Steen Christensen

Gas transport in a confined unsaturated zone during atmospheric pressure cycles

Gas transport induced by temporal pressure fluctuations in the atmosphere can be an important mechanism for transport of atmospheric oxygen within the unsaturated zone. Moreover, the presence of oxygen in the unsaturated zone may be a factor controlling oxidation of sulphide minerals and other redox processes. A field study was carried out in a glacial aquifer with a 10–12 m thick sandy unsaturated zone to explore gas exchange between the atmosphere and the unsaturated zone. The exchange occurs through a “geological window” in a till layer which covers the sandy unsaturated zone. Observed pressure distribution and oxygen concentrations within the unsaturated zone were compared to numerical simulations with SUTRA, a finite element and fluid density dependent groundwater flow model. The simulations were carried out by modeling the gas pressure distribution within the unsaturated zone based on atmospheric pressure time series. The spatial variation in permeability observed from borehole logging was implemented in the model. The analysis demonstrated a good match between the field observations and the numerical simulations. During an atmospheric pressure cycle, atmospheric oxygen migrated more than 10 md−1 horizontally in the capped unsaturated zone. The analysis shows that both the amplitude and the length of the period of pressure variations are important for the transport of oxygen, and it shows that the combined effects determine the extent of a subsurface zone where atmospheric oxygen can reach.

Evaluation of prediction intervals for expressing uncertainties in groundwater flow model predictions

We tested the accuracy of 95% individual prediction intervals for hydraulic heads, streamflow gains, and effective transmissivities computed by groundwater models of two Danish aquifers. To compute the intervals, we assumed that each predicted value can be written as the sum of a computed dependent variable and a random error. Testing was accomplished by using a cross-validation method and by using new field measurements of hydraulic heads and transmissivities that were not used to develop or calibrate the models. The tested null hypotheses are that the coverage probability of the prediction intervals is not significantly smaller than the assumed probability (95%) and that each tail probability is not significantly different from the assumed probability (2.5%). In all cases tested, these hypotheses were accepted at the 5% level of significance. We therefore conclude that for the groundwater models of two real aquifers the individual prediction intervals appear to be accurate.

Evaluation of confidence intervals for a steady-state leaky aquifer model

Abstract The fact that dependent variables of groundwater models are generally nonlinear functions of model parameters is shown to be a potentially significant factor in calculating accurate confidence intervals for both model parameters and functions of the parameters, such as the values of dependent variables calculated by the model. The Lagrangian method of Vecchia and Cooley [Vecchia, A.V. & Cooley, R.L., Water Resources Research, 1987, 23(7), 1237–1250] was used to calculate nonlinear Scheffe-type confidence intervals for the parameters and the simulated heads of a steady-state groundwater flow model covering 450 km2 of a leaky aquifer. The nonlinear confidence intervals are compared to corresponding linear intervals. As suggested by the significant nonlinearity of the regression model, linear confidence intervals are often not accurate. The commonly made assumption that widths of linear confidence intervals always underestimate the actual (nonlinear) widths was not correct. Results show that nonlinear effects can cause the nonlinear intervals to be asymmetric and either larger or smaller than the linear approximations. Prior information on transmissivities helps reduce the size of the confidence intervals, with the most notable effects occurring for the parameters on which there is prior information and for head values in parameter zones for which there is prior information on the parameters.

Joint inversion of aquifer test, MRS, and TEM data

This paper presents two methods for joint inversion of aquifer test data, magnetic resonance sounding (MRS) data, and transient electromagnetic data acquired from a multilayer hydrogeological system. The link between the MRS model and the groundwater model is created by tying hydraulic conductivities (k) derived from MRS parameters to those of the groundwater model. Method 1 applies k estimated from MRS directly in the groundwater model, during the inversion. Method 2 on the other hand uses the petrophysical relation as a regularization constraint that only enforces k estimated for the groundwater model to be equal to MRS derived k to the extent that data can be fitted. Both methodologies can jointly calibrate parameters pertaining to the individual models as well as a parameter pertaining to the petrophysical relation. This allows the petrophysical relation to adapt to the local conditions during the inversion. The methods are tested using a synthetic data set as well as a field data set. In combination, the two case studies show that the joint methods can constrain the inversion to achieve estimates of k, decay times, and water contents for a leaky confined aquifer system. We show that the geophysical data can assist in determining otherwise insensitive k, and vice versa. Based on our experiments and results, we mainly advocate the future application of method 2 since this seems to produce the most reliable results, has a faster inversion runtime, and is applicable also for linking k of 3-D groundwater flow models to multiple MRS soundings.

