The efficiency of geothermal energy extraction from reservoirs depends not only on temperature, but also on the porosity and permeability of the target rocks. Data from boreholes can constrain these parameters well at specific points but not across the entire geothermal targets. In this paper, the electrical resistivity information of the subsurface obtained from magnetotelluric data is calibrated by laws relating electrical conductivity to porosity and permeability, with the aim of extrapolating the porositypermeability values measured in the boreholes to the whole area of study. A new approach, based upon the Generalized Archie’s law, was created to determine the cementation exponent of different types of Irish rocks from porosity and electrical conductivity data. Using sandstone as the target rock, the permeability values were calculated from porosity, electrical conductivity and cementation exponent data using different approaches. The influences of the parametric variables were tested, showing which variables create more instability in the final results. Synthetic electrical resistivity models of the subsurface were also created to analyze the sensitivity of MT data to differences in the porosity - permeability of the target rock. These results of this work will be used within the geothermal IRETHERM geothermal project to assist with the project aims of identifying those areas of Ireland with the most potential for geothermal energy provision.
Abstract The integration of passive electromagnetic geophysical data and well‐log data for basin characterization and interpretation has been investigated in the Clare Basin, western Ireland. The Clare Basin is overmature and has a clear contrast in electrical resistivity between the Clare Shale Formation, a widespread organic rich shale unit, and the surrounding stratigraphy. The electrical resistivity distribution beneath the Clare Basin was determined by means of three‐dimensional (3‐D) joint inversion of three distinct and differently sensitive electromagnetic parameters: (1) the MT impedance tensor (Z), (2) the geomagnetic transfer function (T), and (3) the interstation horizontal magnetic transfer function (H). Well‐log data from a local exploration well, Doonbeg‐1, were analyzed by means of multivariate statistical methods identifying three groups with distinct resistivity values. The groups were propagated along the basin using the 3‐D electrical resistivity model, showing those regions in the basin with significant organic content at high maturity stage. The lack of continuity of these regions supports the hypothesis of advective fluid heating as the cause of the high maturity levels. The results also help to define the geometry of the basin at depth and have identified an area within the basin, near the Loop Head, where organic‐rich clay/shale is either poorly developed, and/or the organic matter is less mature and less conductive. Finally, the potential of the basin for both CO 2 storage and geothermal energy was considered, supporting the use of the Clare Basin as a potential site for geothermal energy but not for the storage of CO 2 .
A hydrogeological conceptual model of the source, circulation pathways and temporal variation of a low-enthalpy thermal spring in a fractured limestone setting is derived from a multidisciplinary approach. St. Gorman's Well is a thermal spring in east-central Ireland with a complex and variable temperature profile (maximum of 21.8 °C). Geophysical data from a three-dimensional(3D)audio-magnetotelluric(AMT) survey are combined with time-lapse hydrogeological data and information from a previously published hydrochemical analysis to investigate the operation of this intriguing hydrothermal system. Hydrochemical analysis and time-lapse measurements suggest that the thermal waters flow within the fractured limestones of the Carboniferous Dublin Basin at all times but display variability in discharge and temperature. The 3D electrical resistivity model of the subsurface revealed two prominent structures: (1) a NW-aligned faulted contact between two limestone lithologies; and (2) a dissolutionally enhanced, N-aligned, fault of probable Cenozoic age. The intersection of these two structures, which has allowed for karstification of the limestone bedrock, has created conduits facilitating the operation of relatively deep hydrothermal circulation (likely estimated depths between 240 and 1,000 m) within the limestone succession of the Dublin Basin. The results of this study support a hypothesis that the maximum temperature and simultaneous increased discharge observed at St. Gorman's Well each winter is the result of rapid infiltration, heating and recirculation of meteoric waters within a structurally controlled hydrothermal circulation system.The online version contains supplementary material available at 10.1007/s10040-021-02393-1.Un modèle conceptuel hydrogéologique de la source, des voies de circulation et de la variation temporelle d’une source thermale à faible enthalpie dans un contexte de calcaire fracturé est dérivé d’une approche multidisciplinaire. St. Gorman’s Well est une source thermale du centre-est de l’Irlande avec un profil de température complexe et variable (maximum de 21.8 °C). Les données géophysiques d’un levé audio-magnétotellurique (AMT) en trois dimensions (3D) sont combinées avec des données hydrogéologiques à intervalles de temps et des informations provenant d’une analyse hydrochimique publiée précédemment pour étudier le fonctionnement de cet intrigant système hydrothermal. L’analyse hydrochimique et les mesures à différentes périodes suggèrent que les eaux thermales s’écoulent à tout moment dans les calcaires fracturés du bassin carbonifère de Dublin, mais présentent une variabilité de débit et de température. Le modèle de résistivité électrique 3D du sous-sol a révélé deux structures importantes: (1) un contact faillé orienté NW entre deux lithologies calcaires; et (2) une faille alignée au Nord, améliorée par dissolution, d’âge cénozoïque probable. L’intersection de ces deux structures, qui a permis la karstification du socle calcaire, a créé des conduits facilitant le fonctionnement d’une circulation hydrothermale relativement profonde (profondeurs estimées vraisemblablement entre 240 et 1,000 m) au sein de la succession calcaire du bassin de Dublin. Les résultats de cette étude appuient l’hypothèse selon laquelle la température maximale et l’augmentation simultanée du débit observés à St. Gorman’s Well chaque hiver sont le résultat d’une infiltration, d’un réchauffement et d’une recirculation rapides des eaux météoriques dans un système de circulation hydrothermale structurellement contrôlé.Se deriva un modelo conceptual hidrogeológico de la fuente, las vías de circulación y la variación temporal de un manantial termal de baja entalpía en un entorno de caliza fracturada a partir de un enfoque multidisciplinar. Gorman’s Well es un manantial termal en el centro-este de Irlanda con un perfil de temperatura complejo y variable (máximo de 21.8 °C). Los datos geofísicos de un estudio audio-magnetotelúrico (AMT) tridimensional (3D) se combinan con los datos hidrogeológicos de un lapso de tiempo y la información de un análisis hidroquímico previamente publicado para investigar el funcionamiento de este intrigante sistema hidrotermal. El análisis hidroquímico y las mediciones a intervalos de tiempo sugieren que las aguas termales fluyen dentro de las calizas fracturadas de la cuenca carbonífera de Dublín en todo momento, pero muestran variabilidad en la descarga y la temperatura. El modelo de resistividad eléctrica tridimensional del subsuelo reveló dos estructuras prominentes: (1) un contacto de falla alineado al NW entre dos litologías calcáreas; y (2) una falla de disolución incrementada, alineada al N, de probable edad cenozoica. La intersección de estas dos estructuras, que ha permitido la karstificación del lecho rocoso calcáreo, ha creado conductos que facilitan el funcionamiento de una circulación hidrotermal relativamente profunda (probablemente a profundidades estimadas entre 240 y 1,000 m) dentro de la sucesión calcárea de la cuenca de Dublín. Los resultados de este estudio apoyan la hipótesis de que la temperatura máxima y el aumento simultáneo de la descarga observados en St. Gorman’s Well cada invierno son el resultado de una rápida infiltración, calentamiento y recirculación de aguas meteóricas dentro de un sistema de circulación hidrotermal estructuralmente controlado.通过多学科方法, 建立了裂缝型石灰岩环境中低焓温泉的来源、循环路径和时间变化的水文地质概念模型。圣戈尔曼井是爱尔兰中东部的温泉, 温度分布复杂多变 (最高21.8 °C)。将三维 (3D) 音频-大地电磁 (AMT) 调查的地球物理数据与延时水文地质数据和先前发布的水化学分析信息相结合, 调查了有趣的热液系统运行情况。水化学分析和延时测量表明, 热水一直在石炭系都柏林盆地的裂缝型石灰岩内流动, 但流量和温度有变异性。地下的 3D 电阻率模型揭示了两个突出的结构:(1) 两个石灰岩岩性之间的 NW 向断层接触; 和 (2) 可能新生代的溶蚀增强的和N向的断层。两个结构的交叉点使石灰岩基岩发生岩溶作用, 形成了通道, 促进了都柏林盆地石灰岩序列内相对较深的热液循环 (估计深度可能在 240 至 1,000 m 之间)作用。通过这项研究发现每年冬天在圣戈尔曼井观察到的最高温度和同时增加的排泄量是结构控制的热液循环系统中大气水快速入渗、加热和再循环的结果。.Um modelo hidrogeológico conceitual da fonte, vias de circulação e variação temporal de uma fonte termal de baixa entalpia em um ambiente de calcário fraturado é derivado de uma abordagem multidisciplinar. O poço de St. Gorman é uma fonte termal no centro-leste da Irlanda com um perfil de temperatura complexo e variável (máximo de 21.8°C). Os dados geofísicos de uma pesquisa áudio-magnetotelúrica (AMT) tridimensional (3D) são combinados com dados hidrogeológicos em intervalos de tempo e informações de uma análise hidroquimica publicada anteriormente, para investigar a operação deste intrigante sistema hidrotérmico. A análise hidroquimica e as medições em intervalos de tempo sugerem que as águas termais fluem de dentro dos calcários fraturados da Bacia Carbonifera de Dublin o tempo todo, mas exibem variabilidade na descarga e na temperatura. O modelo de resistividade elétrica 3D da subsuperfície revelou duas estruturas proeminentes: (1) um contato defeituoso alinhado a NO entre duas litologias de calcário; e (2) uma falha dissolucionalmente aumentada, alinhada a N, de provável idade Cenozóica. A intersecção dessas duas estruturas, que permitiu a carstificação da rocha calcária, criou condutos que facilitam a operação de circulação hidrotérmica relativamente profunda (profundidade estimada entre 240 e 1,000 m) dentro da sucessão de calcário da Bacia Dublin. Os resultados desse estudo suportam a hipótese de que a temperatura máxima e o aumento simultâneo da descarga observada no poço de St. Gorman a cada inverno é o resultado da rápida infiltração, aquecimento e recirculação de águas meteóricas dentro de um sistema de circulação hidrotérmica estruturalmente controlado.
