Geology Reference
In-Depth Information
groundwater investigations and development work. Systematic geo-exploration
has also increased the rate of success in water well drilling in hard rock
areas.
Studies on the hydrogeology of hard/fractured rocks are being pursued for
different purposes and with widely differing objectives, such as:
(i) Development of safe groundwater supplies for domestic and irrigation
purposes.
(ii) Contaminant migration studies, in order to estimate the movement of
pollutants through fractures etc.
(iii) Tapping of geothermal resources involving estimation of extractable
amount of hot fluids from the natural geothermal gradients.
(iv) Development of petroleum and gas reservoirs.
(v) Underground nuclear waste disposal.
(vi) Construction of underground rock cavities for storing water, oil and
gas etc. and underground passages such as tunnels.
(vii) In several other geo-technical problems, e.g. stability of rocks, slopes
and seepage from dams and tunnels, triggering of earthquakes etc.
All the above studies require a clear understanding and proper description
of' the hydrological characteristics of these rocks. A comparison of the
hydrogeological characteristics of porous and hard (fractured) rock aquifers
is given in Table 1.
Table 1:
Comparison of Granular and Fractured Hard Rock Aquifers
Aquifer Characteristics
Aquifer Type
Granular Rock
Fractured Rock
Effective porosity
Mostly primary
Mostly secondary through
joints, fractures etc.
Isotropy
More isotropic
Mostly anisotropic
Homogeneity
More homogeneous
Less homogeneous
Flow
Laminar
Possibly rapid and
turbulent
Flow predictions
Darcy's law usually
Darcy's law may not apply,
applies
cubic law applicable
Recharge
Dispersed
Primarily dispersed with
some point recharge
Temporary head variation
Minimal variation
Moderate variation
Water quality variation
Minimal variation
Greater variation
ROCK DISCONTINUITIES AND FRACTURES
Hard rocks are characterized by various types of rock discontinuities of
various scales varying from few mm size joints to major fault zones and
lineaments. The main rock discontinuities are foliation, fractures (joints),
faults and lineaments.
