Geology Reference
In-Depth Information
bottom of the aquifer was chosen at the top of the massive granite layer.
Flowmeter tests show that transmissive fractures only exist above this fresh
basement.
Flowmeter Tests
The location of hydraulically conductive fissures was obtained using vertical
profiles of flowmeter measurements. In order to increase the vertical extension
of the investigated zone, the measurements are done during injection tests
and not pumping. A water tanker is brought to the site and water is directed
to a drum connected to the well through a pipe with a valve. A constant head
is maintained in the drum thanks to overflowing water flow. This allows
injecting constant flow rate. During the injection at constant rate
Q
when a
pseudo steady state is reached, the flowmeter, which measures the vertical
flow within the screen, is lowered close to the bottom of the bore-well and
a measurement of velocity is obtained. The flow meter is then raised a few
decimetres, another reading taken, raised another few decimetres, and so on.
As illustrated in the lower portion of Fig. 6, the final result is a series of data
points giving vertical discharge
Q
(
z
) within the screen as a function of depth
z
. Just above the top of the screen the meter reading should be equal to
Q
,
the steady injection rate that is measured independently on the surface with
a water meter. The procedure may be repeated several times to ascertain that
readings are stable.
As usually assumed, one considers the aquifer as a series of horizontal
layers. The difference between two successive meter readings constitutes the
net radial flow (
q
rj
) entering each layer of the aquifer:
QQ
e
i 1
i
q
rj
=
(4)
where
e
is thickness of the layer.
Then, the Dupuit formula for a confined aquifer and horizontal flow to a
well is applied to each layer, leading to the estimation of the layer hydraulic
conductivity:
q
R
hr
rj
K
i
=
ln
(5)
2
where
h
is the drawdown,
R
the investigation radius and
r
the well radius.
The aggregated permeability of the profile is obtained:
n
)
Ke
jj
j
1
K
p
=
(6)
n
)
e
j
j
1
