Abstract
Groundwater models simulating flow in buried valleys interacting with regional aquifers are often based on hydrogeological models interpreted from dense geophysical datasets and scarce borehole data. For three simple synthetic cases, it is demonstrated that alternative methods of inversion of transient electro-magnetic (TEM) data can lead to very different interpretations of the hydrogeology inside and surrounding a buried valley. The alternative interpreted hydrogeological models are used in numerical modelling of groundwater flow to a pumping well. It is demonstrated that the alternative models result in quite different groundwater-model predictions of capture zone, recharge area, and groundwater age for the pumping well. It is briefly demonstrated that model calibration against hydraulic head data is not likely to improve the predictions or to identify the structural error of the interpreted hydrogeological models. It is therefore concluded that when TEM-based resistivity models are interpreted to construct the hydrogeological framework of a groundwater model, it must be done cautiously with support from deep borehole information. Too much reliance on geophysical mapping can lead to seriously wrong hydrogeological models and correspondingly wrong groundwater-model predictions.
Résumé
Les modèles simulant l’écoulement dans des vallées enfouies interagissant avec des aquifères régionaux sont souvent basés sur des modèles hydrogéologiques construits à partir d’ensembles denses de données géophysiques et de rares données de forage. Dans trois cas synthétiques et simples, il est démontré que les méthodes alternatives d’inversion des données électromagnétiques transitoires (TEM) peuvent conduire à des interprétations très différentes de l’hydrogéologie à l’intérieur et dans l’encaissant d’une vallée enfouie. Les modèles alternatifs d’interprétation hydrogéologique sont utilisés en modélisation numérique de l’écoulement souterrain vers un puits de pompage. Il est démontré que les modèles alternatifs conduisent à des prévisions tout à fait différentes pour la zone de captage, l’aire de recharge, et l’âge de l’eau souterraine pour le puits de pompage. Il est démontré brièvement que le calage du modèle sur des données piézométriques ne va vraisemblablement pas améliorer la réponse ni identifier l’erreur de structure des modèles d’interprétation hydrogéologique. On conclu que lorsque des modèles de résistivité basés sur la méthode TEM sont utilisés pour construire le cadre hydrogéologique d’un modèle de nappe souterraine, cela doit être fait prudemment avec l’appui de l’information de forage profond. Une trop grande confiance en la cartographie géophysique peut mener à des modèles hydrogéologiques sérieusement erronés et par conséquent à des prédictions erronées de modèle de nappe.
Resumen
Los modelos de de agua subterránea que simulan el flujo en valles enterrados que interactúan con acuíferos regionales están a menudo basados en modelos hidrogeológicos interpretados a partir de densos conjuntos de datos geofísicos y escasos datos de perforaciones. Se demuestra para tres casos sintéticos simples, que los métodos alternativos de inversión de datos transitorios electromagnéticos (TEM) pueden conducir a muy diferentes interpretaciones dentro de la hidrogeología y los alrededores de un valle enterrado. Los modelos hidrogeológicos alternativos interpretados se usan en la modelación numérica del flujo subterráneo hacia pozos de bombeo. Se demuestra que los modelos alternativos dan como resultado muy diferentes predicciones de la modelación de agua subterránea de la zona de captura, el área de recarga y de la edad del agua para los pozos de bombeo. Se demuestra brevemente que no es probable la calibración del modelo en función de los datos de carga hidráulica para mejorar las predicciones o identificar el error estructural de los modelos hidrogeológicos interpretados. Además se concluye que cuando los modelos de resistividad basados en TEM son interpretados para construir el marco hidrogeológico de un modelo de agua subterránea, debe hacerse cautamente con el apoyo de la información de pozos profundos. Demasiada confianza en los mapeos geofísicos puede conducir a modelos hidrogeológicos seriamente equivocados y correspondientes a predicciones erróneas de los modelos de agua subterránea.
摘要
模拟和区域含水层之间有相互作用的地下河谷地下水流的模型通常是基于水文地质模型的,这些水文地质模型是根据大量的地球物理数据和稀少的钻孔数据解译的。三个简单的综合分析例子表明瞬变电磁(TEM)数据反演的替代方法可以得出地下河谷内部和周围水文地质条件的不同解释。这个替代解释水文地质模型常常用于抽水井的地下水流数值模拟。结果表明替代模型得出完全不同的地下水模型预测出的抽水井的捕获区,补给区和地下水年龄。它简单的说明了依据水头数据来校正模型不大可能改善预测结果或者识别出解释水文地质模型的结构性的错误。因此得出的结论是,当利用基于TEM的电阻率模型来建立地下水模型的水文地质框架时,在有深部钻孔信息的帮助下,我们必须慎重对待。过多的依赖地球物理测绘会得出有严重错误的水文地质模型以及相应错误的地下水模型预测结果。
Resumo
Os modelos de simulação de fluxo subterrâneo em vales enterrados que interagem com aquíferos regionais baseiam-se frequentemente em modelos hidrogeológicos interpretados a partir de vastos conjuntos de dados geofísicos e escassa informação de logs de sondagem. Demonstra-se, para três casos sintéticos simples, que a utilização de métodos alternativos de inversão de dados electromagnéticos transientes (TEM) pode levar a interpretações muito diferentes da hidrogeologia da zona interior e envolvente do vale enterrado. Utilizam-se os modelos conceptuais alternativos de interpretação hidrogeológica na modelação numérica do escoamento subterrâneo em direcção a um furo de extracção. Demonstra-se que as interpretações alternativas resultam em previsões do modelo muito diferentes da zona de captura, da área de recarga e da idade da água subterrânea para o furo de extracção. É demonstrado de forma sucinta que não é provável que a calibração dos modelos com dados de potencial hidráulico resulte em melhorias de previsão ou na identificação do erro estrutural dos modelos hidrogeológicos interpretados. Conclui-se por esta razão que, quando os modelos de resistividade baseados no TEM são interpretados para construir o enquadramento hidrogeológico de um modelo de fluxo subterrâneo, tal deve ser feito de forma cautelosa e com o apoio de informação de sondagens profundas. A confiança em demasia no mapeamento geofísico pode levar a modelos conceptuais hidrogeológicos seriamente errados e consequentemente a previsões erradas pelo modelo de fluxo subterrâneo.
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Acknowledgements
The authors gratefully thank FIVA (International Research School of Water Resources, www.fiva.dk), GEUS (Geological Survey of Denmark and Greenland), and Aarhus University for funding this research. We also thank Joakim Hollenbo Westergaard and Lars Hjortshøj Jacobsen for valuable assistance. Finally, the authors would like to thank the journal reviewers whose constructive and relevant comments significantly improved the paper.
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Andersen, T.R., Poulsen, S.E., Christensen, S. et al. A synthetic study of geophysics-based modelling of groundwater flow in catchments with a buried valley. Hydrogeol J 21, 491–503 (2013). https://doi.org/10.1007/s10040-012-0924-5
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DOI: https://doi.org/10.1007/s10040-012-0924-5