A controlled column experiment was conducted to investigate the geophysical response of gasoline spills into the partially saturated sand column. The column was 0.61 diameter (ID) and 2 m high cylindrical polyvinyl chloride, which was packed with the Borden sand to a height of 1.95 m, flushed with CO2, saturated, and drained to a height of 0.73 m. The monitoring techniques used for this experiment was DC resistivity and time domain reflectometry (TDR) methods. The column was equipped with resistivity electrodes and TDR probes, which were placed on the column wall vertically with 3 cm intervals, on opposite sides, two monitoring wells, an injection well, a manometer, an outlet/inlet system, and a vent. A total amount of 5 liters of standard API 91-01 gasoline was added to the system in steps of 1, 2, and 2 liters to examine the geophysical response to different amounts of gasoline. Measurements were taken before and after each injection and also during subsequent fluctuation of the water table. Both monitoring techniques were able to record even the minor changes in the trend of conductivity and permittivity profiles due to the addition of the small amount of gasoline during the first spill. The conductivity and permittivity profiles obtained before lowering the water level below the original level and those obtained after the water level reached to the original level do not match, which is an indication of entrapped gasoline inside the pores. Two core samples was taken from the sand symmetrically after each water table fluctuation and analyzed for total petroleum hydrocarbon (TPH) analysis and the results were compared to the conductivity and permittivity results. The conductivity profile obtained using DC resistivity method was compared to that of obtained using TDR method. The profiles match in the saturated zone where all of the pores are connected with water and therefore electrolytic conduction is predominant. In the unsaturated zone, where there is low pore water connectivity, TDR measured conductivity values are higher than those measured using the resistivity method. Water saturation values were calculated using conductivity and permittivity values before and after each injection. Different values of saturation exponent (n) were tested for Archie’s law until an appropriate value was found which gave the best water saturation from conductivity data for clean Borden sand. Then, the water saturation obtained from permittivity values using Topp’s equations for different materials were compared to that of obtained from conductivity values using Archie’s equation. Topp’s equation for 30 µm glass beads provided the best match. Furthermore, other equations developed by other researchers were examined to obtain water saturation profiles from the permittivity values; all of them overestimate the water saturation for Borden sand. The water saturation profiles after the gasoline spills obtained using both Archie’s law and Topp’s equation do not match, perhaps because both equations were developed for three-phase (water-solid-air) systems.

A controlled column experiment was conducted to investigate the geophysical response of gasoline spills into the partially saturated sand column. The column was 0.61 diameter (ID) and 2 m high cylindrical polyvinyl chloride, which was packed with the Borden sand to a height of 1.95 m, flushed with CO2, saturated, and drained to a height of 0.73 m. The monitoring techniques used for this experiment was DC resistivity and time domain reflectometry (TDR) methods. The column was equipped with resistivity electrodes and TDR probes, which were placed on the column wall vertically with 3 cm intervals, on opposite sides, two monitoring wells, an injection well, a manometer, an outlet/inlet system, and a vent. A total amount of 5 liters of standard API 91-01 gasoline was added to the system in steps of 1, 2, and 2 liters to examine the geophysical response to different amounts of gasoline. Measurements were taken before and after each injection and also during subsequent fluctuation of the water table. Both monitoring techniques were able to record even the minor changes in the trend of conductivity and permittivity profiles due to the addition of the small amount of gasoline during the first spill. The conductivity and permittivity profiles obtained before lowering the water level below the original level and those obtained after the water level reached to the original level do not match, which is an indication of entrapped gasoline inside the pores. Two core samples was taken from the sand symmetrically after each water table fluctuation and analyzed for total petroleum hydrocarbon (TPH) analysis and the results were compared to the conductivity and permittivity results. The conductivity profile obtained using DC resistivity method was compared to that of obtained using TDR method. The profiles match in the saturated zone where all of the pores are connected with water and therefore electrolytic conduction is predominant. In the unsaturated zone, where there is low pore water connectivity, TDR measured conductivity values are higher than those measured using the resistivity method. Water saturation values were calculated using conductivity and permittivity values before and after each injection. Different values of saturation exponent (n) were tested for Archie’s law until an appropriate value was found which gave the best water saturation from conductivity data for clean Borden sand. Then, the water saturation obtained from permittivity values using Topp’s equations for different materials were compared to that of obtained from conductivity values using Archie’s equation. Topp’s equation for 30 µm glass beads provided the best match. Furthermore, other equations developed by other researchers were examined to obtain water saturation profiles from the permittivity values; all of them overestimate the water saturation for Borden sand. The water saturation profiles after the gasoline spills obtained using both Archie’s law and Topp’s equation do not match, perhaps because both equations were developed for three-phase (water-solid-air) systems.

