Rita Deiana

An experiment of non-invasive characterization of the vadose zone via water injection and cross-hole time-lapse geophysical monitoring

The characterization of the vadose zone, i.e. the part of the subsurface above the water table, is a challenging task. This zone is difficult to access with direct methods without causing major disturbance to the natural in-situ conditions. Hence the increasing use of geophysical methods capable of imaging the water presence in the vadose zone, such as ground-penetrating radar (GPR) and electrical resistivity tomography (ERT). This type of monitoring can be applied both to processes of natural infiltration and to artificial injection (tracer) tests, by collecting multiple data sets through time (time-lapse mode). We present the results of a water-injection experiment conducted at a test site in Gorgonzola, east of Milan (Italy). The site is characterized by Quaternary sand and gravel sediments that house an extensive unconfined aquifer, potentially subject to pollution from industrial and agricultural sources. ERT and GPR profiles were acquired in 2D cross-hole configuration and time-lapse mode over a period of several days preceding and following the injection of 3.5 m 3 of fresh water in a purpose-excavated trench. A 3D model of the water-infiltration experiment was calibrated against the time-lapse cross-hole data, particularly focusing on the ability of the model to reproduce the vertical motion of the centre of mass of the injected water as imaged by GPR and ERT. This model calibration provided an estimate of the isotropic hydraulic conductivity of the sediments in the range of 5–10 m/d. However, all isotropic models overpredict the measured excess of moisture content, caused by water injection, as imaged by GPR. The calibration of anisotropic models for the vertical hydraulic conductivity, with the horizontal hydraulic conductivity determined by direct measurement, also leads to a good fit of the sinking of the centre of mass, with a better mass balance in comparison with field data. The information derived from the experiment is key to a quantitative assessment of aquifer vulnerability to pollutants infiltrating from the surface.

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

A tracer test in a shallow heterogeneous aquifer monitored via time-lapse surface electrical resistivity tomography

We illustrate a case study of a saline tracer test in a shallow, highly heterogeneous aquifer, monitored by means of surface time-lapse ERT. The test was aimed at identifying the system’s hydraulic properties. Some of the expected limitations of the method — particularly caused by the strong decrease in ERT resolution with depth — and the consequent problems with mass balance and moment calculation could be partly balanced by the use of direct measurements of groundwater electrical conductivity and tracer concentration at one selected location. The vast heterogeneity of the system, ranging in lithology from clay to gravel at a scale of meters to tens of meters, reflects itself in the tracer migration and distribution over time: The tracer is trapped in the low-permeability regions and from these it is slowly released over time. High-resolution surface ERT proves effective at picturing this system behavior over time. The extreme heterogeneity is also a challenge in the attempt to translate bulk electrical ...

Coupled and uncoupled hydrogeophysical inversions using ensemble Kalman filter assimilation of ERT-monitored tracer test data

Recent advances in geophysical methods have been increasingly exploited as inverse modeling tools in groundwater hydrology. In particular, several attempts to constrain the hydrogeophysical inverse problem to reduce inversion errors have been made using time-lapse geophysical measurements through both coupled and uncoupled (also known as sequential) inversion approaches. Despite the appeal and popularity of coupled inversion approaches, their superiority over uncoupled methods has not been proved conclusively; the goal of this work is to provide an objective comparison between the two approaches within a specific inversion modeling framework based on the ensemble Kalman filter (EnKF). Using EnKF and a model of Lagrangian transport, we compare the performance of a fully coupled and uncoupled inversion method for the reconstruction of heterogeneous saturated hydraulic conductivity fields through the assimilation of ERT-monitored tracer test data. The two inversion approaches are tested in a number of different scenarios, including isotropic and anisotropic synthetic aquifers, where we change the geostatistical parameters used to generate the prior ensemble of hydraulic conductivity fields. Our results show that the coupled approach outperforms the uncoupled when the prior statistics are close to the ones used to generate the true field. Otherwise, the coupled approach is heavily affected by “filter inbreeding” (an undesired effect of variance underestimation typical of EnKF), while the uncoupled approach is more robust, being able to correct biased prior information, thanks to its capability of capturing the solute travel times even in presence of inversion artifacts such as the violation of mass balance. Furthermore, the coupled approach is more computationally intensive than the uncoupled, due to the much larger number of forward runs required by the electrical model. Overall, we conclude that the relative merit of the coupled versus the uncoupled approach cannot be assumed a priori and should be assessed case by case.

An iterative particle filter approach for coupled hydro-geophysical inversion of a controlled infiltration experiment

The modeling of unsaturated groundwater flow is affected by a high degree of uncertainty related to both measurement and model errors. Geophysical methods such as Electrical Resistivity Tomography (ERT) can provide useful indirect information on the hydrological processes occurring in the vadose zone. In this paper, we propose and test an iterated particle filter method to solve the coupled hydrogeophysical inverse problem. We focus on an infiltration test monitored by time-lapse ERT and modeled using Richards equation. The goal is to identify hydrological model parameters from ERT electrical potential measurements. Traditional uncoupled inversion relies on the solution of two sequential inverse problems, the first one applied to the ERT measurements, the second one to Richards equation. This approach does not ensure an accurate quantitative description of the physical state, typically violating mass balance. To avoid one of these two inversions and incorporate in the process more physical simulation constraints, we cast the problem within the framework of a SIR (Sequential Importance Resampling) data assimilation approach that uses a Richards equation solver to model the hydrological dynamics and a forward ERT simulator combined with Archies law to serve as measurement model. ERT observations are then used to update the state of the system as well as to estimate the model parameters and their posterior distribution. The limitations of the traditional sequential Bayesian approach are investigated and an innovative iterative approach is proposed to estimate the model parameters with high accuracy. The numerical properties of the developed algorithm are verified on both homogeneous and heterogeneous synthetic test cases based on a real-world field experiment.

