Geology Reference
In-Depth Information
4
Geophysical Characterization of
Hard Rock Aquifers
N.S. Krishnamurthy, Subash Chandra and
Dewashish Kumar
IFCGR, National Geophysical Research Institute
Hyderabad-500007, India
INTRODUCTION
Geophysics plays a major role for characterizing the hard rocks for
groundwater studies. The qualitative and quantitative application has increased
since past few years due to rapid development and advancement in
microprocessors and associated numerical modelling solutions. Although
geophysics has ability to probe deep earth interior (say >1000 m), but its
application for groundwater studies is usually restricted to depths less than
and around 250 m below the surface. These include mapping the depth and
thickness of aquifers, mapping aquitards or confining layers, locating fractures
and fault zones and mapping contamination to the groundwater such as that
from saltwater intrusion. The theoretical and practical background to
geophysics has been extensively reviewed and can be studied in standard
texts on the subject, for example Kearey & Brooks (1991); Telford et al.
(1976); Parasnis (1979); Dobrin (1976); Grant and West (1965); Reynolds
(1997); Miller et al. (1996); Murali et al. (1998); etc.
IMPORTANCE OF FRACTURES IN HARD ROCK AREAS
Fractures often serve as major conduits for movement of water and dissolved
chemicals through hard rocks in the underground. The geological structure
normally encountered in hard rock areas of places such as in India is granite
or granite gneiss overlain by a variable thickness of weathered material. The
latter is a regolith produced by the in-situ weathering of the basement rock
(Acworth, 1987). The regolith normally grades into solid unfractured basement
over several tens of metres, although often the boundary between the two
