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The subject of this dissertation is use of computer models as data analysis tools in several different geoscience settings, including integrated surface water/groundwater modeling, tephra fallout modeling, geophysical inversion, and hydrothermal groundwater modeling. The dissertation is organized into three chapters, which correspond to three individual publication manuscripts. In the first chapter, a linear framework is developed to identify and estimate the potential predictive consequences of using a simple computer model as a data analysis tool. The framework is applied to a complex integrated surface-water/groundwater numerical model with thousands of parameters. Several types of predictions are evaluated, including particle travel time and surface-water/groundwater exchange volume. The analysis suggests that model simplifications have the potential to corrupt many types of predictions. The implementation of the inversion, including how the objective function is formulated, what minimum of the objective function value is acceptable, and how expert knowledge is enforced on parameters, can greatly influence the manifestation of model simplification. Depending on the prediction, failure to specifically address each of these important issues during inversion is shown to degrade the reliability of some predictions. In some instances, inversion is shown to increase, rather than decrease, the uncertainty of a prediction, which defeats the purpose of using a model as a data analysis tool. In the second chapter, an efficient inversion and uncertainty quantification approach is applied to a computer model of volcanic tephra transport and deposition. The computer model simulates many physical processes related to tephra transport and fallout. The utility of the approach is demonstrated for two eruption events. In both cases, the importance of uncertainty quantification is highlighted by exposing the variability in the conditioning provided by the observations used for inversion. The worth of different types of tephra data to reduce parameter uncertainty is evaluated, as is the importance of different observation error models. The analyses reveal the importance using tephra granulometry data for inversion, which results in reduced uncertainty for most eruption parameters. In the third chapter, geophysical inversion is combined with hydrothermal modeling to evaluate the enthalpy of an undeveloped geothermal resource in a pull-apart basin located in southeastern Armenia. A high-dimensional gravity inversion is used to define the depth to the contact between the lower-density valley fill sediments and the higher-density surrounding host rock. The inverted basin depth distribution was used to define the hydrostratigraphy for the coupled groundwater-flow and heat-transport model that simulates the circulation of hydrothermal fluids in the system. Evaluation of several different geothermal system configurations indicates that the most likely system configuration is a low-enthalpy, liquid-dominated geothermal system.

Sites in a 66-station geodetic network in central Greece have been occupied up to six times since 1989 using GPS surveying, and accurate positions have been computed using fiducially-improved or precise orbits. Site velocities are calculated under the assumption that they are constant with time, after correcting for co-seismic effects, and that the position of the fixed base station (and hence the entire network) may be subject to small errors. Low-order polynomial expressions do not fit the velocity field well. The pattern of observed strain closely resembles that derived from independent geodetic observations made over a hundred-year time-scale. Significant geodetic strain is observed across the Gulf of Korinthos, even after the co-seismic displacement field of the Ms=6.2 1995 Egion earthquake has been removed by forward modelling. Geodetic strain is higher in the western than eastern Gulf, in contrast to the seismic strain which is similar throughout. Seismic strain matches geodetic strain in the east, but a significant deficit of seismic moment exists in the west which may represent a high earthquake hazard in the medium term. The Ms=6.6 1995 Grevena earthquake struck a previously seismically quiet region well covered by a recent triangulation / trilateration survey. Ninety-one points from this network were reoccupied with GPS immediately after the earthquake, and site displacements computed. To invert for the earthquake source parameters from the geodetic displacement field, a novel inversion scheme is used which combines the Monte-Carlo and simplex approaches. A priori parameters are not required, even though the inverse problem is strongly nonlinear. The resulting focal mechanism agrees well with the global CMT solution and locally observed aftershocks, but implies a significantly higher scalar moment than do seismological studies. A network for observing post-seismic deformation has been established, which in view of the low background seismicity seems likely to provide significant results.

