SUBSURFACE LOGGING
Geophysical logging is used for the determination of physical properties of formations adjacent to boreholes and for preparation of geological sections. It includes all techniques of lowering sensing devices in a borehole and recording some physical parameter, which gives information about characteristics of the rock, the fluids contained in the rocks and construction of the well
The drillers log mainly gives a physical description, when combined with geophysical logs, hydrogeological characteristics can be interpreted and the refinement can be made of the lothological cross section already available from geological studies. The modified lithological log is used for designing the tube well.
These are different methods of logging based on different physical principles and have different operational techniques. The important logging methods that are most widely used in Central Ground Water Board are Electrical and Natural Gamma which are discussed below.
Electrical Logging
In electrical well logging two electrical properties are measured in the borehole, one is self-potential and other is point resistivity.
Spontaneous Potential (SP)
It is a continuous record of the natural voltage potential between surface ground and a point electrode as the electrode is lowered into on raised out of the borehole. Opposite the shale the readings of the SP curve are usually fairly constant and tend to follow a straight line on the log called shale base line. Opposite to the permeable formations, in thick beds SP curve shows another deflection, which is constant defining a sand line.
Resistivity Logging
Normal / Lateral logs record the resistivity of a known on assumed volume of earth materials in and around the borehole column. Depending on the positions of current potential and earth electrodes, the extent of horizontal coverage is increased. Alternating current of low frequency is used for this measurement as the logging electrode travels in the hole, changes in the formation resistivity curve changes in electrode resistance, which is in turn cause voltage change in logging circuit and is recorded as resistivity curve.
Natural Gamma Logging
This log measures the natural radioactivity of the formation; in sedimentary formation it reflects the clay/ Shale content of the formation. This is because the radioactive elements tend to concentrate in clay and shale. Clean formation usually has a very low level of radioactivity. The log can be run in cased, uncased boreholes with or with out fluid contents. The record of natural background radiation is required while gamma logging is done.
Gamma Gamma ray Logging
This method is used to detect lithology and porosity.
Neutron Logging
This method is used to detect hydrogen content or moisture content of bore hole environment.
The drillers log mainly gives a physical description, when combined with geophysical logs, hydrogeological characteristics can be interpreted and the refinement can be made of the lothological cross section already available from geological studies. The modified lithological log is used for designing the tube well.
These are different methods of logging based on different physical principles and have different operational techniques. The important logging methods that are most widely used in Central Ground Water Board are Electrical and Natural Gamma which are discussed below.
Electrical Logging
In electrical well logging two electrical properties are measured in the borehole, one is self-potential and other is point resistivity.
Spontaneous Potential (SP)
It is a continuous record of the natural voltage potential between surface ground and a point electrode as the electrode is lowered into on raised out of the borehole. Opposite the shale the readings of the SP curve are usually fairly constant and tend to follow a straight line on the log called shale base line. Opposite to the permeable formations, in thick beds SP curve shows another deflection, which is constant defining a sand line.
Resistivity Logging
Normal / Lateral logs record the resistivity of a known on assumed volume of earth materials in and around the borehole column. Depending on the positions of current potential and earth electrodes, the extent of horizontal coverage is increased. Alternating current of low frequency is used for this measurement as the logging electrode travels in the hole, changes in the formation resistivity curve changes in electrode resistance, which is in turn cause voltage change in logging circuit and is recorded as resistivity curve.
Natural Gamma Logging
This log measures the natural radioactivity of the formation; in sedimentary formation it reflects the clay/ Shale content of the formation. This is because the radioactive elements tend to concentrate in clay and shale. Clean formation usually has a very low level of radioactivity. The log can be run in cased, uncased boreholes with or with out fluid contents. The record of natural background radiation is required while gamma logging is done.
Gamma Gamma ray Logging
This method is used to detect lithology and porosity.
Neutron Logging
This method is used to detect hydrogen content or moisture content of bore hole environment.
posted by DR. JOJI at
12:15 AM
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