Post on 27-Mar-2015
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Caratterizzazione degli acquiferi: l’integrazione di
metodologie geoelettriche ed idrogeologiche
D. Nieto Yàbar, A. Affatato,A. Bratus, G. De Carlo, E. Marin, D. Rapti-Caputo, G. Santarato, C. Vaccaro
Istituto Nazionale di Oceanografia e Geofisica Sperimentale, TriesteAcque del Basso Livenza S.p.A., Annone Veneto (VE)Dipartimento di Scienze della Terra, Università di Ferrara
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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Contents:
1. Scope of the work2. Hydrogeological outline3. Geophysical methods4. Results5. Discussion.
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
www.cami-life.net
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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
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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2. Hydrogeological outline:
the study area
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
Foto di Arno Mohl - WWFAustria
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unconfined indifferentiated aquifer
(recharge area: pollution possible!)
Conceptual
Hydrogeological
Model
coarse
conglomerates
A
resurgence line test area
Sketch model hydr. section A Sketch model hydr. section B
4 Km
0
-100
100
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multi-aquifer system A0
A1
A2
Confined aquifers
20
50
180
200m
0m Unconfined aquifer
A2
A1
A0
resurgence line
Con
cep
tual H
yd
rog
eolo
gic
al M
od
el
coarse sand
clay
test area
B
Sketch model hydr. section A Sketch model hydr. section B
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Aquifer systems (100-507 m b.g.l.)
Torrate
sand
gravel
sandy-gravel
clay
aquifer system
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The test area: the “Torrate” exploitation field of “Acque del
Basso Livenza S.p.A.”
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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 (TEM-TDEM, about 50-500 m b.g.l.)
In-hole resistivity measurements (60 to 507 m b.g.l.)
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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ERT
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
Equipment: Syscal R2Array: Wenner-SchlumbergerElectrodes: up to 128Electrode spacing: 5 mAcquisition mode: resistivity
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The 2D profiles(resistivity)
Inversion method: smooth (Loke’s RES2DINV)
First confined aquifer “A1”
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
clay
gravel
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La forma del primo La forma del primo acquiferoacquifero
Inversion method: smooth (Loke’s RES3DINV)
First confined aquifer “A1”
The 3D ERT resistivity model
-->Sub-horizontally layered geometry
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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TDEM: layout of the first survey (October 2005)
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 1 3
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T a v o l a 2
G r u p p op o m p e
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 1 3
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 1 3
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 2
T D 1
T D 4
T D 3
T D 5
T D 9
T D 6T D 8T D 7
T D 1 2
T D 1 / a g o s t o
T D 2 / a g o s t o
T D 1 1
T D 1 0
T D 1 4
T D 1 3
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T a v o l a 2
G r u p p op o m p eG r u p p op o m p e
pumps
200 m
Equipment: Geonics TEM57/PROTEMFrequency bands: 25, 6, 2.5 HzTransmitter loop: 200x200 m, current 10 to 12 A (M~4.105 to 5.105 A.m2)Receiver loop: 100 coils, 1 m diameterReceiver layout: centre and out of loop on each side
2D ERTprofiles
TD1_t
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TDEM: layout of the repeated
survey (April 2006)
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T a v o la 3
G r u p p op o m p e
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
T D 9
T D 5T D 8
T D 6
T D 1 2
T D 1 1 T D 1 0
T D 1 3
T D 1 4
T D 4
T D 1
T D 2
T D 3
T D 7
L o o p n ° 1
L o o p n ° 2
L o o p n ° 3
T a v o la 3
G r u p p op o m p eG r u p p op o m p epumps
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
200 m
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TDEM data quality:
0.01 0.1 1 10
ms
1E-005
1E-004
1E-003
1E-002
1E-001
1E+000
1E+001
1E+002
mV
Data(10 A)
noise
sounding TD06/05(central loop, high
frequencies)
0.01 0.1 1 10
ms
1E-005
1E-004
1E-003
1E-002
1E-001
1E+000
1E+001
1E+002
1E+003
mV
sounding TD08/05(out of loop, high frequencies)
Data(10 A)
noise
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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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
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In-hole resistivity measurements(lateral array)
sand
gravel
sandy-gravel
clay
aquifer system
from Rapti-Caputo et al., Hydrogeology Journal, 2008, in print
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aquifer bulk resistivity
(m)
water conductivit
y(S/cm)
thickness (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 - -
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
Geophysical and hydrogeological parameters of the aquifers
from Rapti-Caputo et al., 2008, in print on Hydrogeology Journal
*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.
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Pompe Area dei pozzi di alimentazione dell’acquedotto
Estimated effective porosity of aquifer A1
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1 10 100 1000m
-1000
-800
-600
-400
-200
0
m
-1000
-800
-600
-400
-200
0
Resistivity variations vs. time
Occam’s 1D inverted models of Soundings at the centre of loop_2 (left) and at the centre of loop_3 (right). Blue line: first survey; red line: repeated survey.
First confined aquifer A1: the resistivity increases about 13%Second confined aquifer A2: too low thickness-to-depth ratio (lack of resolution)
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.
1 10 100 1000m
-1000
-800
-600
-400
-200
0
m-1000
-800
-600
-400
-200
0
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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Resistivity variations vs. time
Electrical Conductivity (E.C) of water in the first confined aquifer measured “in situ”,
october 2005: about 530 S/cm in the test
area
E.C. measured on may 2006 in the test area :460 S/cm
Observed decrease: 13%, in excellent agreement with TDEM data.
from Rapti-Caputo et al., 2008
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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.
GNGTS - 27° Convegno NazionaleTrieste 6-8 ottobre 2008
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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
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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.