Afin de cartographier la structure souterraine des volcans et détecter des ressources géothermiques de haute température, on utilise souvent l’imagerie de résistivité électrique. La résistivité électrique des volcans est affectée par plusieurs facteurs: volume et salinité de l’eau interstitielle, abondance de minéraux conducteurs, température de la roche et présence de magma. Ce travail de thèse tente de contraindre l'interprétation des structures de résistivité électrique autour des volcans actifs, afin de développer des outils innovants pour l'exploration des ressources géothermiques. La contribution des minéraux conducteurs est au cœur de la thèse: conducteurs ioniques solides (minéraux argileux, en particulier la smectite) ou semi-conducteurs électroniques (pyrite, oxydes de fer), mais l’influence de la porosité, de la salinité, de la température et de la présence de magma est aussi étudiée. La thèse utilise le volcan Krafla comme terrain d’étude pour affiner les interprétations des structures de résistivité électriques, du fait de la disponibilité de carottes, de données, de bibliographie et d’infrastructure. La smectite et la pyrite sont formées par altération hydrothermale des roches volcaniques et témoignent ainsi des convections hydrothermales. Les oxydes de fer en revanche sont plutôt formés lors de la cristallisation du magma et sont dissous lors des circulations hydrothermales. La contribution de la smectite à la conductivité électrique de roches volcaniques, saturées en eau à différentes salinités, est d'abord étudiée en laboratoire (à température ambiante) par spectroscopie d’impédance électrique « résistivité complexe ». Des variations non linéaires de la conductivité électrique à 1 kHz avec la salinité sont observées et discutées. La conduction interfoliaire est suggérée comme un mécanisme important par lequel la smectite conduit le courant électrique. L'influence de la pyrite et des oxydes de fer sur les effets de polarisation provoquée est ensuite analysée en utilisant l'angle de phase de l'impédance, qui dépend de la fréquence. Un angle de phase maximal supérieur à 20 mrad est attribué à la pyrite si la roche est conductrice et aux oxydes de fer si la roche est résistive. L'angle de phase maximal augmente d'environ 22 mrad pour chaque pourcent de pyrite ou d'oxyde de fer. Ces résultats de laboratoire en domaine fréquentiel sont appliqués à l’interprétation de tomographies de résistivité complexe sur le terrain en domaine temporel. Smectite, pyrite et oxydes de fer ont pu être identifiés jusqu'à 200 m de profondeur. La température in-situ, plus élevées qu’en laboratoire, semble augmenter la conductivité de la smectite. De manière générale, la tomographie de résistivité complexe est recommandée comme méthode complémentaire aux sondages électromagnétiques pour l'exploration géothermique. / Electromagnetic soundings are widely used to image the underground structure of volcanoes and look for hightemperature geothermal resources. The electrical resistivity of volcanoes is affected by several characteristics of rocks: volume and salinity of pore fluid, abundance of conductive minerals, rock temperature and presence of magma. This thesis aims at improving the interpretation of electrical resistivity structures around active volcanoes, in order to develop innovative tools for the assessment of geothermal resources. I focus on conductive minerals, which can either be solid ionic conductors (clay minerals, in particular smectite) or electronic semi-conductors (pyrite and iron-oxides), but I also investigate the effects of porosity, salinity, temperature and presence of magma. I use Krafla volcano as a laboratory area, where extensive literature, borehole data, core samples, surface soundings and infrastructures are available. Smectite and pyrite are formed upon hydrothermal alteration of volcanic rocks and thus witness hydrothermal convection. On the other hand, iron-oxides are mostly formed during the primary crystallization of magma and dissolved by hydrothermal fluids. The contribution of smectite to the electrical conductivity of volcanic rocks saturated with pore water at different salinity is first investigated in the laboratory (room temperature) by electrical impedance spectroscopy “complex resistivity”. Non-linear variations of the conductivity at 1 kHz with salinity are observed and discussed. Interfoliar conduction is suggested as an important mechanism by which smectite conducts electrical current. The influence of pyrite and iron-oxides on induced polarization effects is then analyzed, using the frequency-dependent phase-angle of the impedance. A maximum phase-angle higher than 20 mrad is attributed to pyrite if the rock is conductive and to ironoxides if the rock is resistive. The maximum phase-angle increases by about 22 mrad for each additional per cent of pyrite or iron-oxide. These laboratory frequency-domain findings are partly upscaled to interpret field time-domain complex resistivity tomography at Krafla: smectite, pyrite and iron-oxides can be identified down to 200 m. The in-situ temperature, higher than in laboratory conditions, appears to significantly increase the conductivity associated to smectite. In general, time-domain complex resistivity measurements are recommended as a complementary method to electromagnetic soundings for geothermal exploration.