Orientador: Cesar Augusto Moreira / Resumo: A área de estudo está localizada na cidade de Lavras do Sul – RS, que é uma antiga pedreira que foi reaproveitada para a deposição dos resíduos oriundo do município. O objetivo principal é realizar o detalhamento geofísico e estrutural da contaminação do antigo lixão e correlacionar a percolação do lixiviado com as zonas fraturadas, com o uso de dois métodos de investigação (Eletrorresistividade e Polarização Induzida) e os dois arranjos (Dipolo-Dipolo e Schlumberger). Foram realizados 12 perfis de resistividade e cargabilidade na direção 45ºN, processados em 2D e 3D, e foi determinado uma possível presença de zonas de contaminação e percolação de lixiviado para fora da área do lixão. Também foi possível determinar onde estavam dispostos os resíduos sólidos e os limites do lixão, zonas fraturadas com a presença de lixiviado e zonas fraturadas com a presença de sulfetos que aprofundam até 36 m. Desde o início da operação do lixão a contaminação atingiu diversos metros de profundidade, porém com o tempo e a atenuação natural, essas zonas de fratura fecharam devido a cristalização de sulfeto. Os dois arranjos apresentaram resultados satisfatórios para os valores de resistividade. Com relação aos valores de cargabilidade, o arranjo Schlumberger apresentou melhores resultados do que o Dipolo-Dipolo. A geofísica demonstrou ser uma ferramenta importante no estudo de contaminações em rochas cristalinas, principalmente pela sua capacidade de detecção de fraturas e sua profundidade ati... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The study area is located in the city of Lavras do Sul - RS, which is an old quarry that was reused for the deposition of waste from the municipality. The main objective is to perform the geophysical and structural detailing of the contamination of the old dump and to correlate the percolation of the leachate with the fractured zones, using the two research methods (DC Resistivity and Induced Polarization) and the two arrangements (Dipole-dipole and Schlumberger). By performing 12 profiles of resistivity and chargeability in the 45ºN direction, processed in 2D and 3D, a possible presence of contamination zones and percolation of leachate outside the dump area was determined. It was also possible to determine where the solid wastes and the boundaries of the dump were located, fractured zones with the presence of leachate and fracture zones with the presence of sulfides that deepen up to 36 m. From the beginning of the operation of the dump the contamination reached several meters deep, but with time and the natural attenuation, these zones of fracture closed due to mineral neoformation of sulfide. Both arrangements presented satisfactory results for the resistivity values, in relation to the chargeability values, the Schlumberger arrangement presented better results than the Dipole-Dipole. Geophysics has been shown to be an important tool in the study of contaminations in crystalline rocks, mainly due to its ability to detect fractures and its depth. / Mestre

Submitted by DIMITRI ILICH KERBAUY VELOSO null (dimitriveloso@hotmail.com) on 2016-05-03T21:03:36Z No. of bitstreams: 1 Dissertação Final.pdf: 8318470 bytes, checksum: 1f59566750ab6e5e25d4b72baa3db621 (MD5) / Approved for entry into archive by Juliano Benedito Ferreira (julianoferreira@reitoria.unesp.br) on 2016-05-06T13:58:33Z (GMT) No. of bitstreams: 1 veloso_dik_me_rcla.pdf: 8318470 bytes, checksum: 1f59566750ab6e5e25d4b72baa3db621 (MD5) / Made available in DSpace on 2016-05-06T13:58:33Z (GMT). No. of bitstreams: 1 veloso_dik_me_rcla.pdf: 8318470 bytes, checksum: 1f59566750ab6e5e25d4b72baa3db621 (MD5) Previous issue date: 2016-04-20 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A contaminação de solos e águas subterrâneas pode colocar em risco a saúde humana além de inviabilizar a exploração de recursos naturais por longos períodos, com sérios prejuízos ambientais, administrativos e financeiros. Este trabalho realizou um estudo em área contaminada no município de Santa Ernestina-SP, com o intuito de avaliar o atual cenário de contaminação deste local por meio da investigação geofísica pelos métodos de Eletrorresistividade e Polarização Induzida. A área foi contaminada em 2007 devido a um acidente ferroviário que causou o derramamento de 58000 litros de óleo diesel. O monitoramento da área foi realizado durante os quatro anos subsequentes ao acidente, a