Giorgio Cassiani

Vadose zone flow model parameterisation using cross-borehole radar and resistivity imaging

Cross-borehole geoelectrical imaging, in particular electrical resistivity tomography (ERT) and transmission radar tomography, can provide high-resolution images of hydrogeological structures and, in some cases, detailed assessment of dynamic processes in the subsurface environment. Through appropriate petrophysical relationships, these tools offer data suitable for parameterising and constraining models of groundwater flow. This is demonstrated using cross-borehole radar and resistivity measurements collected during a controlled vadose zone tracer test, performed at a field site in the UK Sherwood Sandstone. Both methods show clearly the vertical migration of the tracer over a 200 h monitoring period. By comparing first and second spatial moments of changes in moisture content predicted from a numerical simulation of vadose zone flow with equivalent statistics from two- and three-dimensional ERT and cross-borehole radar profiles the effective hydraulic conductivity is estimated to be approximately 0.4 m/d. Such a value is comparable to field estimates from borehole hydraulic tests carried out in the saturated zone at the field site and provides valuable information that may be utilised to parameterise pollutant transport models of the site.

Relationship between spectral induced polarization and hydraulic properties of saturated and unsaturated sandstone

There is growing interest in the use of geophysical methods for hydrological model parameterization. Empirical induced polarization (IP)–hydraulic conductivity (K) relationships have been developed, but these are only applicable to sediments in which the IP response shows limited variation with electrical current frequency. Here we examine the spectral IP response of samples taken from a UK sandstone aquifer and compare measured parameters with physical and hydraulic properties. We demonstrate the limited value of existing IP-K models due to the inherent IP frequency dependence of these samples. Our results show how the mean relaxation time, τ, is a more appropriate measure of IP response for these sediments. A significant inverse correlation between the surface area to pore volume ratio and τ is observed, suggesting that τ is a measure of a characteristic hydraulic length scale. This is supported by a measured strong positive correlation between log τ and log K. Our measurements also reveal evidence of a relationship between τ and a dominant pore throat size, which leads to postulations about the parallelism between the spectral IP behavior and unsaturated hydraulic characteristics. Additional experiments show how the relaxation time is affected by degree of fluid saturation, indicating that saturation levels must be accounted for if our empirical relationships are applied to vadose zone studies. Our results show clear evidence of the potential value of frequency-based IP measurements for parameterization of groundwater flow models.

Monitoring the hydrologic behaviour of a mountain slope via time-lapse electrical resistivity tomography

Catchment and hillslope hydrology is a major research area in geoscience and the understanding of its underlying processes is still poor. Direct investigation of steep hillslopes via drilling is often infeasible. In this paper, we present the results of non-invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Central Alps. The hillslope considered is steep (30–35°), covered with grass and a soil layer 1–1.5 m thick above a variably fractured metamorphic bedrock. The key hydrologic question is whether rainfall infiltrates mainly into the underlying fractured bedrock, thus recharging a deeper hydraulic system, or flows in the soil layer as interflow towards the stream channel a few hundred metres downhill. In order to respond to this question, we applied 2200 mm of artificial rain on a 2 m × 2 m slope box over about 18 hours. We estimated the effective infiltration by subtracting the measured runoff (7% of total). Due to the limited irrigation time and the climate conditions, the evapotranspiration was considered as negligible. The soil moisture variation and the underlying bedrock were monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT experiment was performed using 2 m deep boreholes purposely drilled and completed with electrodes in the irrigated plot. A larger scale (35 m long) 2D surface ERT survey was also continuously acquired across the irrigated area. Monitoring continued up to 10 days after the experiment. As a result, we observed a very fast vertical infiltration through the soil cover, also favoured by preferential flow patterns, immediately followed by infiltration into the fractured bedrock. The surface layer showed a fast recovery of initial moisture condition nearly completed in the first 12 hours after the end of irrigation. The lateral transmission of infiltrating water and runoff were negligible as compared to the vertical infiltration. These experiment results confirm that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study. We concluded that deep water circulation is the key pathway to hillslope processes at this site.important

Spectral induced polarization for the characterization of free-phase hydrocarbon contamination of sediments with low clay content