A caracterização geológica e geotécnica dos locais de implantação de aterros sanitários é fundamental, tanto para fornecer subsídios adequados ao projeto de instalação das células de deposição dos resíduos sólidos urbanos (RSU), quanto para que se conheçam as características da área antes de ser modificada pela obra. Neste sentido, o presente trabalho visou mostrar a contribuição que a geofísica, com o uso da técnica da eletrorresistividade e do potencial espontâneo, associado a informações de investigação geotécnica direta pode dar para o estabelecimento de um background da área de instalação do aterro sanitário da cidade de São Carlos-SP. Foram realizados 7 caminhamentos elétricos (CE) com o arranjo dipolo-dipolo com eletrodos espaçados de 10 metros e três sondagens elétricas verticais (SEV), utilizando o arranjo Schlumberger, com profundidade de investigação de até 100 metros. Para processamento dos modelos bidimensionais dos CE utilizou-se o programa RES2DINV, e enquanto que para as SEV foi utilizado o modelo unidimensional. A interpretação dos modelos geoelétricos foi aperfeiçoada pela confrontação com informações geológico-geotécnicas prévias da área, obtidas nos estudos de impacto ambiental (EIA/RIMA) e resultados de sondagens de simples reconhecimento (SPT), realizadas na fase de estudo para implantação do aterro sanitário. Com isso foi possível definir com clareza a profundidade do topo rochoso e das zonas de maior umidade nos CE e SEV. O conjunto de seções de CE foi georeferenciada com o auxílio de GPS de precisão, o que permitiu o tratamento espacial das informações e o estabelecimento dos mapas da superfície do topo rochoso, das zonas de fluxo de água subterrânea e da espessura dos materiais inconsolidados. O conjunto de informações obtido permitiu identificar locais possíveis para a instalação futura de poços de monitoramento, assim como os dados geoelétricos das várias camadas servirá como padrão para identificar eventuais contaminações do solo ou da água subterrânea no futuro. A identificação da presença do topo rochoso em pequena profundidade, em algumas áreas do aterro, permite o melhor planejamento das escavações para instalação das células de deposição do RSU. / The geological and geotechnical characterization of landfill implantation areas is crucial; even to provide appropriate subsidies of urban solid residues (USR) installation project of deposition cells and for knowing the characteristics of the area before being modified by the work. In this sense, the present work aimed to show the contribution that the geophysics, with the use of electrical resistivity technique and the spontaneous potential, associated with direct geotechnical investigation information can give to the establishment of a background of landfill installation area from São Carlos-SP city. Were performed 7 electrical imaging (EI) with the dipole-dipole array with electrodes spaced by 10 meters and three vertical electrical sounding (VES), using the Schlumberger array with depth research up to 100 meters. For processing of two-dimensional models of the EI was used the RES2DINV program and while for the vertical electrical sounding (VES)-was used one-dimensional model. Interpretation of geoelectric models was enhanced by the confrontation with geological and geotechnical information of prior area, obtained in the environmental impact assessment (EIA) and results of a simple reconnaissance probes, performed in the study phase for the landfill implementation. It was possible to clearly define the depth of the bedrock and the higher humidity areas in the EI and VES. The set of sections electrical Imaging (EI) was georeferenced with the aid of GPS accuracy that allowed the spatial treatment of Information and the setting maps of the bedrock surface, the groundwater flow zones and the thickness of the unconsolidated materials. The obtained set of information enabling identification of possible locations for the wells future monitoring installation, as well as the geoelectrical data from several layers will be use as a standard to identify possible contamination of the soil or groundwater in the future. The identification of bedrock presence in small depth, in some areas of landfill, allows better planning of excavation for installation of MSW deposition cells.

