An electrostratigraphic cross-section across the central Po plain : bearings on subsurface geology and hydrostratigraphy

A 72-km-long N–S cross-section was surveyed with direct current resistivity soundings through the Quaternary clastic infill of the Apennine foredeep in the Po plain of Lombardy (Northern Italy). The aims are (1) to test the potential of DC resistivity methods at the regional scale; (2) to draw the electrostratigraphic picture of the basin fill, down to the pre-Quaternary substratum; (3) to refine and test the methodology and concepts of hierarchic electrostratigraphy at the basin scale; (4) to contribute to characterize the regional hydrostratigraphy of a relevant hydrogeological basin. 163 resistivity-versus-depth profiles with a maximum investigation depth of 450 m were obtained from vertical electrical soundings. To draw the electrostratigraphic cross-section, the polarity of the resistivity contrast across resistivity interfaces (i.e. resistive over conductive layers or vice versa) was considered and the discontinuities were correlated according to the persisting pattern from north to south. Doing so, the hierarchic assemblage of four major electrostratigraphic boundaries and the five intervening groups of electrostratigraphic units (EsUs) was established. The resulting picture was compared to the available subsurface reconstructions from the literature, using the 1-D borehole data only to check the geophysical image at some random sample sites. The results show that the electrostratigraphic picture captures the geological and hydrostratigraphic complexity of the subsurface of the basin comparing well with the available direct subsurface geological and hydrostratigraphic reconstructions. At the regional scale of the cross-section, two hierarchic orders of EsUs and correlative boundaries could be drawn (first-order groups and second-order individual EsUs), while the lowermost third-order rank, the electrolayer, is recognized at the single-VES sites only.