Borehole geophysical logging employs different techniques in exploring detailed information of the subsurface. However when radiation technique is utilized this log is known as Radiation logging. Today I want to let you know in brief through this post about this type of borehole geophysical technique.
What does Radiation Logging involve?
radiation (nuclear) logging: it measures the total intensity of natural gamma radiation that presents in rocks and sediments. It also known as Radiometric Logging
It is related to radioactivity of radionuclides such as Potassium (40K), Thorium (232Th) and Uranium (238U).
They are of two (3) types,
(i) Natural Gamma Logging
This uses a gamma ray counter to measure the concentration rates of radionuclides present in natural materials. You can refer here to get more details of how the radiation counter works.
If the count rates is higher, means the higher concentration of radionuclides, which represent the fine grained deposits (clay)
If the count rates is lower, means the lower concentration of radionuclides, which represent the very coarse grained deposits (sand)
(ii) 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.
(iii) Density Logging
This uses backscattered electrons to determine the bulk density of the formation.
It works using the same principle as Gamma- gamma logging, when the gamma radiation from the source (Cs-137) is sent to formation and interacts with the electrons within it, the same as Compton scattering. The backscattered electrons are sent to the receiver (detector) at which it's density (electron density) can be obtained and then used to calculate the bulk density of the formation.
You can use the following expression to calculate the bulk density,
De = 2Bd (z/A)
Where Bd - Bulk density, De - electron density, A - Atomic number and z - is the atomic mass.
In most cases the ratio z/A is 1/2, except for some element like hydrogen at which this ratio is unit (1).
Bulk density is the density which includes minerals and water contents within pore spaces of a material.
Figure 1: Simple representation of a density Log
Note: The depth values used in figure 1, are inverted, and used here for representation purpose only, however in real practical the depth must increase towards downwards from the ground surface.
Advantage of Radiation logging over electrical Logging.
- It may be made in cased holes filled with any fluid or even in dry holes.
- It can identify formation boundaries between units.
- It can correlate rock units in different boreholes. Example regional correlation.
- It can determine clay contents units
N.B: In radiation logging lowering radioactivity, leads the Log trace to deflect to the left, while increasing radioactivity leads the Log trace to deflect to the right.
Radiation Logging can provide useful information about subsurfaces, however you have to strictly adhere to safety and health rules when working with this technique, because gamma rays are toxic and may lead you into harmful effects.
Thank you for reading!
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