partir de sondagens manuais a trado, poços de monitoramento, coleta de amostras de água subterrânea e levantamento de dados geológicos e hidrogeológicos, integrados em sucessivos relatórios disponibilizados na Agência Ambiental (CETESB) de Araraquara-SP. A escolha dos métodos de Eletrorresistividade e Polarização Induzida foi feita com o objetivo de analisar as relações de contrastes entre as propriedades elétricas do meio subsuperficial e dos contaminantes em solo e água subterrânea. Foram realizadas 11 linhas paralelas de imageamento elétrico em arranjo Wenner, com geração de modelos de inversão geofísica 2D na forma de seções de resistividade e cargabilidade, e modelos integrados de visualização 3D, a partir dos quais foram gerados mapas de profundidade (horizontes de 1m, 3m, 5m e 7m). Os resultados de resistividade elétrica indicaram zonas rasas de baixa resistividade (valores em torno de 150Ω.m) que ocorrem de forma restrita, próximas aos principais locais de infiltração de contaminantes. Esta assinatura geofísica foi correlacionada à presença de resquícios de contaminantes em zona insaturada, onde os processos de atenuação natural conduziram à queda da resistividade do meio geológico a partir da diluição e dissolução de compostos orgânicos e iônicos. Os resultados para cargabilidade viabilizaram um novo ponto de vista investigativo sobre o monitoramento de áreas contaminadas. Por meio destes dados, foi possível associar a ocorrência de biodegradação à provável presença de minerais metálicos neoformados (sulfetos e óxidos) durante e após a ação dos processos de atenuação natural. A formação destes minerais deve estar associada principalmente à alteração das condições físico-químicas do ambiente geológico e à disponibilidade de íons em solos tropicais contaminados por hidrocarbonetos, tais como Fe+3 e, principalmente, S-2 . O conjunto dos resultados aponta para um panorama geral de avançado estágio de descontaminação da área, com atuação efetiva dos processos de atenuação natural, decorridos sete anos do acidente. / Soil and groundwater contamination presents hazards for human health and can spoiling natural resources for long time giving serious environmental, administrative and financial damages. This paper carried out a study in a contaminated area in Santa Ernestina-SP, in order to evaluate the present scenery of contamination by geophysical methods of DC Resistivity and Induced Polarization methods. The contamination site is due to a railway accident ocurred in 2007 that spilled 58.000 liters of diesel. Since the accident, site monitoring had been done during four years and comprised manual probing, monitoring wells, groundwater samples and geological and hydrogeological data, integrated in successive reports provided by the Environmental Agency (CETESB/ Araraquara - SP). Geophysical methods of DC Resistivity and Induced Polarization were used to detect contrasts in physical properties and its connection with soil and groundwater contamination. It was carried out an electrical imaging that consisted in 11 parallel lines organized in Wenner array. This acquisition allowed generating 2D inversion models for electrical resistivity and chargeability, in addiction to an integrated 3D display model for each parameters, that allowed generating depth maps (1m, 3m , 5m and 7m horizons). The electrical resistivity results indicated low resistivity shallow zones (values around 150Ω.m) that occurs confined, near the sites of contaminant infiltration. This geophysical signature was correlated to traces of contaminants presents into the unsaturated zone, which natural attenuation process led to reduction of the environmental resistivity due to dilution and dissolution of organic and ionic compounds. The chargeability results had enabled a new side of investigation related to the monitoring of contaminated sites. Through this data, it was possible to link the occurrence of biodegradation to likely presence of metallic minerals neoformation (sulfides and oxides) during and after the natural attenuation activity. Minerals neoformation is propably associated with changes in physicochemical environmental‟s conditions in addict to availability of ions (Fe+3 and, mainly, S-2 ) in tropical contaminated soils.The results‟overview suggests an advanced decontamination stage of the site, for which one the natural attenuation processes has been effective more than seven years since the accident.

