GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
Caratterizzazione degli acquiferi:
l’integrazione di metodologie
geoelettriche ed idrogeologiche
D. Nieto Yàbar, A. Affatato,A. Bratus, G. De Carlo, E. Marin, D. RaptiCaputo, G. Santarato, C. Vaccaro
Istituto Nazionale di Oceanografia e Geofisica Sperimentale, Trieste
Acque del Basso Livenza S.p.A., Annone Veneto (VE)
Dipartimento di Scienze della Terra, Università di Ferrara
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
Contents:
1.
2.
3.
4.
5.
Scope of the work
Hydrogeological outline
Geophysical methods
Results
Discussion.
www.cami-life.net
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
1. Scope of the work
•To delineate the whole sequence of aquifers, which
spans several hundreds of meters
•To estimate effective porosity of the aquifers, where
data of water conductivity are available,
•To monitorize the evolution of the aquifers with time
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
2. Hydrogeological
outline:
the study area
Foto di Arno Mohl - WWFAustria
4
+140
160-0001
Conceptual
181-0046
51
+160
181-0029
181-0026
+120
32
Hydrogeological
coarse
A
+80
Model
+60
conglomerates
unconfined
indifferentiated aquifer
(recharge area: pollution
possible!)
test area
resurgence line
+80
82
208-0029
165-0008
+100
+60
-120
169-0212
32
+120
-100
100
1428
181-0046
51
+140
169-0026
+160
-80
169-0036
4
+180
181-0029
181-0026
-60
169-0022
-40
1263C
985B
985A
-20
208-0059
0.00
208-0050
+20
160-0001
+40
169-0006
+100
169-0051
4
+180
+40
+20
0-140
0.00
-20
-40
-160
-60
N24°
-80
-100
N22°
N163°
N9°
N22°
-100
4 Km
-120
-140
-160
N24°
N22°
N163°
N9°
N22°
N175°
0
N145°
1000 m
Sketch model hydr. section A
N5°
N4°
N16°
N169°
N169°
N147°
N178°
N53°
N2°
2000 m
5
Sketch model hydr. section B
Unconfined aquifer
A0
+40
20
A1
+20
A1
0.00
50
169-0212
+60
1428
+80
169-0051
169-0026
169-0022
169-0006
1263C
985B
985A
0m
169-0036
208-0059
A0
coarse sand
-20
B
-40
Confined aquifers
clay
-60
-80
180
-100
A2
-120
A2
-140
200m
-160
N178°
N22°
N53°
N163°
N2°
169-0212
1428
169-0051
169-0026
169-0036
+60
169-0022
+80
82
208-0029
165-0008
+100
169-0006
resurgence line
1263C
985B
985A
32
+120
N147°
N24°
208-0059
+140
N169°
208-0050
181-0046
51
+160
160-0001
+180
N169°
181-0029
N16°
181-0026
N4°
4
Conceptual Hydrogeological Model
multi-aquifer system
+100
+40
+20
0.00
-20
-40
-60
-80
test area
-100
-120
-140
-160
N24°
N22°
N163°
N9°
N22°
N175°
0
N145°
1000 m
Sketch model hydr. section A
N5°
N4°
N16°
N169°
N169°
N147°
N178°
N53°
N2°
2000 m
6
Sketch model hydr. section B
Aquifer systems (100-507 m b.g.l.)
Torrate
sand
sandy-gravel
gravel
aquifer system
clay
7
The test area:
the “Torrate” exploitation field of “Acque del
Basso Livenza S.p.A.”
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
3. The test area: geo-electrical
methods
 2D and 3D Electrical Resistivity Tomography (ERT: 0 to
100 m b.g.l.)
 Transient (Time Domain) Electromagnetism (TEMTDEM, about 50-500 m b.g.l.)
 In-hole resistivity measurements (60 to 507 m b.g.l.)
