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2D tomographic inversion of complex resistivity data on cylindrical models
No full textThe resistive and capacitive response of a multiphase subsoil can be analysed by amplitude and phase models of the electrical complex resistivity. The main goal of this work is to extend the 2D transformed formulation used for electrical site investigations for cylindrical laboratory models, solving the complex resistivity forward problem starting from the Complete Electrode Model approach. This formulation is tested by a comparison with the full 3D solution and is proven to be stable and accurate. Inversion of complex resistivity data is achieved through a Matlab interface included in the EIDORS environment, with the addition of numerous new functions. Three synthetic examples are discussed, to understand the potential and limits of this approach in comparison with the 3D inversion. Laboratory experiments on a cylindrical laboratory model with a horizontal cross-section of 10 electrodes validated synthetic results. The model having a height of 1 m and a diameter of 500 mm is made by sand contaminated from the top by an engineered fluid with electrical properties similar to chlorinated solvents
Complex resistivity and spectral induced polarization techniques for environmental applications
No full textLa tesi concerne la valutazione delle potenzialità e dei limiti delle tecniche di Resistività Complessa e di Polarizzazione Indotta Spettrale per l'individuazione di contaminazione del sottosuolo, a partire dalle condizioni controllate di laboratorio.
La soluzione del problema diretto 2D per indagini in laboratorio è stata realizzata tramite una formulazione trasformata e utilizzando il Complete Electrode Model per definire le condizioni al contorno. L'algoritmo agli elementi finiti, implementato in Matlab per risolvere numericamente queste equazioni, ha dimostrato di essere stabile e accurato attraverso un confronto con la formulazione 3D inclusa nel software EIDORS e per mezzo di simulazioni sintetiche con rumore gaussiano aggiunto. Di conseguenza, questa formulazione può essere utilizzata per produrre modelli 2D su dispositivi cilindrici di laboratorio provvisti di una sezione orizzontale in mezzeria (p.es. lisimetri). L'intero processing geofisico (elaborazione dei dati, modellazione e inversione) è stato incluso all'interno di EIDORS tramite un algoritmo in Matlab interattivo, per mezzo dell'aggiunta di numerose nuove funzioni. Simulazioni con rumore gaussiano, eseguite su modelli eterogenei con anomalie in ampiezza e fase, hanno fornito dei modelli finali con valori limitati di errore e una buona ricostruzione delle anomalie sia in forma che in posizione. Pertanto il software EIDORS può costituire un codice di base per l'inversione tomografica di dati nel dominio del tempo e nel dominio della frequenza, sia per indagini in laboratorio che in sito, a causa della sua flessibilità e dell'affidabilità raggiunta dalla modellazione e dall'inversione, con l'obiettivo primario di condividere i dati e promuovere la collaborazione tra i diversi gruppi di ricerca.
Per i fini suddetti è stato progettato un nuovo dispositivo sperimentale per eseguire misure 2D e 3D su modelli fisici di laboratorio, capaci di simulare le configurazioni tipiche del sottosuolo. La simmetria cilindrica permette di minimizzare gli effetti di bordo e di ricavare delle sezioni orizzontali all'interno dei singoli strati. Modelli preliminari di acqua e sabbia hanno messo in luce i principali problemi nelle campagne di misura provocati dalla saturazione, dal materiale costituente gli elettrodi, dagli array e dal range di frequenza. La strumentazione sperimentale in corrente alterenata, dopo la calibrazione e la minimizzazione dei suddetti errori, fornisce misure di ampiezza e fase affidabili, stabili e ripetibili. Le misurazioni non sono affette dall'accoppiamento induttivo per frequenze fino a 5-10 Hz. I modelli fisici costruiti in laboratorio hanno riguardato la preparazione di una tipica alternanza sabbia-argilla-sabbia, simulando un acquifero superficiale per saturazione del livello superiore. Lo sversamento di HFE 7100, avente caratteristiche fisiche simili ai solventi clorurati, è stato condotto dall'alto, ipotizzando la rottura di un piccolo serbatoio di stoccaggio.
