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Caratterizzazione degli acquiferi: l’integrazione di metodologie geoelettriche ed idrogeologiche

This study focuses on the integration of geoelectrical and hydrogeological methods to characterize aquifers. The scope of the work includes delineating the aquifers, estimating effective porosity, and monitoring aquifer evolution over time.

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Caratterizzazione degli acquiferi: l’integrazione di metodologie geoelettriche ed idrogeologiche

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  1. 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. Rapti-Caputo, 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

  2. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Contents: • Scope of the work • Hydrogeological outline • Geophysical methods • Results • Discussion. www.cami-life.net

  3. 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

  4. Foto di ArnoMohl - WWFAustria GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 2. Hydrogeological outline:the study area

  5. test area 4 Km Conceptual Hydrogeological Model coarse A conglomerates unconfined indifferentiated aquifer (recharge area: pollution possible!) resurgence line 100 0 -100 Sketch model hydr. section B Sketch model hydr. section A

  6. B resurgence line test area multi-aquifer system A0 0m Unconfined aquifer A0 20 A1 A1 coarse sand 50 Confined aquifers clay 180 A2 A2 200m Conceptual Hydrogeological Model Sketch model hydr. section B Sketch model hydr. section A

  7. sand sandy-gravel aquifer system gravel clay Aquifer systems (100-507 m b.g.l.) Torrate

  8. The test area: the “Torrate” exploitation field of “Acque del Basso Livenza S.p.A.”

  9. 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 (TEM-TDEM, about 50-500 m b.g.l.) • In-hole resistivity measurements (60 to 507 m b.g.l.)

  10. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 ERT Equipment: Syscal R2 Array: Wenner-Schlumberger Electrodes: up to 128 Electrode spacing: 5 m Acquisition mode: resistivity

  11. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 The 2D profiles(resistivity) gravel Inversion method: smooth (Loke’s RES2DINV) clay First confined aquifer “A1”

  12. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 The 3D ERT resistivity model First confined aquifer “A1” La forma del primo acquifero Inversion method: smooth (Loke’s RES3DINV) -->Sub-horizontally layered geometry

  13. 200 m TDEM: layout of the first survey (October 2005) TD1_t 2D ERT profiles pumps 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

  14. 200 m GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 TDEM: layout of the repeated survey (April 2006) pumps

  15. 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) Data (10 A) Data (10 A) noise noise

  16. 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

  17. In-hole resistivity measurements (lateral array) sand gravel sandy-gravel clay aquifer system from Rapti-Caputo et al., Hydrogeology Journal, 2008, in print

  18. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Geophysical and hydrogeological parameters of the aquifers *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. from Rapti-Caputo et al., 2008, in print on Hydrogeology Journal

  19. Pompe Area dei pozzi di alimentazione dell’acquedotto Estimated effective porosity of aquifer A1

  20. GNGTS - 27° Convegno Nazionale Trieste 6-8 ottobre 2008 Resistivity variations vs. time 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. 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.

  21. 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. from Rapti-Caputo et al., 2008

  22. 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.

  23. 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

  24. 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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