Clay material characterization is of importance for many geo-engineering and environmental applications, and geo-electrical methods are often used to detect them in the subsurface. Spectral induced polarization (SIP) is a geo-electric method that non-intrusively measures the frequency-dependent complex electrical conductivity of a material, in the mHz to the kHz range. We present a new SIP dataset of four different types of clay (a red montmorillonite sample, a green montmorillonite sample, a kaolinite sample, and an illite sample) at five different salinities (initially de-ionized water, ~10-3, ~10-2, ~10-1, and 1 mol/L of NaCl). We propose a new laboratory protocol that allows the repeatable characterization of clay samples. The complex conductivity spectra are interpreted with the widely used phenomenological double-Pelton model. We observe an increase of the real part of the conductivity with salinity for all types of clay, while the imaginary part presents a non monotonous behavior. The decrease of polarization over conduction with salinity is interpreted as evidence that conduction increases with salinity faster than polarization. We test the empirical petrophysical relationship between σ”surf and σ’surf and validate this approach based on our experimental data and two other datasets from the literature. With this dataset we can better understand the frequency-dependent electrical response of different types of clay. This unique dataset of complex conductivity spectra for different types of clay samples is a step forward toward better characterization of clay formations in situ.

In volcanic islands, a crucial step in managing watershed water resources is the characterization of groundwater aquifers from local to regional scales. Airborne geophysical data provide high-resolution images down to hundreds of meters below the surface, over large areas. Yet, the production of an accurate interpretation of regional geophysical imagery may be time consuming or limited by the low density of geological and hydrological field observations. Here, we propose an approach combining airborne electromagnetic and magnetic data in order to reduce geophysical ambiguities and provide a multiscale hydrogeophysical characterization of Piton des Neiges volcano (Réunion Island). With limited calibration data, this methodology produces a geological model more accurate than using airborne electromagnetic data alone. Through the continuous coverage of both methods, we demonstrate the influence of volcanic unit geometries on groundwater flows within the critical zone and we highlight major structures impacting groundwater flows at both local and regional scales.

The use of geophysical tools for subsurface characterization is a common practice in environmental studies and georesources engineering. The electrical conductivity of the subsurface is strongly influenced by the different properties of the subsurface such as pore fluid chemistry, and consequently, by subsurface processes that affect the spatial distribution of that chemistry, such as the mixing dynamics of pore fluids. In the context of freshwater-saline water interaction in coastal areas, changes in solute spatial distribution are coupled to density-driven flow, which can thus be monitored via geoelectrical measurements. Here, we study the Rayleigh Taylor instability and subsequent convection occurring due to the density difference between two miscible liquids when the lighter one is positioned on top of the denser one, a configuration that is relevant for saltwater-freshwater interactions in coastal aquifers. We simulate the convective process and monitor it numerically by computing the transverse apparent conductivity of the medium in time, as the convection develops. We then look for correlations between the geoelectrical signal and a global scalar measure of the convective process' advancement, namely the variance of the solute concentration field.

Estimating evolution of exchanges within the stream-aquifer interface is frequently tackled with the help of numerical models. Yet, the definition of boundary conditions is generally based on poorly constrained assumptions and restrained to the location of piezometers. We suggest here to stretch the modeling domain and build stronger constraints, both in space and time, by using a multi-method approach. On a hotspot of the Orgeval Critical Zone observatory (France), we show how a thorough interpretation of high-resolution geophysical images, combined with geotechnical data, helps describing the spatial heterogeneities of the shallow aquifer. It provides a detailed distribution of hydrofacies, valuable prior information about the associated hydrodynamic properties and makes it possible to expand the modeling window in space. We show how the local temporal dynamic of the water table can be captured with high resolution time-lapse seismic acquisitions. Time-lapse variations in seismic data are discriminated from noise or measurement errors to be interpreted, regarding hydrological observations, as temporal changes in the saturated-unsaturated zone continuum. Each seismic snapshot is then thoroughly inverted to actually image spatial water content variations and delineate the water table outside the limits defined by the piezometers. This posterior geophysical information is then suggested as initial and boundary conditions of the expanded hydrogeological modeling domain. We finally calibrate and provide plausible ranges of hydraulic parameters to reproduce the water table and improve the estimation of stream-aquifer exchanges.