Submitted by RODOLFO BUSOLIN ALVES DOS SANTOS null (rodolfo_busolin@hotmail.com) on 2018-03-27T01:32:36Z No. of bitstreams: 1 Dissertação.pdf: 7973963 bytes, checksum: 2e90678c451c6ded29475c9bcd42029c (MD5) / Approved for entry into archive by Ana Paula Santulo Custódio de Medeiros null (asantulo@rc.unesp.br) on 2018-03-27T12:27:06Z (GMT) No. of bitstreams: 1 santos_rba_me_rcla.pdf: 7696907 bytes, checksum: b5a23c52c7fbb3f6024f32da8c4b4dcc (MD5) / Made available in DSpace on 2018-03-27T12:27:06Z (GMT). No. of bitstreams: 1 santos_rba_me_rcla.pdf: 7696907 bytes, checksum: b5a23c52c7fbb3f6024f32da8c4b4dcc (MD5) Previous issue date: 2018-02-06 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / A área de estudo está localizada na cidade de Lavras do Sul – RS, que é uma antiga pedreira que foi reaproveitada para a deposição dos resíduos oriundo do município. O objetivo principal é realizar o detalhamento geofísico e estrutural da contaminação do antigo lixão e correlacionar a percolação do lixiviado com as zonas fraturadas, com o uso de dois métodos de investigação (Eletrorresistividade e Polarização Induzida) e os dois arranjos (Dipolo-Dipolo e Schlumberger). Foram realizados 12 perfis de resistividade e cargabilidade na direção 45ºN, processados em 2D e 3D, e foi determinado uma possível presença de zonas de contaminação e percolação de lixiviado para fora da área do lixão. Também foi possível determinar onde estavam dispostos os resíduos sólidos e os limites do lixão, zonas fraturadas com a presença de lixiviado e zonas fraturadas com a presença de sulfetos que aprofundam até 36 m. Desde o início da operação do lixão a contaminação atingiu diversos metros de profundidade, porém com o tempo e a atenuação natural, essas zonas de fratura fecharam devido a cristalização de sulfeto. Os dois arranjos apresentaram resultados satisfatórios para os valores de resistividade. Com relação aos valores de cargabilidade, o arranjo Schlumberger apresentou melhores resultados do que o Dipolo-Dipolo. A geofísica demonstrou ser uma ferramenta importante no estudo de contaminações em rochas cristalinas, principalmente pela sua capacidade de detecção de fraturas e sua profundidade atingida. / The study area is located in the city of Lavras do Sul - RS, which is an old quarry that was reused for the deposition of waste from the municipality. The main objective is to perform the geophysical and structural detailing of the contamination of the old dump and to correlate the percolation of the leachate with the fractured zones, using the two research methods (DC Resistivity and Induced Polarization) and the two arrangements (Dipole-dipole and Schlumberger). By performing 12 profiles of resistivity and chargeability in the 45ºN direction, processed in 2D and 3D, a possible presence of contamination zones and percolation of leachate outside the dump area was determined. It was also possible to determine where the solid wastes and the boundaries of the dump were located, fractured zones with the presence of leachate and fracture zones with the presence of sulfides that deepen up to 36 m. From the beginning of the operation of the dump the contamination reached several meters deep, but with time and the natural attenuation, these zones of fracture closed due to mineral neoformation of sulfide. Both arrangements presented satisfactory results for the resistivity values, in relation to the chargeability values, the Schlumberger arrangement presented better results than the Dipole-Dipole. Geophysics has been shown to be an important tool in the study of contaminations in crystalline rocks, mainly due to its ability to detect fractures and its depth.

