Geoscience Reference
In-Depth Information
map the fracture traces on the borehole wall and determine their density
and orientations.
2
,
8
However, it is well known that not all of these traces
will correspond to water conducting fractures. Since many of the fractured
rocks of interest are of low permeability, the flow rate from a packed interval
can be very low. Packed-off test intervals are usually larger than individual
water-conducting zones, thus leading to uncertainty in the location of a
water-bearing fracture. In general, the flowing fractures contain fluids with
different chemical composition and ion content from each other and hence
have different electric conductivities.
9
Like an experimental technique, EC
logging is subject to detection limits. For inflowing formation water is deter-
mined by the ability to properly identify EC changes caused by the inflow.
Microprobe could successfully detect discharges from both the shallow,
local flow system and the deeper, more regional, bedrock flow systems.
10
Revil and Leroy showed the groundwater flow is responsible for an electric
field called the streaming potential, which is the main component of the so
called self-potential anomalies.
11
In the present context EC logs has been used for identifying the depth of
weathered zone/water bearing fractured rocks in bore wells. Present work
summaries the results from 17 observational bore wells in three different
hard rock problematic areas, and findings are compared with other geophys-
ical/geological outputs. The present study describes a technique involving
the use of electrical conductivity logs of bore well water (without of packers)
for identifying the water bearing fractures in hard rock areas. In addition
to this, technique may provide valuable information about the water table,
temperature of water at different depths, EC at different depths, quality of
water and saturation index at aquifer level etc.
2. Background of the Areas
The study area compared with three different locations in India (Fig. 1).
(a) Maheshwaram watershed is situated in the R.R. district of Andhra
Pradesh, India, about 30 km south of Hyderabad, covering an area 60 km
2
,
is a representative watershed in granitic rocks. The depth of water level in
this area varies from 14 to 26.25 m. Groundwater flow is mainly associated
with fractured rock aquifers. (b) Wailpally watershed is located about 60 km
towards east of Hyderabad, covering about 50 km
2
, also represented granitic
terrain. Groundwater occurs in shallow-weathered and deep-fractured gran-
ite and in weathered gneisses. The depth to water level varies from a
few meters to about 26 m below ground level. The groundwater is mostly
