The Variable Density Log (VDL) displays acoustic waveforms to assess sonic or ultrasonic measurements, often used alongside cement-bond logs for better interpretation. It relies on gamma ray emissions and Compton scattering to measure formation density, with tools including long and short spaced detectors for accurate readings. The VDL is particularly reliable for identifying porosity in sandstones and limestones, though it has limitations related to uniformity of conditions in the borehole.

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.