Vertical radar profiling for the assessment of landfill capping effectiveness

In this paper we present the results of the characterization of a large landfill cap by means of ground-penetrating radar (GPR) measurements. The GPR data were collected in boreholes, using a vertical radar profile (VRP) configuration, where one antenna was kept at the ground surface while the other was progressively lowered into the borehole. This yields a vertical profile of GPR velocity from which a moisture content profile can be obtained. VRPs were conducted in 15 boreholes available on-site, all having been drilled through the protective cap and the waste mass into the underlying native soil. The separation between the boreholes (many tens of metres) makes it infeasible to characterize the site via other forms of hole-to-hole GPR measurements, with the exception of a few pairs of holes drilled at the periphery of the waste mass and therefore of limited usefulness. The VRP data allowed for the characterization of the moisture content profile across the waste body down to the water table, very close to the natural land surface, providing evidence that the waste is generally fairly dry (moisture content less than 10% on average). In order to assess the effectiveness of the cap, two surveys where conducted in March and April 2005 using all the boreholes with the aim of identifying moisture content changes due to natural rainfall and especially artificial irrigation over a limited area surrounding one of the boreholes. The time-lapse VRP results show that in the irrigated area a measurable amount of water seeps through the cap and changes the waste moisture content. Elsewhere, we basically observed no changes in moisture content due to natural infiltration during the same period: this fact does not confirm or exclude that some degree of leakage can occur. Direct in-situ permeability measurements, albeit more localized than VRP data, confirm that the landfill cap is not totally impermeable, thus corroborating the results derived from geophysical measurements.

Noninvasive characterization of the Trecate (Italy) crude-oil contaminated site: links between contamination and geophysical signals

The characterization of contaminated sites can benefit from the supplementation of direct investigations with a set of less invasive and more extensive measurements. A combination of geophysical methods and direct push techniques for contaminated land characterization has been proposed within the EU FP7 project ModelPROBE and the affiliated project SoilCAM. In this paper, we present results of the investigations conducted at the Trecate field site (NW Italy), which was affected in 1994 by crude oil contamination. The less invasive investigations include ground-penetrating radar (GPR), electrical resistivity tomography (ERT), and electromagnetic induction (EMI) surveys, together with direct push sampling and soil electrical conductivity (EC) logs. Many of the geophysical measurements were conducted in time-lapse mode in order to separate static and dynamic signals, the latter being linked to strong seasonal changes in water table elevations. The main challenge was to extract significant geophysical signals linked to contamination from the mix of geological and hydrological signals present at the site. The most significant aspects of this characterization are: (a) the geometrical link between the distribution of contamination and the site’s heterogeneity, with particular regard to the presence of less permeable layers, as evidenced by the extensive surface geophysical measurements; and (b) the link between contamination and specific geophysical signals, particularly evident from cross-hole measurements. The extensive work conducted at the Trecate site shows how a combination of direct (e.g., chemical) and indirect (e.g., geophysical) investigations can lead to a comprehensive and solid understanding of a contaminated site’s mechanisms.

The Influence of Subsoil Structure and Acquisition Parameters in MASW Mode Mis-identification

ABSTRACT Inversion of surface wave dispersion properties is commonly used to derive shear wave velocity depth profiles. However, one of the critical and yet rarely considered issues in this ill-posed inversion process is mode contamination. Rayleigh dispersion modes are the theoretically possible solutions of motion. Experimentally, we define Rayleigh dispersion properties from spectra energy maxima in some domain (as f–k), thus possibly producing only apparent experimental dispersion curves, where energy spreads onto several modes. If this phenomenon is not recognized, the inversion of an apparent dispersion curve can produce results unrelated to the actual subsurface structure. In this work, we present the results of synthetic tests that highlight the most common subsoil conditions and acquisition pitfalls that can give rise to surface wave mode contamination. In particular, we consider three typical subsoil structures that can produce this phenomenon: 1) a simple two-layer system with a strong impedanc...

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.

Structural Investigations and Modelling of Seismic Behaviour on Ruins in the Monumental Area of Hierapolis of Phrygia

The archaeological site of Hierapolis of Phrygia presents a number of ruins, which date back to the II century B.C. They are located in a medium-high seismic area crossed by a system of faults still active. The valorisation plan of the site aims at establishing an archaeo-seismological area, i.e., a site where tourists can visit structures keeping the signs of past earthquakes in proper safety conditions. To this purpose, the knowledge phase is a fundamental step in the preservation process. The paper presents the results of a series of investigation procedures applied to several incomplete structures still standing in the area, namely: the Martyrion of St. Philip, the tombs A18 and A20, and the monumental latrines. The geometrical survey and the elaboration of images were used in combination, as base for the evaluation of the conservation conditions, the interventions applied in the past, and the deterioration and damage states of the structures. To integrate the analysis, soil and structures were also inspected on site with NDT procedures. In particular, tests provided the seismic soil classification (by MASW, HVSR) and the qualification of masonry elements (by SPVT, GPR). Limit analysis and FEM approach were adopted to characterize the actual seismic behaviour of the structural assemblages and to predict their possible future damage. Results pointed out the high vulnerability of large portions of the structures still standing in the site.