Hydrogeophysical imaging of alluvial aquifers: electrostratigraphic units in the quaternary Po alluvial plain (Italy)

Resistivity imaging of Pleistocene alluvial aquifers in a contractional tectonic setting: A case history from the Po plain (Northern Italy)

“…They consist of multilayer freshwater aquifers, including the phreatic and the non-saturated zone, delimited by a water table few meters deep in the LFP. From bibliographic data, electrical conductivity of freshwater extracted from water wells in the Lodi alluvial plain ranges between 500 μS/cm and 620 μS/cm corresponding to an average electrical resistivity of 18 Ωm (Mele et al, 2012). Groups C and D roughly correspond to the lower part of the Middle Pleistocene and Lower Pleistocene marine-to-transitional succession ("Villafranchiano" Auct.…”

“…VES data were processed with RES1D software (Loke, 2001) to determine the local electrostratigraphic sequence (i.e., vertical stacking of resistive above conductive electrolayers or vice versa; Mele et al, 2012) at each VES station. The models assume that the subsurface resistivity changes only with depth and no lateral resistivity variations exist, i.e., 1-D model.…”

“…A two-step correlation procedure was applied (Mele et al, 2012): (1) definition of the 1-D electrostratigraphic sequence; and (2) correlation of the resistivity interfaces between VES electrolayers, according to their polarity, in order to bound the subsurface volumes that are characterized by the minimum internal variation of electrical resistivity. These resistivity interfaces define an assemblage of quasi 3-D electrostratigraphic units (EsUs; Mele, 2009;Mele et al, 2012). EsUs are geophysical entities delimited by Fig.…”

“…Until now, the investigation has mainly focussed on multiplescale geoelectrical imaging of the most common sedimentary units encountered in alluvial hydrostratigraphy, like meandering rivers or braided rivers (Baines et al, 2002;Bersezio et al, 2007;Bowling et al, 2005Bowling et al, , 2007Bratus and Santarato, 2009;Mele, 2009;Mele et al, 2012;Sinha et al, 2012;Yadav et al, 2010). These results encouraged a new effort to use geoelectrical methods to investigate the regional architecture of alluvial hydrostratigraphy in a complex tectono-sedimentary setting.…”

“…1; 2) and 1-D resistivity inverse modeling was used to define a high-resolution electrostratigraphic model. The concept of "electro-stratigraphic units" (EsUs), as it was redefined by Mele et al (2012), was adopted to link the vertical and lateral heterogeneities of electrical properties to hydrostratigraphic heterogeneity (external shape, size, lateral continuity and connectivity of sedimentary bodies; textures and fine-to-coarse grained sediments ratios; facies associations and internal structure of the sedimentary bodies; distribution of diagenetic vs. pedogenetic features). Calibration to stratigraphic logs and borehole electric logs allowed correlation of the electrostratigraphic image to the stratigraphy of alluvial units and their marine substratum.…”