TM 5-3820-256-10
(3)
The unknows are, generally, (a), (b), (c), and (d), above. Granular materials will have high resistivity
compared to fine materials, such as silt and clay; crystalline materials, such as limestone and granite, will
have high resistivity compared to granular materials.
(4)
The quality of the formation water greatly affects the measured resistivity, and generally, the resistivity of a
formation varies in an inverse proportion to the total dissolved solids. For example, all other conditions
remaining the same, if the total solid content increases, the formation resistivity decreases. Therefore, a
clean sand, filled with salty water, may actually have extremely low resistivity.
(5)
Porosity of the formation also has an effect on the resistivity. It is not as pronounced as the effect from
water quality. In the logging of chemical precipitates, such as limestone, changes in porosity may enable
you to detect the water producing zones. Increased porosity will lower the formation resistively; therefore, in
such material, a low resistive zone (where no shale is present) indicates increased porosity and enhances
the possibility of that zone producing water.
(6) The exact range of values for clean sand, gravel, or sandstone is something learned by experience in your
geographical area. In the midwest United States, for example, clean sand and gravel generally exhibit
resistivity values in the range of from 350 to 1000 ohm-feet. The lower values apply to formations having
water quality in the range of 300 to 400 ppm (parts per million) total solids and the upper values apply for
formation waters having 100 to 150 ppm total solids. The above example is general and included for
guidance only.
f. Selecting Formation Contact. When "picking" the formation boundaries, the 0.25-foot curve should be used
wherever possible. The inflection point (the point midway between changes in curvature of the resistivity curve) of the
resistivity curve is used to make the contact between different formations.
g. Correlation by Electrical Logs.
(1)
A useful application of the electrical logs is in correlating formation thickness and depths from one well to
another. For example, two wells within a few feet of each other invariably will give identical electrical logs.
When the wells are farther apart, the correlation will still be recognizable and the changes which do occur,
e. g. thinning or thickening of formations, are exactly the information needed to guide further explorations.
(2)
Correlation is commonly possible to considerable distances in bedrock formation, in the order of thousands
of feet. Because of the variable nature of unconsolidated glacial and alluvial deposits, do not expect such
distances, except in special cases of a single, widespread type of deposit.
h. Effect of Metal on Resistivity Logs.
(1)
Because metal is such a good conductor, its presence in a zone being measured will cause a major
decrease in the resistivity and make the log unusable for determining formation type. This effect, however,
may be used in locating steel lost in the well.
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