Borehole geophysics is the science of recording and analyzing measurements of physical properties made in wells or test holes. Probes that measure different properties are lowered into the borehole to collect continuous or point data that is graphically displayed as a geophysical log. Multiple logs typically are collected to take advantage of their synergistic nature--much more can be learned by the analysis of a suite of logs as a group than by the analysis of the same logs individually. Borehole geophysics is used in ground-water and environmental investigations to obtain information on well construction, rock lithology and fractures, permeability and porosity, and water quality. The geophysical logging system consists of probes, cable and drawworks, power and processing modules, and data recording units. State-of-the-art logging systems are controlled by a computer and can collect multiple logs with one pass of the probe

1. Borehole Geophysical Techniques
Abhijith V & Mohit Kumar

2. Spontaneous Potential (SP) Log
● Measures natural potential
difference between a movable
electrode in the borehole and the
fixed potential of a surface
electrode
● +ve deflection- Cations- Mud -
Membrane Potential
● -ve deflection-Anions-SSt, Porous
Lst- Liquid Junction Potential
● Applications:
-correlation
-lithology indication
-porosity and permeability
-resistivity
-salinity

3. Resistivity Log
● Measures the resistivity of a
formation by passing electricity
through electrodes placed at
standard spacings
● Shale, Clay & Saltwater –> Low
● Freshwater sand –> Moderate to
high
● Cemented sandstone & Nonporous
limestone –> High
● Various resistivity tools in use
● Applications:
-porosity and permeability
-fluid characteristics
-oil detection
-metallic mineral exploration

4. Sonic Log
● Measures the time of first
arrival of sound pulses from a
transmitter at a receiver
● The arrival time depends on the
density of the formation, which
in turn depends on the porosity
and permeability
● Applications:
-Density of the formation
-Porosity calculation for known
lithology
-Calibration of seismic data

5. ● It measures the natural radioactivity
of the formation.
● It provides a measurement of the
muddiness of a unit.
● Quartz arenite and clean carbonate
sediments -> low log response.
● Mudstone, volcanic ash and granite
wash -> high log response.
● It can be run in open as well as cased
holes.
● Applications:
-Identification of Lithology
-Facies correlation.
-Evaluation of radioactive minerals.
-Identify shale volumes.
Gamma Logging

6. Neutron Logging
● It measures H+ content of the
borehole environment.
● A fast neutron source is used to
bombard the rock. When any
individual neutron collides with a H+
ion, some of the neutron’s energy lost
and it slows down.
● A large number of slow neutrons
recorded indicates a large amount of
fluid i.e., high porosity
● Applications:
-measures formation water,
hydrocarbons, and bound water in
clay minerals, gypsum, etc.

7. Density Logging
● This tool operate by emitting gamma
radiation and detecting the
proportion of the radiation that
returns to detectors on the tool.
● Denser the formation, more electrons
are present, and more energy is lost due
to collisions.
● If matrix density is known, then energy
loss is directly related to porosity.
(density decreases with increasing porosity)
● Applications:
-Identification of lithology.
-Measurement of bulk density &
porosity.

8. THANK YOU