Geology Reference
In-Depth Information
may be fairly sharp. Hydrogeologically the weathered overburden has a high
porosity and contains a significant amount of water, but, because of its
relatively high clay content, it has a low permeability. The bedrock on the
other hand is fresh but frequently fractured, which gives it a high permeability.
But as fractures do not constitute a significant volume of the rock, fractured
basement has a low porosity. For this reason a good borehole, providing long
term high yields, is one which penetrates a large thickness of regolith, which
acts as a reservoir, and one which additionally intersects fractures in the
underlying bedrock, the fractures providing the rapid transport mechanism
from the reservoir and hence the high yield. Boreholes which intersect
fractures, but which are not overlain by thick saturated regolith, cannot be
expected to provide high yields in the long term. Boreholes which penetrate
saturated regolith but which find no fractures in the bedrock are likely to
provide sufficient yield for a hand pump only.
HARD ROCK AQUIFER CHARACTERISTICS
The most significant features of the hard rock aquifers are: 1. A topographical
basin or a sub-basin generally coincides with ground water basin. Thus, the
flow of ground water across a prominent surface water divide is very rarely
observed. 2. The aquifer parametres like Storativity (
S
) and Transmissivity
(
T
) often show erratic variations within small distances. 3. The saturated
portion of the mantle of weathered rock or alluvium or laterite, overlying the
hard fractured rock, often makes a significant contribution to the yield obtained
from a dug-well or bore-well. 4. Only a modest quantity of ground water,
in the range of one cu.m. to a hundred cu.m. or so per day, is available at
one spot. 5. Draw down in a pumping dug-well or bore-well is often almost
equal to the total saturated thickness of the aquifer.
STUDY AREA
The case studies shown in this part are from the geophysical studies carried
out in the Maheshwaram watershed having an area of about 60 km
2
, situated
at about 30 km south of Hyderabad, India. It lies in between geographical
coordinates having 17° 06
30 to 78° 29 00 East longitudes and forms part of Survey of India toposheet
56K/8. The geology of the Maheshwaram watershed is mainly granites of
Archean age intersected by dolerite dykes and quartz veins. They have
undergone variable degrees of weathering with depths extending up to even
20 m followed by fracturing at many places. The dyke located in the extreme
northern part strikes east-west with about 15 m width. Another dyke exposed
about 1 km south of the first one, strikes N60°E-S60°W with a width of
about 20 m at places. A quartz vein of about 20 m width with a strike of
ENE-WSW is exposed in the drainage divide in the southern boundary of
the watershed. Ground water in the area occurs under water table conditions
20
to 17° 11
00
North latitudes and 78° 24
