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Physics of the earth and atmosphere is
geophysics. Knowing the earth by investigating
its physical properties is geophysical exploration,
and understanding groundwater by investigating
the physical properties of earth is the over all
domain of ground water geophysics.
Geophysical methods can be mainly grouped into
two categories.
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Objectives
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Decipher thickness
of weathered formation & delineation of
fractures. |
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Determine depth to bed rocks
and bed rock topography. |
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Locate caverns in limestone
areas. |
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Locate sub-surface structures
favourable for ground water accumulation |
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Demarcate fresh-saline water interface. |
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Demarcate lateral extents of
saturated fracture zones in hard rock areas. |
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Delineate resistive
granular zones in sedimentaries. |
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Assess ground water pollution
and movement of pollution plume in terms of
electrical conductivity. |
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Define vadose zone
characteristics to identify areas for artificial
recharge to ground water |
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Identify waste disposal sites
& impacts. |
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Assess soil salinity |
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Demarcate sea water intrusion |
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To determine direction of ground
water movement to estimate hydraulic parameters
of aquifers |
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Activities
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Electrical
( Resistivity & Self potential) methods
of Geophysical investigation for pin ponting
suitable sites forbore wells and tube well
drillings |
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Geophysical investigation
in coastal, hard rock areas. |
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Geophysical investigation in Andaman &
Nicobar islands for delineation of fresh water
aquifer. |
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Geophysical investigations
to locate sites for Artificial Recharge structures |
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Deep Drilling in problematic areas |
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Design and execution of well field and testing
for special operations |
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Formulation of development strategies/
recommendations |
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Acheivements
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C.G.W.B conducted deep & shallow resistivity
investigation in different geologic terrain
of West Bengal, Sikkim & UT of A&N
island.
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Objectives
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Recording
physical parameters and interpretation
in terms of characteristic of the
rocks and its fluid content. |
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To identify distinct
boundaries between different hydrogeological
units |
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To decide the proper
position of the screen against productive
aquifers. |
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To
determine the lithology, geometry,resistivity,
formation factor, bulk density,
porosity,permeability,moisture content. |
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To determine the source,
movement , chemical and physical characteristics
of ground water. |
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To Demarcate the boundaries
between fresh and saline water |
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Used in ground water
quality control and management for
monitoring ground water pollution |
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Assess ground water pollution and
movement of pollution plume in terms
of electrical conductivity. |
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To correlate
the geologic strata.
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Activities
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Self Potential
logging for determining bed boundaries
and quality of formation water |
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Point
resistance ( PR) logging device
used for determining bed boundaries
and changes in lithology and identifying
fractures in resistive rocks.
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Resistivity
logging ( 16” short normal
and 64” long normal) to determine
bed boundaries and to calculate
formation factor and porosity. |
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Natural
gamma logging for identification of
lithology and stratigraphic correlation
in open and cased hole. |
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Flow meter
logging for the measurements of movement
of water in the borehole column under
natural gradients. |
Acheivements
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About
350 No. of Geophysical logging has
been done in different hydrogeologic
terrain of west Bengal, Sikkim and
Andaman and Nicobar island. |
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Thin
productive aquifers, at depth, which was
not identified on drill cut samples was
identified on geophysical logs and proper
assembly is being designed.
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The gamma
log of Sripur, Gaighata block, N-24 Parganas
indicates that within thick clay which starts
from 226 mbgl, there are several thin sand
layers at deeper depths (i.e. below 435m)
whereas these zones were identified as mixed
zone in drill cut sample.
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Gamma log of rural
health center complex, Beldanga, Murshidabad,
several sand zones have been identified
at depth which could not be detected on
hand specimen.
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Detection
of even thin sand layers by geophysical
methods, becomes, all the more important
in arsenic affected areas.
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In coastal area of
Digha fresh water bearing thin sand beds
were identified by Electrical & gamma
logging.
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