The identification of organic pollutants in the soil and the subsurface is a goal of primary importance in the management of contaminated sites. However, only a few non-invasive techniques can be useful towards this goal. One such technique is spectral induced polarization. In this study, we investigate the spectral induced polarization effect of changing fluid saturation in a well-characterized porous medium, analysing the difference between air and hydrocarbons, at different degrees of water saturation. The experiments were conducted on fine eolic sand samples coming from an experimental site near Turin, Italy. Octanol and benzene were used as non-aqueous phase liquids. Samples were initially saturated with water having controlled electrical conductivity and subsequently de-saturated stepwise with injection of air at known pressure. The eolic sand samples were then re-saturated with the same water contaminated with hydrocarbons and then a non-aqueous phase liquid phase (either octanol or benzene) was injected in volumetric steps, in order to compare the effects of air and non-aqueous phase liquid invasion. At each saturation step, spectral induced polarization measurements were conducted in the 0.01 Hz to 1 kHz range using the ZEL-SIP04 impedance meter developed at the Forschungszentrum Juelich. The measurement setup guaranteed a 1 mrad phase precision for the entire frequency measurement range. Measurements were conducted under temperature controlled conditions at 20 (±0.5)° C. All spectral induced polarization curves show a peak in the range 0.01–1 Hz that changes in intensity and frequency with varying saturation and a high-frequency phase shift increase dominated by capacitive coupling effects of the measuring system. A multiple Cole-Cole model was fitted to the data. The effects of de-saturation on the low-frequency Cole-Cole parameters are that a) resistivity increases with decreasing water saturation but increases less with non-aqueous phase liquid than with the same volume of air; b) chargeability increases with decreasing water saturation but in presence of non-aqueous phase liquids its value is sometimes lower, sometimes higher and sometimes similar to the one observed in presence of air; c) the time constant τ increases with decreasing water saturation and is consistently larger with non-aqueous phase liquid than with air. These differences between air and non-aqueous phase liquid injection can be explained in terms of differences in non-aqueous phase distribution within the porous medium, as observed by X-ray micro-CT: while air is homogeneously distributed, non-aqueous phase liquids segregate under density effects. In summary, all spectral induced polarization effects of air and non-aqueous phase liquid injection in the considered porous medium are volumetric, i.e., are not due to interaction with grain surfaces or other electrical-chemical effects but are caused by pore obstruction by the electrically non-conductive phase.

Integrated Geophysical Characterization of a Hydrocarbon Contaminated Site

The characterization of contaminated sites requires that direct investigations be supplemented with a suitable set of less-invasive, and more extensive, measurements. A combination of geophysical methods and direct push penetrometric techniques has been recently proposed as the backbone of site characterization within the EU FP7 project ModelPROBE. Here we present the first results of the investigations conducted at a field site in Trecate (NW Italy) which was affected in 1994 by crude oil contamination from a well blowout. The investigations include surface GPR, ERT, IP, SIP and SP surveys, together with direct push sampling and EC logs and limited cross-hole measurements. Many of the geophysical measurements have been conducted in time-lapse mode in order to separate static and dynamic signals, the latter particularly linked to strong seasonal changes in water table elevations. The goal is to identify (a) the structural characteristics that controlled the contaminant penetration into the subsurface and its current possible movements, and (b) assess possible correlation between measured geophysical properties and contamination levels and/or biodegradation of contaminants. Our preliminary results help provide a reasonable description of the contaminant infiltration mechanisms into the subsurface, while further analyses are necessary to establish a direct link with contamination and biodegradation.

Stochastic Analysis of Cross-hole GPR Data for Subsurface Characterization

Ground penetrating radar (GPR) is a well-established geophysical technique, used in particular via specific relationships to estimate hydrological parameters in vadose zone, i.e. moisture content, both from the surface and in boreholes. The principal aim of this work is to have a more complete view of how boreholes GPR ZOP measurements are informative of the subsoil geometry and distribution of relative permittivity. For this purpose an electromagnetic (EM) wave simulator has been applied within a stochastic Monte Carlo framework. In this manner both averaging and critically refracted wave effects are taking into account. Results from synthetic and real ZOP datasets are statistically analysed to deduce what kind of subsoil er-distributions are resolvable and well defined with a degree of uncertainty. The results illustrate how the often employed direct-wave approach is not able to take into account the complexity of the system. It usually reconstructs a smooth profile, sometimes underestimating the real permittivity: particularly a high er layer might be underestimated, when it is enclosed between low er media. Results show that care must be used inverting ZOP data for physical parameter estimation, subsurface stratification could be more complex than appears from direct ZOP evidences.