Orientador: Cesar Augusto Moreira / Resumo: Uma barragem de rejeitos é constituída por uma barreira física, cuja função é a acumulação de produtos residuais do processo industrial de beneficiamento mineiro. Os métodos de projeto para barragens de rejeitos diferem das barragens de retenção de água pela construção do barramento em diferentes etapas. Deste modo, os problemas de estabilidade em uma barragem são de grande importância, principalmente quando se trata de barragens construídas para armazenamento destes rejeitos de mineração, uma vez que a possibilidade de falhas não apenas ameaça a segurança das pessoas e da propriedade industrial, mas também pode causar impactos ambientais substanciais. No Estado de Minas Gerais, aproximadamente 30 km a sudoeste do município de Caldas, está localizada a Mina Osamu Utsumi de responsabilidade das Indústrias Nucleares do Brasil (INB), a qual tem um sistema para contenção de rejeitos provenientes da exploração e processamento de urânio, atualmente desativado. O Sistema consiste em uma barragem de rocha e terra a qual apresenta a existência de surgências no maciço rochoso à jusante do barramento, cujo fluxo ocorre em sistema fraturado. O objetivo do trabalho é reconhecer possíveis zonas de fluxo no interior e na base da barragem por meio de um conjunto de ensaios geofísicos de eletrorresistividade adquiridos com a técnica de tomografia elétrica através de modelos de inversão 2D e modelamento 3D. Foram adquiridas 5 linhas de tomografia elétrica com espaçamento entre eletrodos de 6 m... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: A tailings dam is constituted by a barrier, whose function is the accumulation of residual products from the mining beneficiation industrial process. Project methods for tailings dams differ from water retention dams by the construction of the dam in different stages. Thus, the stability problems in a dam are of great importance, especially when it comes to dams built for the storage of these mining waste, since the possibility of failures not only threatens the safety of people and industrial property, but also cause negative and substantial effects to the local environment. The Osamu Utsumi uranium mine, administrated by the state company Industrias Nucleares do Brasil (INB), is located 30 km southwest of Caldas, in the State of Minas Gerais, and its contention system of tailings, generated by the exploration and processing of uranium ore, is currently out of operation. The system is composed of a rock-soil dam, which presents the existence of water upwellings downstream in the bedrock, with water flux confined in fractures. This research aims the recognition of possible flux zones into the base of the dam through geophysical surveys applying DC Resistivity, in addition to Electrical Resistivity Tomography technique in the generation of 2D inversion models and 3D modeling. The acquisition was composed of five lines of electrical resistivity tomography with 6 m of spacing between electrodes, using Schlumberger array. The results are presented by 2D and 3D geophysical models ... (Complete abstract click electronic access below) / Mestre

Geophysical methods are essential for exploration and monitoring of subsurface formations, e.g. in carbon dioxide sequestration or enhanced geothermal energy. One of the keys to their successful application is the knowledge of how the measured physical quantities are related to the desired reservoir parameters. The work presented in this thesis shows that the presence of carbon dioxide (CO2) in pore space gives rise to multiple processes all of which contribute to the electrical rock conductivity variation. Basically, three mechanisms take place: (1) CO2 partially replaces the pore water, which is equivalent to a decrease in water saturation. (2) CO2 chemically interacts with the pore water by dissolution and dissociation. These processes change both the chemical composition and the pH of the pore filling fluid. (3) The low-pH environment can give rise to mineral dissolution and/or precipitation processes and changes the properties of the grain-water interface. Investigations on the pore water phase show that the reactive nature of CO2 in all physical states significantly acts on the electrical conductivity of saline pore waters. The physico-chemical interaction appears in different manifestations depending mainly on the pore water composition (salinity, ion types) but also on both temperature and pressure. The complex behaviour includes a low- and a high-salinity regime originating from the conductivity increasing effect of CO2 dissociation, which is opposed by the conductivity decreasing effect of reduced ion activity caused by the enhanced mutual impediment of all solutes. These results are fundamental since the properties of the water phase significantly act on all conduction mechanisms in porous media. In order to predict the variation of pore water conductivity, both a semi-analytical formulation and an empirical relationship for correcting the pore water conductivity, which depends on salinity, pressure and temperature, are derived. The central