Mauro Mele

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

The integration of surface geological and geomorphological information with borehole point-data and geophysical (e.g., geoelectrical) images of the subsurface yields spatially consistent representations of alluvial aquifers heterogeneity at different scales, from depositional systems to basin fills. Such an approach requires a conceptual framework to match the stratigraphic units with their evidence from ground-based DC resistivity methods to effectively fill the gaps between sparse borehole data and to obtain valid representations of sedimentary heterogeneities. Such an approach is applied to characterize two sites of the Quaternary aquifers of the central Po Plain (Italy), which represent (1) the middle-upper Pleistocene braided to meandering river depositional systems sitting on Southalpine crust and (2) their down-current counterparts, where they are involved by the latest uplift and deformation due to the tectonic activity of the Apennine frontal thrusts. Electrical resistivity was considered as a proxy of the litho-textural properties of hydrofacies and their major hierarchical association at depth and was interpreted in accordance with the depth-decreasing resolution of ground-based resistivity methods. Thus, it was possible to identify the geophysical signature of hydrostratigraphic units through “Electrostratigraphic Units”, i.e., sedimentary volumes identified by resistivity contrasts that spatially preserve the vertical polarity. Hydrostratigraphy and electrostratigraphy were then joined together through a site-specific relationship between electrical resistivity and hydraulic conductivity, which takes into account the prevailing process of current conduction, the litho-textural properties of hydrofacies and the groundwater electrical conductivity. At the scales of aquifer systems and complexes, this approach permitted to establish the conceptual framework to match hydrostratigraphy, electrostratigraphy, average hydrodynamic properties and distribution of heterogeneities.

Relating electrical conduction of alluvial sediments to textural properties and pore-fluid conductivity

Electrical conductivity of alluvial sediments depends on litho-textural properties, fluid saturation and porewater conductivity. Therefore, for hydrostratigraphic applications of direct current resistivity methods in porous sedimentary aquifers, it can be useful to characterize the prevailing mechanisms of electrical conduction (electrolytic or shale conduction) according to the litho-textural properties and to the porewater characteristics. An experimental device and a measurement protocol were developed and applied to collect data on eight samples of alluvial sediments from the Po plain (Northern Italy), characterized by different grain-size distribution, and fully saturated with porewater of variable conductivity. The bulk electrical conductivities obtained with the laboratory tests were interpreted with a classical two-component model, which requires the identification of the intrinsic conductivity of clay particles and the effective porosity for each sample, and with a three-component model. The latter is based on the two endmember mechanisms, surface and electrolytic conduction, but takes into account also the interaction between dissolved ions in the pores and the fluid-grain interface. The experimental data and their interpretation with the phenomenological models show that the volumetric ratio between coarse and fine grains is a simple but effective parameter to determine the electrical behaviour of clastic hydrofacies at the scale of the representative elementary volume.

Mapping the spatial variation of soil moisture at the large scale using GPR for pavement applications

The characterization of shallow soil moisture spatial variability at the large scale is a crucial issue in many research studies and fields of application ranging from agriculture and geology to civil and environmental engineering. In this framework, this work contributes to the research in the area of pavement engineering for preventing damages and planning effective management. High spatial variations of subsurface water content can lead to unexpected damage of the load-bearing layers; accordingly, both safety and operability of roads become lower, thereby affecting an increase in expected accidents. A pulsed ground-penetrating radar system with ground-coupled antennas, i.e., 600-MHz and 1600-MHz center frequencies of investigation, was used to collect data in a 16 m × 16 m study site in the Po Valley area in northern Italy. Two ground-penetrating radar techniques were employed to nondestructively retrieve the subsurface moisture spatial profile. The first technique is based on the evaluation of the dielectric permittivity from the attenuation of signal amplitudes. Therefore, dielectrics were converted into moisture values using soil-specific coefficients from Topp’s relationship. Groundpenetrating-radar-derived values of soil moisture were then compared with measurements from eight capacitance probes. The second technique is based on the Rayleigh scattering of the signal from the Fresnel theory, wherein the shifts of the peaks of frequency spectra are assumed comprehensive indicators for characterizing the spatial variability of moisture. Both ground-penetrating radar methods have shown great promise for mapping the spatial variability of soil moisture at the large scale.