Stochastic Analysis of Cross-hole Gpr Data for Subsurface Characterization

We analyze the relationship between cross-hole GPR travel times and the spatial distribution of dielectric properties that conditions such data. The most common approach is to translate directly travel times into electromagnetic velocities and these in turn into dielectric constants and soil volumetric moisture content. This approach is known to lead to over-smoothed moisture content profiles that are not necessarily compatible with the true soil moisture content that should be reproduced by water flow models. We used an approach based on full waveform propagation and Monte Carlo simulations to assess, in both synthetic and real case studies, how reliable this direct wave approach is, and to develop an alternative stochastic inversion procedures that proves to provide robust estimates of soil moisture content profiles, also in presence of sharp boundaries.

Electrical Resistivity Tomography Time-lapse Monitoring of Three-dimensional Synthetic Tracer Test Experiments

Time-lapse electrical resistivity tomography (ERT) represents a powerful tool for subsurface solute transport characterization since a full picture of the spatio-temporal evolution of the process can be obtained. However, the quantitative interpretation of tracer tests is difficult because of the uncertainty related to the geo-electrical inversion and the constitutive models linking geophysical and hydrological quantities. Here a new approach based on the Lagrangian formulation of transport and the ensemble Kalman filter (EnKF) data assimilation technique is applied to assess the spatial distribution of hydraulic conductivity K by incorporating time-lapse cross-hole ERT data. Under the assumption that the solute spreads as a passive tracer, for high Peclet numbers the spatial moments of the evolving plume are dominated by the spatial distribution of the hydraulic conductivity. The assimilation of the electrical conductivity 4D images allows updating of the hydrological state as well as the spatial distribution of K. Thus, delineation of the tracer plume and estimation of the local aquifer heterogeneity can be achieved at the same time by means of this interpretation of time-lapse electrical images from tracer tests.

Static and dynamic aspects of near surface characterization through physics-based integration of GPR, ERT, SIP and SP data in the time-lapse mode

The use of geophysics for the characterization of the near surface is requiring more and more frequently that data be analysed quantitatively to offer meaningful information for the specific discipline object of investigation. This is true for all applications, including environmental studies, hydrology, soil science and geotechnics. This tendency leads substantially to overtaking of the classical approach to geophysics as a pure imaging technique, and requires in-depth understanding of the information contained in each specific physical measurement. Irrespective of the specific application, the geophysical response of the near surface is essentially controlled by a combination of geological (“static”) and ambient (“dynamic”) factors. The latter include moisture content and temperature variations. The separation of static and dynamic factors is the key step towards a quantitative use of near surface geophysics, as individual disciplines and applications may be interested selectively in one or more of the static or dynamic aspects, or combinations. Physico-mathematical modelling is often a fundamental tool that helps to discriminate between static and dynamic aspects, extracting the factors of specific interest for the application at hand. A link between measured geophysical quantities and the corresponding quantities of practical interest can only be established in the form of quantitative constitutive relationships. As many applications can benefit from the joint application of multivariate geophysical measurements (e.g. ERT, GPR, SIP etc) it would be highly advantageous to develop constitutive laws that in turn depend on few parameters that can be independently measured and that have a common, albeit different, impact on several geophysical data. In this contribution we illustrate the above general framework with a number of applications including catchment hydrology, digital soil mapping, contaminated site characterization and subsurface hydrology.

Non Invasive Characterization of a Small Mountain Catchment for Hydrological Purposes

The description of catchment response during storms and droughts is a fundamental issue in geosciences. In particular, the subsoil characterization is a crucial step towards understanding hydrological processes in mountain regions and for the calibration of models that can describe and predict water flow patterns. Non-invasive (or minimally invasive) geophysical techniques are helpful to provide spatially extensive data, whereas traditional borehole-based sampling is often limited because of the localized nature of such measurements and the disturbance induced to samples. The aim of this study is the characterization of a small-scale mountain catchment to obtain detailed knowledge about its hydrogeological and hydrological system behavior. The experimental site is located at Carre near Vicenza (North-Eastern Italy) in the pre-alpine hill region named Bregonze. For this purpose we used several geophysical techniques: Electrical Resistivity Tomography (ERT), Seismic refraction survey, Seismic surface wave investigation. The former techniques highlight the shallowest subsoil discontinuities (from 0 to 2 m); the latter one provides knowledge about the underlying layers and the associated lateral heterogeneities. The combination of geophysical methods can help characterize a complex hydrogeological system, at least from the structural viewpoint.