We present multi-dimensional modelling of the magnetotelluric (MT) data collected in the Newcastle area, west of Dublin, Ireland, in the frame of the IRETHERM project (www.iretherm.ie). IRETHERM’s overarching objective is to develop a strategic and holistic understanding of Ireland's geothermal energy potential through integrated modelling of new and existing geophysical, geochemical and geological data. The Newcastle area is situated on the southern margin of the Dublin Basin, close to the largest conurbation on the island of Ireland. A description of data processing methods, 2-D and 3-D geoelectrical models of the area and integrated modelling with other geophysical data are presented here. The MT soundings were carried out in the highly urbanized Dublin suburb in 2011 and 2012. The MT time series data were heavily noise-contaminated and distorted due to electromagnetic noise from nearby industry and DC tram/railway systems. Time series processing using several modern robust codes was able to obtain reasonably reliable and interpretable MT impedance and geomagnetic transfer function estimates at most of the locations. The most “quiet” 4-hour subsets of data during the night time, when the DC tram system was not operating, were used in multi-site and multi-variate processing, with the most reliable sounding curves spanning the frequency range 10 kHz to 0.001 Hz. A novel technique using inter-station transfer functions has been implemented as a processing tool to obtain broader primary data for final multi-dimensional modelling. In order to reduce distortion in the MT data, the dimensionality, and if two-dimensional (2-D) then the regional 2-D strike direction, must be determined. Tensor distortion decomposition was applied at each site to determine if the data are suitable for 2-D modelling, with most data at most sites accepting a 2-D description as valid. The final 2-D models underwent examination using a new stability technique, and the final two 2-D profiles with reliability estimations, expressed through conductance and resistivity, were prepared. As a conclusion to the modelling, 3-D models of all MT data in the Newcastle area have been performed and further on-going 3-D modelling along with the inter-station magnetic transfer functions have been performed. The integrated modelling with existing seismic reflection profiles and gravity data in the area reveals that the Blackrock to Newcastle Fault (BNF) is visible in the models as a conductive area to depths of 4 km and is highly fractured. Generally, the southern area is more resistive and compact with a horizontal conductive layer at approximately 1 km depth, with a very thin sedimentary layer on top. The structures north of the BNF are more heterogeneous, with deeper conductive layers (2-3 km) and thicker (several hundred meters) sedimentary layers above.
The Okavango Delta of northern Botswana is one of the world's largest inland deltas or megafans. To obtain information on the character of sediments and basement depths, audiomagnetotelluric (AMT), controlled-source audiomagnetotelluric (CSAMT) and central-loop transient electromagnetic (TEM) data were collected on the largest island within the delta. The data were inverted individually and jointly for 1-D models of electric resistivity. Distortion effects in the AMT and CSAMT data were accounted for by including galvanic distortion tensors as free parameters in the inversions. By employing Marquardt–Levenberg inversion, we found that a 3-layer model comprising a resistive layer overlying sequentially a conductive layer and a deeper resistive layer was sufficient to explain all of the electromagnetic data. However, the top of the basal resistive layer from electromagnetic-only inversions was much shallower than the well-determined basement depth observed in high-quality seismic reflection images and seismic refraction velocity tomograms. To resolve this discrepancy, we jointly inverted the electromagnetic data for 4-layer models by including seismic depths to an interface between sedimentary units and to basement as explicit a priori constraints. We have also estimated the interconnected porosities, clay contents and pore-fluid resistivities of the sedimentary units from their electrical resistivities and seismic P-wave velocities using appropriate petrophysical models. In the interpretation of our preferred model, a shallow ∼40 m thick freshwater sandy aquifer with 85–100 Ωm resistivity, 10–32 per cent interconnected porosity and <13 per cent clay content overlies a 105–115 m thick conductive sequence of clay and intercalated salt-water-saturated sands with 15–20 Ωm total resistivity, 1−27 per cent interconnected porosity and 15–60 per cent clay content. A third ∼60 m thick sandy layer with 40–50 Ωm resistivity, 10–33 per cent interconnected porosity and <15 per cent clay content is underlain by the basement with 3200–4000 Ωm total resistivity. According to an interpretation of helicopter TEM data that cover the entire Okavango Delta and borehole logs, the second and third layers may represent lacustrine sediments from Paleo Lake Makgadikgadi and a moderately resistive freshwater aquifer comprising sediments of the recently proposed Paleo Okavango Megafan, respectively.
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