The document summarizes acoustic logging, which uses sonic tools to measure the speed of sound waves through rock formations. It discusses the principles of acoustic logging, including measuring interval transit time and classifying compressional and shear waves. It then covers the quantitative uses of calculating porosity from transit times and identifying lithology, as well as qualitative uses like fracture and secondary porosity identification.

1. - Priyansh Bhimajiyani
Intern, Halliburton
Acoustic Logging

2.  Contents
1. Acoustic Log
2. Priciple Uses
3. Principle of Measurements
4. Log Characteristics
5. Quantitative Uses
6. Qualitative uses

3. Acoustic logs or Sonic logs measure the slowness of elastic waves of
the formation.
In sonic log, the capacity of formation to transmit sound waves is
measured.
The capacity varies with porosity, lithology, and rock texture.
Travel time of a wave is the distance that the wave travels times the
slowness of the medium, Thus,
Slowness = 1/Velocity
 Acoustic Log

4.  Principle Uses

5. The sonic log is a porosity log that measures
interval transit time (Δt) of a compressional
sound wave travelling through one foot of
formation.
Interval transit time (Δt) in microseconds per
foot is the reciprocal of the velocity of a
compressional sound wave in feet per second.
Acoustic tools measure the speed of sound
wave in subsurface formations.
The sonic tool works by sending a sound pulse
into the formation and measuring interval
transit time (sound wave to traverse 1ft of
formation).
 Principles Of Measurements

6. The particles of the medium do not travel
with the wave, but only vibrate around
their mean central position.
Acoustic waves are classified according to
the direction of particle displacement with
respect to the direction of wave
propagation as either:
 Compressional (P waves): Particle displacement is
parallel to the direction of propagation.
 Shear (S Waves): Direction of particle displacement is
perpendicular to the direction of propagation.
The presence of the borehole excites two
additional acoustic energy modes, called
guided waves:
 Normal (pseudo-Rayleigh)
 Tube (Stoneley) waves
 Principles Of Measurements

7.  Principles Of Measurements

8.  Principles Of Measurements

9.  Principles Of Measurements

10. 1. Depth of Investigation
The path of the compressional waves detected by
sonic tools is essentially along the borehole wall with
very little penetration, generally between about
2.5cm-25cm (1‖-10‖) from the borehole wall.
The penetration is independent of receiver separation
and depends on the signal wavelength.
Therefore, for a perticular velocity, penetration is
greater with low frequency.
Wavelength (λ) = Velovity (v) / Frequency
 Log characteristics

11. 2. Bed Resolution
The vertical resolution of the sonic is the span between the receivers
for the borehole compensated tools and should be similar for the
long-spacing tools. This is frequently 2 feet (61cm).
Beds less than 60cm thickness will be registered on the sonic log, but
a true velocity will not be recorded. Specialist tools now exist with
much higher resolutions
 Log characteristics

12. 3. Unwanted Logging Effect
 Cycle Skipping : In some cases the signal arriving at the receiver is too low to trigger the
detection on the first arrival. The detection then occurs at the 2nd or 3rd arriving cycle. Therefore,
we missed or skipped cycles. This shows as a sudden and abrupt increase of the interval transit
time. This happens in poor holes.
 Noise Spikes : This is reverse of cycle skipping. This happens when noise signals trip a receiver
which causes noise spikes on the log. This happens in hard formations like limestone.
 Log characteristics

14.  Quantitative Uses
  mal ttt   1
1. To calculate Porosity:
From Wyllie time average Equation:
Δt = interval transit time
Ф = Porosity
Δtl = Transit time of interstial fluid
Δtma = Transit time of matrix material
mal
ma
tt
tt

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

15. • From equation, it can be said that the
transit time measured by the tool is
the sum of the time spent in the fluid
and in the solid rock matrix.
• Therefore, there is linear relationship
between porosity and measured
transit time which will vary depending
upon matrix material
 Quantitative Uses

16. • But when in sandstone, porosities are
very high, the effect which causes
most deviation is lack of compaction.
• For a particular set of known matrix
velocities, the time average formula
always over-estimates the porosity.
• For unconsolidated sediments, there
are compaction correction coefficients
based on cross plotting sonic
porosities and density of neutron log
porosities.
 Quantitative Uses

17. • When gas is pores of the formation, the
Wyllie time average formula no longer
applies as the gas it lowers the rock rigidity
more than its density and decreases sonic
velocity.
• But this effect can be used to identify
gaseous hydrocarbon when a gas/water
contact is present.
• To estimate the real porosity in the presence
of gas, the porosity calculated from raw log
should be multiplied by 0.7
 Quantitative Uses
7.0*wylliecorr  

18. 1. Lithology Identification:
 There is too much variation
within each type of lithology and
too much overlap between
different types of lithology.
Velocities in
Carbonate>Sandstone>Shale
 Qualitative Uses

19. 2. Fracture Identification
Both compressional and shear waves are
reduced in amplitude while travelling
across fractures, the latter being more
sensitive because the fluids with in the
open fractures are incapable of
conducting shear waves.
Attenuation depends upon the angle
which a fracture plan makes with the
vertical travelling acoustic signals. While
shear waves are highly attenuated by
horizontal fractures. The compressional
waves effect by high angle fractures.
 Qualitative Uses

20. 3. Secondary Porosity
 Neutron & Density logs respond to total porosity regardless of its
form of distribution.
 Sonic logs tend to ignore irregular porosity (such as vugs) and
fracture porosity.
 Compressional waves passing through the formation, find a path
through the rock matrix around the water-filled vugs & fractures
that is faster than the path through the water in them.
 Since the sonic tool registers the first arrival time of the
compressional wave, it follows that ØS will represent the primary
porosity.
 A comparison of ØD & ØS should indicate when secondary porosity
is present.
 If ØD> ØS, vugs and/or fractures are likely to be present in the
formation.
 Qualitative Uses

21. 4. Compaction Study
 Using general compaction
trends, it is possible to estimate
erosion at uncornformities or
the relative amount of
upliftment as the compaction is
generally accompanied by
diagenetic effects which are
irreversible and stay frozen
during upliftment.
 Qualitative Uses

22. 5. High Pressure Identification
 An increase in pore pressure or
overpressure is indicated by a drop in
sonic velocity.
 It is possible to calculate the amount
of overpressure from the extent of
deviation of sonic velocity from the
normal compaction trend or by
equivalent depth method.
 Qualitative Uses

23. 6. Borehole Damage
 There is difference between the
sonic and the seismic velocities
because the sonic velocities can
be affected by mechanical or
chemical damage immediately
around the wellbore
 Qualitative Uses

24. 7. Cement Bond Evaluation
 Qualitative Uses

25. Thank You