This document discusses log quality control and image artifacts. It covers four aspects of LQC including acquisition tool data quality, processing dip data quality, image quality and artifacts, and image grading confidence limits. It then describes various types of artifacts that can impact image logs including acquisition artifacts from drilling or logging, borehole wall artifacts, processing artifacts, and derived artifacts. Specific artifact examples are provided for each category with images to illustrate common issues like washouts, breakouts, mudcake buildup, processing errors, and geological features that appear artifact-like. Recognition of artifacts is important for proper log quality control and interpretation.

1. LQC & image artefacts/1
Image log & dipmeter
analysis course
Dip & image LQC basics

2. LQC & image artefacts/2
Four aspects of LQC:
1. Acquisition/tool data quality
2. Processing dip data quality
3. Image quality, artefact images
4. Image grading - confidence limits
Image log and dip data LQC
LQC – Log Quality Checks

3. LQC & image artefacts/3
• Is the tool working properly?
– caliper reading in casing
– all sensors working?
– depth-match with other logs
– directional readings (inclinometers)
– repeatability
– cross-borehole correlation and lithology match
• Bad hole
• Curve character
• Tool rotation
• Mudcake build-up
• Pad pressure
• Hole deviation
Basic quality indicators – raw image data

4. LQC & image artefacts/4
Items to check for calculated dips:
– Processing parameters suitable?
– Do dips look similar on the repeat log?
– Borehole orientation and magnetic declination ok?
– Mathematical dips present?
– Mirror dips present?
– Dip quality appropriate for the observed curve activity?
Items for check for manual dips:
– Is the correct bit-size used for dip calculation?
Basic quality indicators – dip data
Use plots at appropriate scales for LQC
Standard plots are 1:500 and 1:200

5. LQC & image artefacts/5
1. Acquisition Artefacts
– Drilling
– Logging
2. Borehole Wall Artefacts
3. Processing Artefacts
4. Derived Artefacts
Basic quality indicators – borehole images

6. LQC & image artefacts/6
• Grade 1
– Features which can be categorically
identified
• Grade 2
– Features which do not have a unique
interpretation
• Grade 3
– Features which are ambiguous,
i.e. probably are non-geological
Image confidence limits

7. LQC & image artefacts/7
Images arising from physical tool problems, whether
intermittent or actual equipment failure, or due to
changes in formation properties resulting in erratic tool
movement:
– Drilling operations
– Logging operations
1. Acquisition artefacts

8. LQC & image artefacts/8
Types of acquisition artefact
• Tool orbiting
• Drill bit slide
• Side track window
• ‘Bit at rest’
• Stabiliser grooving
• Hole deviation
• Conveyance eccentralisation
• Standoff
• Button/flap death
• Mud smear
• Noise
– 60 Hz
– ‘Woodgrain’
– ‘Honeycomb’
• Tool speed irregularities
• Tool stick
• Signal loss
– Acoustic
– Resistivity
• Tool rotation
• Extreme conditions
Drilling Logging

9. LQC & image artefacts/9
Microresistivity image
Ultrasonic image with caliper
Low <= resistivity => High
Low <= amplitude => High
Spiral hole

10. LQC & image artefacts/10
Microresistivity image
Bit
Stabiliser
Ultrasonic image
& caliper
bit
stabiliser
‘Bit at rest’

11. LQC & image artefacts/11
RAB image
Ultrasonic image
Window
Slide
Sidetrack window Drill slide

12. LQC & image artefacts/12
Ultrasonic image Microresistivity image
stick
Tool stick

13. LQC & image artefacts/13
Ultrasonic image
Inclinometry irregularities

14. LQC & image artefacts/14
Microresistivity images
Smear
Dead Flap (FMI)
Dead flap Mud smear

15. LQC & image artefacts/15
Microresistivity image Ultrasonic image
(visible at low EMEX)
Tool noise (60 Hz)

16. LQC & image artefacts/16
Honeycomb
noise on
amplitude
image only
Cause - error in amplitude gains -
tool changes gain on each shot to collect statistics
to make the next correction.
Ultrasonic amplitude image
Noise – ‘honeycomb’

17. LQC & image artefacts/17
Ultrasonic Image
Cause - systematic error in peak interpolation
Woodgrain effect
Noise – ‘woodgrain’

18. LQC & image artefacts/18
In water
In attenuative mud
Ultrasonic image
Signal loss 1

19. LQC & image artefacts/19
UBI time window with
signal dispersion
Ultrasonic image
Amplitude Transit time
some signal out of window
Signal loss 1

