borehole Images.
Before Clustering After Clustering of “Blue” and “Grey”
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Optical Imaging.
Ability to separate optical colour image components.
Red, Green, Blue spectra
Luminance, Saturation, Hue
Correlate to geochemical, stratigraphic,
geophysical & geological data?
Can we correlate optical image properties to
mineral phases,
grain size distribution,
porosity.
Borehole Imaging: Developments - Borehole Images.
Geophysical logging & Borehole Imaging – understanding the measurement, 7th October 2016.
• Geophysical logs respond to the physical rock properties surrounding a borehole.
characterise, identify, depth, apparent thickness, grade, limiting factors.
• Geophysical logs support and enhance other datasets collected by the drilling program.
a precise dataset.
• Geophysical logging is a cost effective service, able to produce calibrated results at the drill site.
results available in paper and/or digital format at the borehole.
• Geophysical logging – Current Trends.
crossplot analysis for automatic lithology generation.
• Borehole Imaging - Service.
Acoustic & optical methods have been available since the late 1990’s.
Are a common, routine service on exploration and/or mine projects.
Routinely used in cored and percussion drilled boreholes for structure orientation.
• Borehole Imaging - Products.
Excellent source of orientated, classified, detailed structure data in cored boreholes.
The only source of orientated, classified, detailed structure data in non-cored boreholes.
Source of insitu stress measurements.
• Borehole Imaging – Current Trends.
Acoustic imaging from inside PVC casing.
Automatic structure picking, an integrated workflow, cluster analysis.
Optical image component analysis
THANK YOU – QUESTIONS ?
Summary.