Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy) |
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| Institution: | 1. Dipartimento Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy;2. Dipartimento di Geoscienze di Padova, Italy;3. CNR-IDPA (Consiglio Nazionale delle Ricerche, Istituto per la Dinamica dei Processi Ambientali), via Mario Bianco 9, I-20131 Milano, Italy;1. Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, via Mangiagalli 34, 20133 Milano, Italy;2. Dipartimento di Scienze, Università degli Studi Roma Tre, 00146, Rome, Italy;3. Department of Geological Sciences, University of Florida, Gainesville, FL 32611, USA;1. Cumhuriyet University, Engineering Faculty, Department of Geophysical Engineering, Campus, 58140 Sivas, Turkey;2. Ankara University, Faculty of Eng., Department of Geophysical Engineering, Geophysical Modeling Group, Golbasi 06830, Ankara, Turkey;1. Sorbonne Universités, UPMC Univ Paris 06, UMR 7619, METIS, F-75005, Paris, France;2. Irstea, UMR G-EAU, Domaine de Lavalette, 361 rue Jean-François Breton, 34196 Montpellier cedex 5, France;3. Irstea, Hydrosystèmes et Bioprocédés, 1 rue Pierre-Gilles de Gennes, CS 10030, 92761 Antony cedex, France;4. Université des Sciences et de la Technologie Houari Boumediene, BP 32 El Alia, 16111, Bab Ezzouar, Alger, Algeria |
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| Abstract: | In this work we present the hydrogeophysical imaging of a key sector of the Quaternary Po foreland basin (northern Italy), focussing on the reconstruction of clastic aquifers and aquitards in a complex tectono-sedimentary subsurface architecture. The study area includes the relic reliefs of Casalpusterlengo and Zorlesco, two smooth morphological features involving uplifted and gently folded Pleistocene marine to alluvial sediments, plausibly linked to the buried Northern Apennines thrust and fold belt. The geophysical data include 35 Direct Current Vertical Electrical Soundings collected over a 37 km2 wide area, acquired with Schlumberger array and maximum half-spacing of 500 m. 1-D resistivity-depth profiles were computed for each VES. An integrated hydrostratigraphic approach was applied, to constrain the interpretation of the geophysical data along several cross-sections, including the comparison of resistivity soundings to stratigraphic logs, borehole electric logs and the pore-water properties.The resistivity interfaces, traceable with the same laterally continuous vertical polarity, were used to develop an electrostratigraphic model in order to portray the stacking of electrostratigraphic units down to 200 m below ground surface. Their vertical associations show a general upward increase of electrical resistivity. This assemblage mimics the regional coarsening upwards depositional trend, from the conductive units of the Plio-Pleistocene marine-to-transitional depositional systems to the resistive units of the Middle–Late Pleistocene fluvial and alluvial plain depositional systems. Middle Pleistocene depositional systems host an alternation of North-dipping, high-to-intermediate permeability aquifer systems (70–180 Ωm, thickness of 5–70 m) separated by low permeability aquitards (20–50 Ωm, thickness up to 40 m). These units pinch out against the Casalpusterlengo and Zorlesco relic reliefs, where they cover the uplifted and folded regional aquitard (20–50 Ωm) formed by Pliocene-Lower Pleistocene clays to sandy silts with gravel lenses in agreement with borehole data. In the deepest part of the local stratigraphy, a broad low-resistivity anomaly (< 10 Ωm) was clearly mapped through the study area. By comparison with electrical borehole logs in deep oil-wells, it could be interpreted as the fresh–saltwater interface due to the presence of connate waters and brines hosted by the marine-to-transitional shales. |
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