Gli esperimenti di laboratorio hanno confermato l'affidabilità dell'algoritmo proposto. Dopo lo sversamento di HFE, l'ampiezza dell'impedenza aumenta progressivamente come previsto, tenendo conto del rapporto percentuale tra il volume globale di HFE sversato e il volume di sabbia. D'altra parte la fase evidenzia un aumento meno marcato ed è probabilmente più sensibile alle particolari condizioni di lavoro e al rapporto volumetrico tra contaminante e materiale contaminato. Inoltre negli spettri di fase si individua un unico picco per frequenze di circa 1-2 Hz. Questi risultati confermano l'affidabilità della tecnica tomografica a resistività complessa per l'individuazione di contaminanti, sottolineando ancora una volta l'importanza di definire dei criteri codificati per l'acquisizione in sito.The potential and limits of the complex resistivity and the Spectral Induced Polarization techniques for environmental applications are investigated starting from the controlled laboratory conditions. The solution of the 2D forward problem for the complex resistivity is achieved on cylindrical-shaped models by a transformed formulation and using the Complete Electrode Model to define boundary conditions. The finite element algorithm, implemented in Matlab to solve numerically these equations, has proven to be accurate and stable through a comparison with the 3D full formulation and synthetic simulations. Accordingly, this formulation can be used to produce 2D reconstruction on cylindrical laboratory devices provided with one horizontal mid-section of electrodes (e.g. lysimeters). The whole geophysical process (data processing, forward modelling and inversion) has been included within EIDORS through an interactive Matlab algorithm with numerous new function added. Simulations with Gaussian noise, performed on heterogeneous models with anomalies included, provide low-error inverted models whereas anomalies are well-detected both for shape and position. EIDORS software could act as a base code for tomographic inversion of time-domain and frequency-domain data, both for laboratory and for in field investigations, because of its high-flexibility and reliability now reached by forward and inversion routines, with the primary aim of to share data and promote collaboration between groups working these fields.
A new experimental device was presented to perform 2D and 3D measurement on cylindrical physical models in order to simulate typical subsoil configurations. The cylindrical symmetry avoids to have boundary effects of prismatic tanks and allows us to provide horizontal cross-section within the different layers. Preliminary water and sand models have highlighted the main problems due to saturation, electrodes, array and frequency range. The experimental AC instrument, after the calibration, provides reliable amplitude and phase dataset with stable and repeatable measurements. Measurements are not affected by the inductive coupling for frequency up to 5-10 Hz. Physical models built up in the laboratory have concerned the preparation of a typical sand-clay-sand multilayer model, simulating a shallow aquifer by saturation of the top layer. The HFE 7100, having physical properties similar to the chlorinated solvents, was spilled from the top to simulate a point source spilling, that is for example a failure of a little storage tank.
Laboratory experiments have confirmed the reliability of the algorithm proposed. After HFE spilling, impedance amplitude increases progressively as expected, taking into account the percentage ratio between the global volume of HFE spilled and the sand volume. On the other hand the phase increases less evidently, since they are probably more sensible to the particular working conditions. Evidences from SIP phase spectra seems to be also convincing as we detect a single peak for frequency of about 1-2 Hz. These results confirm that complex resistivity tomography can play an important for contaminant detection, stressing once more the importance to define codified criteria for acquiring CR ad SIP in field data
Tomografia elettrica di resistività e caricabilità nel dominio del tempo a scala di laboratorio
No full textLe tecniche di tomografia elettrica di resistività (ERT), tomografia elettrica d’impedenza
(EIT) e polarizzazione indotta (IP) sono ampiamente utilizzate in ambito geofisico per
applicazioni geologiche, ingegneristiche e ambientali. In particolare, negli ultimi quindici anni
molti progetti di ricerca (p.es. Vanhala, 1997; Ramirez et al., 1999; Chambers et al., 2004; Cassiani
et al. 2009) sono stati sviluppati su modelli di laboratorio, con differenti geometrie di acquisizione
e diversi approcci alla soluzione dei problemi diretto ed inverso. In questa nota verrà investigato a
scala di laboratorio l’intero processo d’indagine tomografica nel dominio del tempo, fornendo una
valutazione degli errori che scaturiscono nelle diverse
fasi (acquisizione, modellazione diretta, inversione),
e presentando un algoritmo risolutivo agli elementi
finiti basato sull’approccio CEM (Complete
Electrode Model) per la risoluzione del problema
diretto (Cheng et al., 1989; Vaukhonen, 1997)
Tomografia di Resistività Complessa ed analisi di Polarizzazione Indotta Spettrale per la caratterizzazione 2D e 3D dei suoli a scala di laboratorio
No full text3D complex resistivity tomography on cylindrical models using EIDORS