part of the laboratory experiments covers the spectral complex conductivity of water-bearing sand during exposure to and flow-through by CO2 at pressures up to 30MPa and temperatures up to 80°C. It is shown that the impact of CO2 on the real part of conductivity of a clean quartz sand is dominated by the low- and high-salinity regime of the pore water. The obtained data further show that chemical interaction causes a reduction of interface conductivity, which could be related to the low pH in the acidic environment. This effect is described by a correction term, which is a constant value as a first approximation. When the impact of CO2 is taken into account, a correct reconstruction of fluid saturation from electrical measurements is possible. In addition, changes of the inner surface area, which are related to mineral dissolution or precipitation processes, can be quantified. Both the knowledge gained from the laboratory experiments and a new workflow for the description and incorporation of geological geometry models enable realistic finite element simulations. Those were conducted for three different electromagnetic methods applied in the geological scenario of a fictitious carbon dioxide sequestration site. The results show that electromagnetic methods can play an important role in monitoring CO2 sequestration. Compared to other geophysical methods, electromagnetic techniques are generally very sensitive to pore fluids. The proper configuration of sources and receivers for a suitable electromagnetic method that generates the appropriate current systems is essential. Its reactive nature causes CO2 to interact with a water-bearing porous rock in a much more complex manner than non-reactive gases. Without knowledge of the specific interactions between CO2 and rock, a determination of saturation and, consequently, a successful monitoring are possible only to a limited extend. The presented work provides fundamental laboratory investigations for the understanding of the electrical properties of rocks when the reactive gas CO2 enters the rock-water system. All laboratory results are put in the context of potential monitoring applications. The transfer from petrophysical investigations to the planning of an operational monitoring design by means of close-to-reality 3D FE simulations is accomplished.

The objective of our studies is the combination of electromagnetic and direct current (DC) resistivity methods in a joint inversion approach to improve the reconstruction of a given conductivity distribution. We utilize the distinct sensitivity patterns of different methods to enhance the overall resolution power and ensure a more reliable imaging result. In order to simplify the work with more than one electromagnetic method and establish a flexible and state-of-the-art software basis, we developed new DC resistivity and electromagnetic forward modeling and inversion codes based on finite elements of second order on unstructured grids. The forward operators are verified using analytical solutions and convergence studies before we apply a regularized Gauss-Newton scheme and successfully invert synthetic data sets. Finally, we link both codes with each other in a joint inversion. In contrast to most widely used joint inversion strategies, where different data sets are combined in a single least-squares problem resulting in a large system of equations, we introduce a sequential approach that cycles through the different methods iteratively. This way, we avoid several difficulties such as the determination of the full set of regularization parameters or a weighting of the distinct data sets. The sequential approach makes use of a smoothness regularization operator which penalizes the deviation of the model parameters from a given reference model. In our sequential strategy, we use the result of the preceding individual inversion scheme as reference model for the following one. We successfully apply this approach to synthetic data sets and show that the combination of at least two methods yields a significantly improved parameter model compared to the individual inversion results. / Ziel der vorliegenden Arbeit ist die gemeinsame Inversion (\"joint inversion\") elektromagnetischer und geoelektrischer Daten zur Verbesserung des rekonstruierten Leitfähigkeitsmodells. Dabei nutzen wir die verschiedenartigen Sensitivitäten der Methoden aus, um die Auflösung zu erhöhen und ein zuverlässigeres Ergebnis zu erhalten. Um die Arbeit mit mehr als einer Methode zu vereinfachen und eine flexible Softwarebasis auf dem neuesten Stand der Forschung zu etablieren, wurden zwei Codes zur Modellierung und Inversion geoelektrischer als auch elektromagnetischer Daten neu entwickelt, die mit finiten Elementen zweiter Ordnung auf unstrukturierten Gittern arbeiten. Die Vorwärtsoperatoren werden mithilfe analytischer Lösungen und Konvergenzstudien verifiziert, bevor wir ein regularisiertes Gauß-Newton-Verfahren zur Inversion synthetischer Datensätze anwenden. Im Gegensatz zur meistgenutzten \"joint inversion\"-Strategie, bei der verschiedene Daten in einem einzigen Minimierungsproblem kombiniert