Combining sedimentological and geophysical data for high-resolution 3-D mapping of fluvial architectural elements in the Quaternary Po plain (Italy)

Current approaches to the reconstruction of the geometry of fluvial sediments of Quaternary alluvial plains and the characterization of their internal architecture are strongly dependent on core data (1-D). Accurate 2-D and 3-D reconstructions and maps of the subsurface are needed in hydrostratigraphy, hydrogeology and geotechnical studies. The present study aims to: 1) improve current methods for geophysical imaging of the subsurface by means of VES, ERGI and GPR data, and calibration with geomorphological and geological reconstructions, 2) optimize the horizontal and vertical resolution of subsurface imaging in order to resolve sedimentary heterogeneity, and 3) check the reliability/uncertainty of the results (maps and architectural reconstructions) by comparison with exposed analogues. The method was applied to shallow (0 to 15 m) aquifers of the fluvial plain of southern Lombardy (Northern Italy). At two sites we studied fluvial sediments of meandering systems of the Last Glacial Maximum and post-glacial historical age. These sediments comprise juxtaposed and superimposed gravel–sand units with fining-upward sequences (channel-bar depositional elements), which are separated by thin and laterally discontinuous silty and sandy clay units (overbank and flood plain deposits). The sedimentary architecture has been studied at different scales in the two areas.At the scale of the depositional system, we reconstructed the subsurface over an area of 4 km2 to a depth of 18 m (study site 1). Reconstructed sequences based on 10 boreholes and water-well stratigraphic logs were integrated with the interpretation of 10 vertical electrical soundings (VES) with Schlumberger arrays and 1570 m long dipole–dipole electrical resistivity ground imaging profiles (ERGI). In unsaturated sediments, vertical and horizontal transitions between gravel–sand units and fine-grained sediments could be mapped respectively at the meter- to decameter scale after calibration of the VES with borehole data. Similar information could be obtained in waterlogged sediments, in which the largest units could be portrayed and the lateral continuity of major hydrostratigraphic units could be assessed. Maps of apparent resistivity were combined with sand-to-clay ratio maps obtained from stratigraphic data, which substantially increased their quality. ERGI profiles added substantial information about the horizontal transitions between fine- and coarse-grained units. At the scale of depositional elements (channel-bar systems) we studied quarry exposures, over an area of about 4000 m2, down to 8 m below ground level (study site 2). In this case, facies analysis was performed on progressing quarry faces and integrated with a network of 165 m long ERGI profiles and 1100 m long ground-penetrating radar (GPR) profiles. Channel boundaries and accretion surfaces of point bars were resolved by both GPR and ERGI, which permitted 3-D mapping of these surfaces.Comparison between the results obtained for the two study sites demonstrates that integration of sedimentological data with geophysical imaging (ERGI and VES) enables the identification of stratigraphic units at the scale of depositional elements. Moreover, fining-upward trends and other internal features of the deposits, such as the transitions from coarse to fine-grained sediments within channel-bar complexes, could be resolved. Hence, the combination of sedimentological and geophysical methods provides a more accurate 3-D reconstruction of hydrostratigraphically significant sedimentary units compared to reconstructions based solely on borehole/point data.