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
ERT
Equipment: Syscal R2
Array: WennerSchlumberger
Electrodes: up to 128
Electrode spacing: 5 m
Acquisition mode:
resistivity
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
The 2D profiles
gravel
(resistivity)
Inversion method: smooth
(Loke’s RES2DINV)
clay
First confined
aquifer “A1”
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
The 3D ERT resistivity model
First confined
aquifer “A1”
La formamethod:
del primo
Inversion
smooth
acquifero
(Loke’s
RES3DINV)
-->Sub-horizontally layered geometry
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TDEM: layout of the
first survey (October
2005)
TD14
Loop n° 3
TD11
TD13
TD10
TD1_t
TD2/agosto
2D ERT
profiles
TD1/agosto
TD12
Gruppo
pumps
pompe
Loop n° 2
TD8
Equipment: Geonics TEM57/PROTEM
Frequency bands: 25, 6, 2.5 Hz
Transmitter loop: 200x200 m, current
10 to 12 A (M~4.105 to 5.105 A.m2)
Receiver loop: 100 coils, 1 m diameter
Receiver layout: centre and out of loop
on each side
TD6
TD7
TD9
TD4
TD3
Loop n° 1
TD1
TD5
TD2
13
200
m
Tavola 2
GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
TD13
TDEM: layout of
the repeated survey
(April 2006)
TD2
TD10
TD11
TD1
TD14
TD3
Loop n° 3
Loop n° 1
TD12
TD4
TD7
pumps
Gruppo
pompe
TD8
TD5
TD6
Loop n° 2
TD9
14
200 m 3
Tavola
GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
TDEM data quality:
sounding TD06/05
(central loop, high frequencies)
sounding TD08/05
(out of loop, high frequencies)
1E+002
1E+003
1E+002
1E+001
mV
1E+000
mV
1E-001
Data
(10 A)
1E+001
Data
(10 A)
1E+000
1E-001
1E-002
1E-002
1E-003
1E-003
noise
noise
1E-004
1E-004
1E-005
1E-005
0.01
0.1
1
ms
10
15
0.01
0.1
1
ms
10
TDEM: results
(view from SW)
Aquifer A1
Potential aquifer
(unknown before
this survey)
Inversion: 1D smooth
(Occam, unconstrained)
3D imaging via kriging
interpolation of 1D models
16
In-hole
resistivity
measurements
(lateral array)
sand
gravel
sandy-gravel
clay
aquifer system
17
from Rapti-Caputo et al., Hydrogeology Journal, 2008, in print
GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
Geophysical and hydrogeological parameters of the aquifers
aquifer
bulk
resistivity
(Wm)
water
conductivit
y
(mS/cm)
thicknes
s (m)
formation
factor
Estimated
porosity
(%;
m=1.3)
transmissivity
(m2/s)
hydraulic
conductivity
(m/sec)
A1
122*
530*
30±3
6.5
24
1.5x10-2
5x10-4
A2
225
380
13
8.5
19
5.2x10-3
4x10-4
A3
250
334
3
8.3
20
8.1x10-5
2.7x10-5
A6
330
296
>20
9.8
17
-
-
*october 2005 data.
average value estimated by 1D constrained inversion with fixed geometry, using bore-hole direct
information and seismic data (Giustiniani et al., 2008, Geophysical Prospecting).
Effective porosity was estimated using Archie’s law: a=1, m=1.3.
18
from Rapti-Caputo et al., 2008, in print on Hydrogeology Journal
Estimated effective porosity of aquifer A1
Pompe
Area dei pozzi di
alimentazione
dell’acquedotto
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GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
Resistivity variations vs. time
Wm
1
Second confined aquifer A2:
too low thickness-to-depth
ratio (lack of resolution)
1000 1
0
0
-200
-200
-400
-400
m
Sixth confined aquifer: large
variations of depth of its base,
due to electrical equivalence
and lesser data quality at the
latest TDEM decay times.
Additional information is
needed.
100
10
Wm
100
m
First confined aquifer A1: the
resistivity increases about 13%
10
-600
-600
-800
-800
-1000
-1000
Occam’s 1D inverted models of Soundings at the centre
of loop_2 (left) and at the centre of loop_3 (right).
20
Blue line: first survey;
red line: repeated survey.
1000
Resistivity variations vs. time
Electrical Conductivity
(E.C) of water in the
first confined aquifer
measured “in situ”,
october 2005: about 530
mS/cm in the test area
E.C. measured on may
2006 in the test area :
460 mS/cm
Observed decrease:
13%, in excellent
agreement with TDEM
data.
21
from Rapti-Caputo et al., 2008
GNGTS - 27° Convegno Nazionale
Trieste 6-8 ottobre 2008
Conclusions
Combining surface and in-hole resistivity data allows a
satisfactory characterisation of a multi-aquifer
sequence and of its main hydrogeological properties
Combined ERT-TDEM are an efficient tool to image
aquifers in an alluvial mattress
TDEM has shown a sufficiently high sensitivity to
resistivity variations, to be considered as a reliable tool
for monitoring purposes;
Aknowledgements:
Research carried out with the financial support of the EC, contract LIFE04 ENV/IT/00500.
22
M. Giustiniani∗, F. Accaino, S. Picotti and U. Tinivella: Characterization of the shallow
aquifers by high-resolution seismic data. Geophysical Prospecting, 2008, 56, 655–666
23
Based on the well known analytical relationship given by Niwas
and Singhal (1981):
T = KσR = KS/σ
(2)
where T = transmissivity, K = hydraulic conductivity, R = tρ (t and
ρ are the thickness and resistivity of the aquifer layer) is the transverse
resistance and S = t/ρ is the longitudinal conductance of the aquifer
layer under study, the transmissivity in the whole surveyed area can be
evaluated for aquifers A1, A2 and A3, bearing in mind that in areas of
similar geologic setting and water quality, the product Kσ remains
fairly constant.
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