werden, was in einem großen Gleichungssystem resultiert, stellen wir schließlich einen sequentiellen Ansatz vor, der zyklisch durch die einzelnen Methoden iteriert. So vermeiden wir u.a. eine komplizierte Wichtung der verschiedenen Daten und die Bestimmung aller Regularisierungsparameter in einem Schritt. Der sequentielle Ansatz wird über die Anwendung einer Glättungsregularisierung umgesetzt, bei der die Abweichung der Modellparameter zu einem gegebenen Referenzmodell bestraft wird. Wir nutzen das Ergebnis der vorangegangenen Einzelinversion als Referenzmodell für die folgende Inversion. Der Ansatz wird erfolgreich auf synthetische Datensätze angewendet und wir zeigen, dass die Kombination von mehreren Methoden eine erhebliche Verbesserung des Inversionsergebnisses im Vergleich zu den Einzelinversionen liefert.

Geophysical methods are essential for exploration and monitoring of subsurface formations, e.g. in carbon dioxide sequestration or enhanced geothermal energy. One of the keys to their successful application is the knowledge of how the measured physical quantities are related to the desired reservoir parameters. The work presented in this thesis shows that the presence of carbon dioxide (CO2) in pore space gives rise to multiple processes all of which contribute to the electrical rock conductivity variation. Basically, three mechanisms take place: (1) CO2 partially replaces the pore water, which is equivalent to a decrease in water saturation. (2) CO2 chemically interacts with the pore water by dissolution and dissociation. These processes change both the chemical composition and the pH of the pore filling fluid. (3) The low-pH environment can give rise to mineral dissolution and/or precipitation processes and changes the properties of the grain-water interface. Investigations on the pore water phase show that the reactive nature of CO2 in all physical states significantly acts on the electrical conductivity of saline pore waters. The physico-chemical interaction appears in different manifestations depending mainly on the pore water composition (salinity, ion types) but also on both temperature and pressure. The complex behaviour includes a low- and a high-salinity regime originating from the conductivity increasing effect of CO2 dissociation, which is opposed by the conductivity decreasing effect of reduced ion activity caused by the enhanced mutual impediment of all solutes. These results are fundamental since the properties of the water phase significantly act on all conduction mechanisms in porous media. In order to predict the variation of pore water conductivity, both a semi-analytical formulation and an empirical relationship for correcting the pore water conductivity, which depends on salinity, pressure and temperature, are derived. The central part of the laboratory experiments covers the spectral complex conductivity of water-bearing sand during exposure to and flow-through by CO2 at pressures up to 30MPa and temperatures up to 80°C. It is shown that the impact of CO2 on the real part of conductivity of a clean quartz sand is dominated by the low- and high-salinity regime of the pore water. The obtained data further show that chemical interaction causes a reduction of interface conductivity, which could be related to the low pH in the acidic environment. This effect is described by a correction term, which is a constant value as a first approximation. When the impact of CO2 is taken into account, a correct reconstruction of fluid saturation from electrical measurements is possible. In addition, changes of the inner surface area, which are related to mineral dissolution or precipitation processes, can be quantified. Both the knowledge gained from the laboratory experiments and a new workflow for the description and incorporation of geological geometry models enable realistic finite element simulations. Those were conducted for three different electromagnetic methods applied in the geological scenario of a fictitious carbon dioxide sequestration site. The results show that electromagnetic methods can play an important role in monitoring CO2 sequestration. Compared to other geophysical methods, electromagnetic techniques are generally very sensitive to pore fluids. The proper configuration of sources and receivers for a suitable electromagnetic method that generates the appropriate current systems is essential. Its reactive nature causes CO2 to interact with a water-bearing porous rock in a much more complex manner than non-reactive gases. Without knowledge of the specific interactions between CO2 and rock, a determination of saturation and, consequently, a successful monitoring are possible only to a limited extend. The presented work provides fundamental laboratory investigations for the understanding of the electrical properties of rocks when the reactive gas CO2 enters the rock-water system. All laboratory results are put in the context of potential monitoring applications. The transfer from petrophysical investigations to the planning of an operational monitoring design by means of close-to-reality 3D FE simulations is accomplished.