No full textComplex resistivity imaging is a relatively new geophysical technique, developed in the last few decades mainly for hydrogeological and environmental applications. The aim of this work is to present an EIDORS application of the 3D complex resistivity tomography on cylindrical laboratory models. EIDORS is an open-source numerical environment developed with the aim of sharing data and promoting collaboration between groups working in these fields. In spite of being a well-recognised software for forward modelling and inversion for medical tomographies, EIDORS still needs to be adapted for geophysical purposes. We discuss the role played by the mesh choice and the contact impedances on the accuracy of the finite-element solution achieved by tetrahedral elements. When a 3D tomography is performed on a standard machine with limited local memory, the dual reconstruction can help to retain a sufficient accuracy without increasing the allocated memory. Although for medical applications on the human body a linear inversion can effectively represent the slight changes in resistivity magnitude, when a subsoil has to be investigated resistivity can vary substantially. Thus we develop an algorithm to add to the non-linear inversion for complex resistivity data, through the integration of the EIDORS basic functions. The algorithm has been validated through four synthetic examples. The reconstructed models, having a growing degree of complexity, are similar to the true ones. We highlight the role played by phase and resolution to detect the anomalies. When the dipole length is enlarged and the embedded anomalies decrease in size, the reconstruction becomes more difficult. We show that EIDORS could act as a base code for tomographic inversion of frequencydomain data (and also of time-domain real-valued data) for laboratory problems, because of its high flexibility and reliability reached by the forward and inversion routines
Assessment of errors from different electrode materials and configurations for electrical resistivity and time-domain IP data on laboratory models
No full textIn the last twenty years Electrical Resistivity Tomography and time-domain Induced Polarization techniques have been widely used for geological, environmental, chemical and hydro-geological applications. As a matter of fact, the choice of electrodes (material and number) to be employed is crucial to avoid large measurement errors. The aim of this work is the quantitative assessment of errors in acquisition with respect to different electrode materials and configurations, as well as a comparison with previous results. To this end, a cylindrical column (height of 280 mm, diameter of 135 mm) is set up and all measurements are performed on a 7 electrode, 2D horizontal cross-section. Resistance, chargeability and self-potential measurements for different electrode materials (steel, iron, aluminium, copper and carbon) are acquired over a cylindrical sample filled by water with known conductivity. A statistical analysis of the experimental data demonstrates that iron and steel provide the best performances both for resistance and for chargeability. Carbon and copper are reliable for resistive surveys, but not for capacitive ones. Standard deviations associated to aluminium electrodes are the highest among the five materials. Changing the number of electrodes (from 7 to 20) results in an exponential increase of resolution of the resistive and chargeable anomalies included in the samples. © 2011 - OGS
Resistivity and full‐decay IP inversion for imaging a coastal aquifer prone to saline intrusion. The Pontina Plain case study (Central Italy)
Full text linkMany coastal areas are affected by groundwater salinization due to the unsustainable use of groundwater resources. For a cost-effective quantitative assessment of groundwater resources, electrical resistivity (ER) tomography is often used as a standalone geophysical technique. In this paper, we present an application of the integration of direct-current ER and full-decay-induced polarization (IP) method at the Pontina Plain (Central Italy). The case study is a coastal area in Central Italy prone to salinization due to both geological and anthropogenic factors. To achieve these goals, we inverted full-decay time-domain electrical data for Cole–Cole parameters. The resulting multi-parameter model provides a first approximation prediction of the permeability, employing well-established empirical relationships with the electrical parameters. We demonstrated that our approach: (i) can locate highly conductive zones directly related to saline intrusion inland using the resistivity as a fast proxy; (ii) can remove the ambiguity in the detection of clay/silt layers in the near-surface; and (iii) permit a prediction of the permeability, employing full-decay inversion of time-domain electrical data. However, the extremely conductive environment prevents the use of IP data for the reconstruction of deep layers or detection of the salt wedge front. Therefore, this approach can be used for hydro-geophysical screening and monitoring of salinization-prone sites, where strong limitations to direct inspection exist due to external constraints (e.g., protected lands)
Tomographic inversion of time-domain resistivity and chargeability data for the investigation of landfills using a priori information
