The document discusses variable density logs (VDL), which provide a color or grayscale representation of acoustic wave amplitude received during sonic or ultrasonic measurements. VDLs are commonly used alongside cement bond logs to provide better interpretation. It also discusses density logs, which measure formation density using gamma rays emitted from a borehole tool. Density is plotted on a bulk density scale and used to indicate porosity, derive total porosity, and identify lithology when used with neutron logs. Limitations include needing uniform mudcake/washouts less than 0.75 inches.

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.