A presentation of the acoustic waveform at a receiver of a sonic or ultrasonic measurement, in which the amplitude is presented in color or the shades of a gray scale. The variable-density log is commonly used as an adjacent to the cement-bond log, and offers better insights into its interpretation.

1. TOPIC
Variable density log

2. WHAT IS VDL
• A presentation of the acoustic waveform at
a receiver of a sonic or ultrasonic measurement,
in which the amplitude is presented in color or
the shades of a gray scale. The variable-density
log is commonly used as an adjacent to
the cement-bond log, and offers better insights
into its interpretation.
 Acoustic waves are a type of longitudinal waves that
propagate by means of adiabatic compression and
decompression

4. PRINCIPLE
• A radioactive source, applied to the borehole wall,
emits medium-energy gamma rays into the
formations. These gamma rays are high velocity
particles that collide with the electrons in the
formation. At each collision a gamma ray loses
some, but not all, of its energy to the electron, and
then continues with diminished energy. This type
of interaction is known as Compton scattering.

5. PRINCIPLE(CONTINUE)
• The scattered gamma rays reaching the detector, at a
fixed distance from the source are counted as an
indication of formation density.
• The number of Compton scattering is directly
related to the number of electrons in the formation.

6. TOOLS
• Long spaced detector
• Short spaced detector
• Source of radiation

8. SCALE
• The density log is generally plotted on linear
scale of bulk density.
• Most often its scale is in between 1.95 to
2.95g/cm3

9. UNITS
• In CGS it is g/cm3 .
• In SI it is kg/m3 .

10. USES
• Most reliable for porosity indicator of sandstone
and limestone because their porosity is well
known.
• On the other hand the density of clay minerals such
as mudstone is highly variable, depending on the
depositional environment, overburden, pressure,
type of clay minerals and many other factors.
• Its main use to derive the total porosity of the
formation.

11. USES(CONTINUE)
• It is also useful in gas-bearing formations and in
the recognition of evaporites.
• It gives the best way to identify the lithology in
borehole with collaboration of Neutron log.

12. LIMITATIONS
• Correction only valid if mudcake or sandoff
(perforating) <0.75 inch and is uniform along pad.
• Abnormally low density(high permeability) is
washed out (>1.7inch) hole or rough hole.