The objective of our studies is the combination of electromagnetic and direct current (DC) resistivity methods in a joint inversion approach to improve the reconstruction of a given conductivity distribution. We utilize the distinct sensitivity patterns of different methods to enhance the overall resolution power and ensure a more reliable imaging result. In order to simplify the work with more than one electromagnetic method and establish a flexible and state-of-the-art software basis, we developed new DC resistivity and electromagnetic forward modeling and inversion codes based on finite elements of second order on unstructured grids. The forward operators are verified using analytical solutions and convergence studies before we apply a regularized Gauss-Newton scheme and successfully invert synthetic data sets. Finally, we link both codes with each other in a joint inversion. In contrast to most widely used joint inversion strategies, where different data sets are combined in a single least-squares problem resulting in a large system of equations, we introduce a sequential approach that cycles through the different methods iteratively. This way, we avoid several difficulties such as the determination of the full set of regularization parameters or a weighting of the distinct data sets. The sequential approach makes use of a smoothness regularization operator which penalizes the deviation of the model parameters from a given reference model. In our sequential strategy, we use the result of the preceding individual inversion scheme as reference model for the following one. We successfully apply this approach to synthetic data sets and show that the combination of at least two methods yields a significantly improved parameter model compared to the individual inversion results. / Ziel der vorliegenden Arbeit ist die gemeinsame Inversion (\"joint inversion\") elektromagnetischer und geoelektrischer Daten zur Verbesserung des rekonstruierten Leitfähigkeitsmodells. Dabei nutzen wir die verschiedenartigen Sensitivitäten der Methoden aus, um die Auflösung zu erhöhen und ein zuverlässigeres Ergebnis zu erhalten. Um die Arbeit mit mehr als einer Methode zu vereinfachen und eine flexible Softwarebasis auf dem neuesten Stand der Forschung zu etablieren, wurden zwei Codes zur Modellierung und Inversion geoelektrischer als auch elektromagnetischer Daten neu entwickelt, die mit finiten Elementen zweiter Ordnung auf unstrukturierten Gittern arbeiten. Die Vorwärtsoperatoren werden mithilfe analytischer Lösungen und Konvergenzstudien verifiziert, bevor wir ein regularisiertes Gauß-Newton-Verfahren zur Inversion synthetischer Datensätze anwenden. Im Gegensatz zur meistgenutzten \"joint inversion\"-Strategie, bei der verschiedene Daten in einem einzigen Minimierungsproblem kombiniert werden, was in einem großen Gleichungssystem resultiert, stellen wir schließlich einen sequentiellen Ansatz vor, der zyklisch durch die einzelnen Methoden iteriert. So vermeiden wir u.a. eine komplizierte Wichtung der verschiedenen Daten und die Bestimmung aller Regularisierungsparameter in einem Schritt. Der sequentielle Ansatz wird über die Anwendung einer Glättungsregularisierung umgesetzt, bei der die Abweichung der Modellparameter zu einem gegebenen Referenzmodell bestraft wird. Wir nutzen das Ergebnis der vorangegangenen Einzelinversion als Referenzmodell für die folgende Inversion. Der Ansatz wird erfolgreich auf synthetische Datensätze angewendet und wir zeigen, dass die Kombination von mehreren Methoden eine erhebliche Verbesserung des Inversionsergebnisses im Vergleich zu den Einzelinversionen liefert.