Full text linkIn this paper, we present a new code for the modelling and inversion of resistivity and chargeability data using a priori information to improve the accuracy of the reconstructed model for landfill. When a priori information is available in the study area, we can insert them by means of inequality constraints on the whole model or on a single layer or assigning weighting factors for enhancing anomalies elongated in the horizontal or vertical directions. However, when we have to face a multilayered scenario with numerous resistive to conductive transitions (the case of controlled landfills), the effective thickness of the layers can be biased. The presented code includes a model-tuning scheme, which is applied after the inversion of field data, where the inversion of the synthetic data is performed based on an initial guess, and the absolute difference between the field and synthetic inverted models is minimized. The reliability of the proposed approach has been supported in two real-world examples; we were able to identify an unauthorized landfill and to reconstruct the geometrical and physical layout of an old waste dump. The combined analysis of the resistivity and chargeability (normalised) models help us to remove ambiguity due to the presence of the waste mass. Nevertheless, the presence of certain layers can remain hidden without using a priori information, as demonstrated by a comparison of the constrained inversion with a standard inversion. The robustness of the above-cited method (using a priori information in combination with model tuning) has been validated with the cross-section from the construction plans, where the reconstructed model is in agreement with the original design
Time-lapse monitoring of an electrokinetic soil remediation process through frequency-domain electrical measurements
Full text linkThe electrokinetic (EK) method is an emerging technique for soil remediation, even though a monitoring system of the contaminant removal through geophysical methods has not been developed yet. In this paper, frequency-domain time-lapse measurements are used on heavy-metal contaminated sediments for monitoring an EK remediation process in a small-scale measuring cell. Our goal is to monitor the development of the electrokinetic process within the sediment and to evaluate the total time needed for the treatment. In fact, frequency-domain electrical monitoring provides complex resistivity spectra at different time steps that can be correlated to changes in the physical properties of the sediments. We perform laboratory spectral induced polarization (SIP) measurements on different samples before, during and after the EK treatment, using different electrolyte solutions (acids and tap water), commonly employed in EK remediation. Direct-current measurements (resistivity and chargeability) were also acquired on one sample for testing the reliability of the system by a comparison with a widespread commercial instrumentation for field measurements. Results indicate that resistivity is a diagnostic parameter as long as it is linked to changes in water saturation, pH and ionic concentration and not to the percentage of metal extraction. The resistivity exhibited well-defined signatures as a function of time that changes depending on the conditioning agent and the grain size distribution. These peculiarities were used to understand the physical processes occurring within the cell and consequently to assess the effectiveness of the electrokinetic treatment. Conversely, the polarization effect was negligible using acids as conditioning agents at the electrolyte chambers. Therefore, the SIP method is not effective under these conditions, being the polarization effect significant only when tap water was used at both ends of the measuring cell. In this case, we were able to correlate changes in water saturation with the time-shift observed on relaxation time distributions (RTDs) after inversion of SIP data and to observe, using normalized chargeability, that polarization is stronger at high pH values. On these basis, resistivity is suitable to monitor the development of the remediation, to optimise the energy levels required for treatment and to assess the end time of the EK process (time when metal mobilization ends). In fact, the end time of treatment can be associated with the time at which resistivity becomes stable. This time is highly dependent on the particular working conditions and sediment grain size as demonstrated by our experiments
Complex Resistivity Tomography and Spectral Induced Polarization analysis on a laboratory device for 2D and 3D soil characterization
No full textElectrical Impedance Tomography EIT and Spectral Induced Polarization SIP are large used techniques for environmental application. The aim of this work is to understand the polarization signatures related to uncontaminated soils using typical subsoil models on laboratory targets and to build a new versatile algorithm suitable for this situations, for a future better estimation of contaminated samples. To this end a new experimental device was build up and SIP measurements were acquired on this new configuration, within ImHz - 12 kHz frequency range. A couple of frequencies was chosen among the entire frequency range depending on sample configuration, and for each frequency value an inversion of complex resistivity data was carried out through a new algorithm developed in EIDORS environment. SIP measurements have highlighted the spectral response of water, sand and clay, in terms of polarization signatures on the phase spectra, while 2D and 3D complex resistivity inversions have shown the good resolution achieved by the EIDORS algorithm and the importance of the device configuration
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