Geology Reference
In-Depth Information
drawdowns in an observation well located at a radial distance
r
from the
4
T
Q
pumping well,
s
DN
=
as a function of:
Tt
Sr
Reduced time
t
z
=
for type A curves;
2
Tt
Sr
Reduced time
t
y
=
for type B curves;
2
y
where
T
is the transmissivity of the aquifer,
S
the storage coefficient,
S
y
the
specific yield,
t
the time since the starting of pumping. The interpretation
using this method consists in fitting the observed drawdowns on the abacus
constituted by two types of curves: type A curve for early times and type B
curve for later times. Both curves are characterised by the same parameter
2
K
K
, the
thickness of the aquifer
b
and the distance
r
between the observation and
pumping wells.
The application of this method (Table 1a) to the observation wells IFP-
1/1 and IFP-1/2 leads to the evaluation of transmissivities, storage coefficients
(
S
) and specific yields (
S
y
). Very similar values are obtained in each observation
wells for
T
A
and
T
B
showing the coherence of the interpretation of this
pumping test using Neuman method. The values obtained for specific yields
are consistent with other ones deduced both from indirect methods by
Magnetic Resonance soundings (Wyns et al., 1995) and from direct methods
by adjustment of water levels fluctuations using a global model (Engerrand,
2002).
rK
b
D
z
r
=
, which is a function of the permeability anisotropy
K
D
=
2
Table 1a:
Transmissivity and storage parameters obtained
by adjustment of drawdown
Observation
Pumping
r
T
A
T
B
T
A
/T
B
T
AB
S
S
y
(m
2
/s)
(m
2
/s)
(m
2
/s)
well
well
(m)
(-)
(-)
(-)
IFP-1/1
IFP-1
28
1.76E-05
1.96E-05
0.90
1.86E-05
7.0E-05 1.7E-03
IFP-1/2
IFP-1
27.5
1.71E-05
1.76E-05
0.97
1.74E-05
3.7E-05 1.5E-03
T
A
: transmissivity obtained by adjustment on type
A
curve,
T
B
: transmissivity obtained
by adjustment on type
B
curve,
T
AB
: average of
T
A
and
T
B
.
The determination of
K
D
needs the knowledge of the aquifer thickness
b
.
Flowmeter measurements during injection tests in eight wells of the basin
have shown that the fresh basement does not contain any conductive fracture.
It is namely the case in pumping well IFP-1. Thus, the top of this layer was
chosen as the bottom of the aquifer. As for classical methods, the uncertainty
on the value of
r
makes difficult any interpretation of drawdown in pumping
wells. The results of this interpretation, included in Table 1b, show an
anisotropy of the permeability tensor, in accordance with geological