Mapping Soil Water Capacity Through EMI Survey to Delineate Site-Specific Management Units Within an Irrigated Field

Abstract An accurate and high-resolution mapping of soil properties allows optimizing the management of irrigation and fertilization at field scale by applying variable amounts of water and nutrients. Site-specific management (SSM) is fundamental to improve crop yield and to use resources more efficiently, improving environmental sustainability. Adoption of site-specific management practices requires the delineation in the field of subregions with similar soil properties affecting yield (site-specific management units (SSMU)). It is common practice to characterize the spatial variability of soil properties through electromagnetic induction (EMI) surveys to obtain soil electrical conductivity (EC) maps that can be used to delineate SSMU. The objectives of this work, carried out over a uniformly drip-irrigated and fertilized maize, were to (i) delineate SSMU from EC maps; (ii) compare the SSMU inferred from measurements with two different EMI sensors; (iii) map the soil-available water-holding capacity (AWC) from EC maps through a regression model between EC and measured AWC; and (iv) evaluate significant differences of crop yield among the SSMU. The EC maps at increasing depths were processed through principal component analysis, and three SSMU were delineated for both EMI sensors using the Management Zone Analyst software. The significant difference in crop yield across the three SSMU, tested through the analysis of variance, suggested that AWC was the main limiting factor in crop yield. This result highlights the importance of a variable-rate irrigation based on SSMU, which could be a solution to save water and increase crop yield.

Exposing high-school students to Geosciences through seminars, laboratory and field demonstrations

To familiarize high school students with the richness of geosciences and their practical applications, Universita degli Studi di Milano has been conducting a number of activities in co-operation with the Liceo G. Bruno (Melzo, MI, Italy). This contribution discusses the number of activities conducted including classroom seminars, laboratory experiments and in-the-field demonstrations.

Comparing EM38 and Profiler-EMP400 for the Delineation of Homogeneous Management Zones within Agricultural Fields

The improvement in crop yield, both in quantity and quality, depends on the adoption of appropriate management strategies for the agronomic and irrigation practices. The adoption of site-specific (SS) management practices is fundamental, not only to improve crop yield, but also for a more efficient use of resources, increasing the environmental sustainability of the agricultural production The SS management requires the delineation of sub-regions with similar yield limiting factors or similar soil properties affecting yield (Site Specific Management Units – SSMU). It is a common practice in precision agriculture (PA) to characterize the spatial variability of soil properties, measuring the soil electrical conductivity through non-invasive electro-magnetic (EM) sensors to obtain high-resolution soil maps for the delineation of SSMUs. Because of the expanding use in the future of the multi-frequency EM sensors in order to more effectively assess the soil variability, the objective of this work is to compare the measurements collected by Geonics EM38 (the most widely used EM sensor in PA) and GSSI Profiler-EMP400 (a multi-frequency EM sensor) in order to assess their reliability to delineate SSMUs. The data from 2-D electrical resistivity imaging were used to compare the response of the two different sensors to soil variability.

Complex Electrical Properties of Sand-clay Mixtures and Alluvial Sediment Samples

The interpretation uncertainty of DC-resistivity surveys is reduced applying Spectral Induced Polarization methods. The aim of this work is to compare the complex electric resistivity behaviour, in the frequency range from 10 mHz to 100 kHz, of saturated sand-clay mixtures prepared in laboratory and alluvial muddy sands or sandy mud samples extracted from Po plain quarries (Italy). The comparison is made on the basis of the coarse-to-fine granulometric fraction ratio, defined with a cut-off diameter of 0.063 mm. Electrical analyses are conducted within a cylindrical sample-holder equipped with impolarizable potential electrodes and results are discussed in terms of magnitude and phase spectra. Large differences in the phase spectra are observed. They are interpreted in terms of mineralogical composition, especially of the fine-grained fraction, and organic matter content. In fact, the effect of water electrical conductivity on these samples is already showed by a series of systematic tests and the textural assemblage is controlled. To evaluate the contribution of sediments’ geochemical properties, data fitting through the Cole-Cole model is proposed with satisfactorily results. Since phase spectrum is quite sensitive to the fitting parameters, also a multi-objective approach that consider separately the fit of the magnitude and phase spectra has been tested.