Basics of Borehole Geophysics explained

Most surface geophysical techniques provide vast information about subsurfaces. However when it comes to wanting to trace detailed information about subsurface in a certain cross- area, it is highly recommended to deploy these techniques in a drilled hole. This is termed as Borehole geophysics

What is Borehole Geophysics?

Borehole geophysics involves the depth wise sequential record of physical parameters such as resistivity, density, temperature, porosity, etc. It may sometimes be known as Geophysical Logging.

Borehole: Is the narrow hole drilled into ground for the purpose either of obtaining subsurface information or extracting fluids such as water, gas, oil, contaminants.

It is the same as a well, but they may differ due to the fact that the borehole is drilled at great depth and has small diameter compared to traditional wells.

The basic Geophysical borehole system is made up of these parts, 

Downhole probe: It consists of sensing instruments/equipment that are lowered down the drilled hole  to record physical parameters.

Multicore cable: It consists of many cables used to lower and rise up sensing instruments in borehole.

Winch system: It used to provide mechanical support for Multi Cables, such as by varying tension to ensure pull up or down.

Control unit: It is used for control of other units such as downhole probes.

Recording unit: It used to assign and record the measured values (data) for further processing

There are two (2) types of boreholes used 

(i) Cased holes : These boreholes have cased materials. The term case is used to refer to a hollow pipe that is inserted into a drilled hole so as to provide mechanical support to ensure hole stability. Also it can be used either to prevent entrance of materials into/from the hole. Most of the case materials are PVC or steel depending on the purpose of use.

(ii) Open holes : These are boreholes with no case.  These boreholes can support deploying of various geophysical techniques.

types of boreholes geophysical tools.

There are many types of boreholes techniques ,however in order get basic knowledge we can classify these tools as follows,

electrical logging: This includes the follows techniques,

Resistivity Logging: it measures changes in resistance between the lead electrode in the borehole and the fixed electrode  at the surface. It deploys different electrodes spacing and configuration. It uses a simple tool device known as Single Point Resistance Device.

Useful: It provides information for distinguishing different types of lithologies example sand versus clay

Self- Potential (SP) Logging: It records depthwise natural electric potential (mV) of a point in a borehole with respect to a fixed point on the surface.

Self- Potential is caused by electrochemical effects between dissimilar layers, electro-kinetic effects produced by movement of borehole fluid through permeable beds.

Useful: It used to measure potential difference (P.d) develops in boreholes at the contacts between clay beds and sands.

radiation (nuclear) logging: it measures the total intensity of natural gamma radiation that presents in rocks and sediments. It is related to radioactivity of radionuclides such as Potassium (K), Thorium (Th) and Uranium (U).

They are of two (3) types,

Natural Gamma Logging: This uses gamma ray counters to measure the concentration rates of radionuclides present in natural materials. It can be used to distinguish lithologies such as sand from clay

Gamma - Gamma Logging: This uses gamma radiation that originates from the source probe and is recorded after it has backscattered and attenuated within the borehole and surrounding formation.

Density Radiation Logging: This uses backscattered electrons to determine the bulk density of the formation.

Note: For radiation logging lowering of radioactivity, leads the Log trace to deflect to the left, while increasing of radioactivity leads the Log trace to deflect to the right.

acoustic/sonic logging: It measures the velocity of sound through the rock surrounding an uncased fluid filled hole. The transit time of sound is measured through the formation around the borehole.

caliper logging: It records the variation in the diameter of the borehole with depth.

Useful: Identifying faults and fractures.

neutron logging: It uses neutron sources and detectors arranged in a single probe which produces a record related to the hydrogen content of the borehole environment.

Since hydrogen content is directly proportional to the interstitial water.

Useful: estimates moisture content.

dipmeter logging: it is depth sequential measures at which resistivity changes indicating dips. Useful: For structural and stratigraphic analysis.

Applications of Borehole geophysical techniques

- Geotechnical engineering: example acoustic logs, caliper log. 

- Groundwater studies: example neuton logs for fluid (H) content evaluation, fluid conductivity logs for water quality studies, Gamma logs and resistivity logs

- Geological studies: example Gamma logs, resistivity log, Self-Potential (SP) logs, Caliper log for structural identification such as faults and fractures.

 - Petroleum engineering studies: example neutron logs, resistivity logs

Although Boreholes geophysical methods provide detailed information about the subsurface section, it has some limitations in cost of operation, we can say some tools are expensive. Also  the need for site drilling as a preliminary condition for its operation reduces its suitability to operate in some environmentally protected areas.

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