A caracterização geológica e geotécnica dos locais de implantação de aterros sanitários é fundamental, tanto para fornecer subsídios adequados ao projeto de instalação das células de deposição dos resíduos sólidos urbanos (RSU), quanto para que se conheçam as características da área antes de ser modificada pela obra. Neste sentido, o presente trabalho visou mostrar a contribuição que a geofísica, com o uso da técnica da eletrorresistividade e do potencial espontâneo, associado a informações de investigação geotécnica direta pode dar para o estabelecimento de um background da área de instalação do aterro sanitário da cidade de São Carlos-SP. Foram realizados 7 caminhamentos elétricos (CE) com o arranjo dipolo-dipolo com eletrodos espaçados de 10 metros e três sondagens elétricas verticais (SEV), utilizando o arranjo Schlumberger, com profundidade de investigação de até 100 metros. Para processamento dos modelos bidimensionais dos CE utilizou-se o programa RES2DINV, e enquanto que para as SEV foi utilizado o modelo unidimensional. A interpretação dos modelos geoelétricos foi aperfeiçoada pela confrontação com informações geológico-geotécnicas prévias da área, obtidas nos estudos de impacto ambiental (EIA/RIMA) e resultados de sondagens de simples reconhecimento (SPT), realizadas na fase de estudo para implantação do aterro sanitário. Com isso foi possível definir com clareza a profundidade do topo rochoso e das zonas de maior umidade nos CE e SEV. O conjunto de seções de CE foi georeferenciada com o auxílio de GPS de precisão, o que permitiu o tratamento espacial das informações e o estabelecimento dos mapas da superfície do topo rochoso, das zonas de fluxo de água subterrânea e da espessura dos materiais inconsolidados. O conjunto de informações obtido permitiu identificar locais possíveis para a instalação futura de poços de monitoramento, assim como os dados geoelétricos das várias camadas servirá como padrão para identificar eventuais contaminações do solo ou da água subterrânea no futuro. A identificação da presença do topo rochoso em pequena profundidade, em algumas áreas do aterro, permite o melhor planejamento das escavações para instalação das células de deposição do RSU. / The geological and geotechnical characterization of landfill implantation areas is crucial; even to provide appropriate subsidies of urban solid residues (USR) installation project of deposition cells and for knowing the characteristics of the area before being modified by the work. In this sense, the present work aimed to show the contribution that the geophysics, with the use of electrical resistivity technique and the spontaneous potential, associated with direct geotechnical investigation information can give to the establishment of a background of landfill installation area from São Carlos-SP city. Were performed 7 electrical imaging (EI) with the dipole-dipole array with electrodes spaced by 10 meters and three vertical electrical sounding (VES), using the Schlumberger array with depth research up to 100 meters. For processing of two-dimensional models of the EI was used the RES2DINV program and while for the vertical electrical sounding (VES)-was used one-dimensional model. Interpretation of geoelectric models was enhanced by the confrontation with geological and geotechnical information of prior area, obtained in the environmental impact assessment (EIA) and results of a simple reconnaissance probes, performed in the study phase for the landfill implementation. It was possible to clearly define the depth of the bedrock and the higher humidity areas in the EI and VES. The set of sections electrical Imaging (EI) was georeferenced with the aid of GPS accuracy that allowed the spatial treatment of Information and the setting maps of the bedrock surface, the groundwater flow zones and the thickness of the unconsolidated materials. The obtained set of information enabling identification of possible locations for the wells future monitoring installation, as well as the geoelectrical data from several layers will be use as a standard to identify possible contamination of the soil or groundwater in the future. The identification of bedrock presence in small depth, in some areas of landfill, allows better planning of excavation for installation of MSW deposition cells.