“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.