20. LQC & image artefacts/20
Speckling due to
8% oil lubricant
additive in WBM
Extremes of resistivity,
intense speckling
Microresistivity images
Signal loss 2

21. LQC & image artefacts/21
Voltage measured
between buttons
Voltage button
electrodes
Current
from
pads
passes
through
formation
Current
electrode
Current
electrode
‘Artefact of the month’

22. LQC & image artefacts/22
Dual OBMI tool length artefact

23. LQC & image artefacts/23
Good
Slightly milled
Heavily milled
EARTH Imager ‘bedding’ artefact

24. LQC & image artefacts/24
Artefacts arising from physical characteristics of the
borehole wall:
– Washout & rugosity
– Breakout
– Key-seat furrow
– Spiral hole
– Drilling induced fractures
– Stabiliser grooving
– Tool marks
– Mudcake
– Cable slap Ultrasonic images are
particularly susceptible
2. Borehole wall artefacts

25. LQC & image artefacts/25
Amplitude Transit time
washout
Ultrasonic images
Rugosity & washout
signal loss in
washout

26. LQC & image artefacts/26
Pads at full
extent -
blurring
Microresistivity image
Washout

27. LQC & image artefacts/27
Ultrasonic image
Microresistivity image
Borehole breakout

28. LQC & image artefacts/28
Ultrasonic image
Microresistivity image
Caliper under gauge
Mudcake
build-up,
blurred image
Mudcake over
permeable sand
Mudcake build-up

29. LQC & image artefacts/29
Microresistivity image
Ultrasonic image
ToH BoH
Key-seat/low-side furrow
ToH BoH
Low-side furrow

30. LQC & image artefacts/30
Grooves
Ultrasonic image
Stabiliser grooving

31. LQC & image artefacts/31
Microresistivity images MSCT coring marks
MSCT Cores
RFT point
Tool marks

32. LQC & image artefacts/32
Ultrasonic image
Cable marks
Cable slap

33. LQC & image artefacts/33
Images directly influenced by image processing:
– Pad match error
– Normalisation windows
– Wrong borehole diameter
– Incorrect dip values
– Multiple-pass offsets
– Incorrect gain corrections
– Faulty circuitry corrections
3. Processing artefacts

34. LQC & image artefacts/34
Microresistivity images
Wrong
Offset
20 cm
Pad match offsets Button offset

35. LQC & image artefacts/35
Artefact of fracture
Window length
Artefact of fracture

36. LQC & image artefacts/36
Microresistivity images
Erroneous button correction Corrected image
Faulty circuitry and speed correction

37. LQC & image artefacts/37
Ultrasonic images
No gain correction With gain correction
Gain correction/overcorrection

38. LQC & image artefacts/38
Images of actual geological features which differ from
core appearance due to borehole geometry, the nature
of tool measurements and the mud filtrate invasion
profile:
– Current gather effects
– Halos
– Nodules (resistive)
– Pyrite (conductive)
– Fracture aureoles
– Cement mottling
– Proximity effects
– Shallow gas effects
– ‘Woodgrain 2’
4. Derived artefacts

39. LQC & image artefacts/39
Microresistivity images
Halo effects

40. LQC & image artefacts/40
Microresistivity images
Resistive mottling
Cement mottling
Fracture aureole

41. LQC & image artefacts/41
Resistive anomaly intersects borehole wall
Resistive anomaly 5mm away from borehole wall
N N
W
E S
N N
W
E S
Microresistivity image
Features away from
borehole wall
Features touching borehole wall
Benthic forams - courtesy
C. Williams.
Proximity effects

42. LQC & image artefacts/42
Microresistivity image
Gas effect

43. LQC & image artefacts/43
Ultrasonic Image
Multiple (Fresnel) effect
(most notable on CBIL images)
Woodgrain effect in mudcake
Noise – ‘woodgrain 2’

44. LQC & image artefacts/44
Residual oil

45. LQC & image artefacts/45
• Systematic artefact recognition is the first step
in LQC for any interpretation...
• Acoustic tools are most effected by borehole
wall & mud fluid.
• Micro-resistivity devices are most susceptible
to effects of current flow.
• As tool design & processing improve – some
artefacts will disappear.
• Often more than one artefact present.
Most are predictable.
• Ability to see through an artefact, ‘ignore’, and interpret!
Artefact LQC summary