Geology Reference
In-Depth Information
in the bore-wells) is measured and is invariably used in conjunction with SP
logging. Measurement of ground resistance serves more as an indicator of
the probable order of resistivity rather than for determining the actual
resistivity of formations. The compact zone shows high resistance while the
others is indicated by lower values. The PR log has some advantages such
as: 1. The anomaly highs or lows are proportional to the resistivity of the
formation. 2. Very thin beds with different resistivities can be identified
provided the borehole diametres are not very large compared to sonde
diameter. 3. The measurement and interpretation of PR logs is simple.
Resistivity Logging:
The most commonly used electrode arrangement is
normal or potential sonde in which one current electrode and two potential
electrodes are located on the sonde. The other current electrode is kept on
the surface. The curves obtained by potential or normal resistivity logs are
symmetrical in form in which the maximum indicates the layer with the
higher resistivity, and the minimum indicates a layer with lower resistivity.
Gamma Ray Log:
The gamma ray log is a record of a formation's
radioactivity. The radiation emanates from naturally occurring uranium,
thorium and potassium. Most rocks are radioactive to some degree. Igneous
and metamorphic rocks are more so than sedimentary rocks. However,
amongst sedimentary rocks, shales have by far the strongest radiation. It is
for this reason that the simple gamma ray log has been called the 'shale log'.
A high gamma ray value frequently means shale. Quartzite shows no
radioactivity. Sandstones usually show low gamma ray values.
The various well logging techniques used in IFP-19 at Maheswaram
watershed are shown in Figs. 9 and 10. The geological sequence encountered
while drilling is the top weathered and semi-weathered layer followed by
fractured granite underlain by compact granite at a depth of about 36 m. The
resistivity sounding result shows a resistivity of 292
m for the semi-
weathered zone followed by fracture granite from 16-30 m with a resistivity
of 496
m, underlain by bed-rock. Self-Potential, Point Resistance,
Temperature log, Short Normal, Long Normal and Gamma logs were done
in this well. S.P. and Temperature logs do not indicate any anomalous zone.
The SN and LN logs indicate low resistivity around 31 m and the PR log
indicates a low resistance around 32 m.The apparent resistivity as observed
against weathered zone are 110 m and 600 m by SN and LN respectively
whereas against fractured zone are 220
m. The gamma log
indicates high activity below 22.5 m onwards, which is in the range of 500
cps compared to that above 22.5 m which is in the range of 300 cps or less.
This high activity can be activated to a fractured zone. Combining all these
results showed that there is a clear fracture zone around 25 m as shown in
Fig. 10. All the electric logs indicate that the bedrock occurs at a depth
of 38 m.
m and 1200