Academic drift in Danish professional engineering education. Myth or reality? Opportunity or threat?

This article examines whether and, if so, to what extent academic drift can be said to have taken place in Danish professional engineering education. If the answer is affirmative, what were the driving forces behind it and what are the consequences – if any? First, a theoretical and conceptual framework for the discussion is introduced. This is followed by a case study of institutional change in higher education in Denmark, with a particular emphasis on two institutional examples, the two previously independent engineering colleges that recently merged with universities. The two examples are based on data gathered in interviews with teachers and management in these two engineering colleges (now both part of a university). In conclusion, based on the findings the questions posed in the title of this article are addressed and possible benefits and drawbacks of increased academisation of professional engineering education are discussed.

A synthetic study of geophysics-based modelling of groundwater flow in catchments with a buried valley

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.RésuméLes 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.ResumenLos 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。ResumoOs 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.

Epistemology, ontology and ethics: 'galaxies away from the engineering world'?

Philosophy of technology/philosophy of science has recently become part of the curriculum of engineering degree programmes in Denmark. However, to what extent do teachers of engineering see it as meaningful for students to work with relatively abstract philosophical concepts such as epistemology, ontology and ethics as part of engineering degree programmes? And what, if any, are the complexities and difficulties in implementing philosophical questioning into engineering curricula? Do teachers tend to see philosophy of science as a kind of ‘Trojan horse’ – an unwelcome idea that will defocus engineering degree courses and steal time from more important subjects? Or do they see it as a necessary and welcome addition to engineering curricula that will result in more qualified and free-thinking engineering graduates? Subsequently these issues are discussed in the light of findings in an empirical case study carried out by the authors at their Institute.

Seismic investigations of buried tunnel valleys on- and offshore Denmark

Abstract Tunnel valleys on- and offshore Denmark have been investigated based on a database of 1000 km two-dimensional (2D) onshore seismic data, 5600 km 2D offshore seismic data and 1200 km2 three-dimensional (3D) offshore seismic data. From the 2D data we identified 216 onshore and 674 offshore seismic tunnel valley intersections, and 55 individual valleys were identified from three 3D surveys. The majority of the valleys have depths ranging from 50 to 200 m and widths between 500 and 1500 m. Up to seven generations of tunnel valleys were identified, indicating repeated erosion and deposition within the study area. The valleys were most likely formed by subglacial meltwater erosion during the last three glaciations. Statistical analyses conducted on the data show that there are no significant differences between the onshore and offshore valleys with respect to their depth and shape; they share morphological and structural characteristics. The onshore seismic data have been analysed in conjunction with lithological information from boreholes. The analyses show that tunnel valley bottoms terminate equally commonly in substrates dominated by clay and sand, and that the valley shapes are similar for the two substrates.

Generation of 3‐D hydrostratigraphic zones from dense airborne electromagnetic data to assess groundwater model prediction error

We present a new methodology to combine spatially dense high-resolution airborne electromagnetic (AEM) data and sparse borehole information to construct multiple plausible geological structures using a stochastic approach. The method developed allows for quantification of the performance of groundwater models built from different geological realizations of structure. Multiple structural realizations are generated using geostatistical Monte Carlo simulations that treat sparse borehole lithological observations as hard data and dense geophysically derived structural probabilities as soft data. Each structural model is used to define 3-D hydrostratigraphical zones of a groundwater model, and the hydraulic parameter values of the zones are estimated by using nonlinear regression to fit hydrological data (hydraulic head and river discharge measurements). Use of the methodology is demonstrated for a synthetic domain having structures of categorical deposits consisting of sand, silt, or clay. It is shown that using dense AEM data with the methodology can significantly improve the estimated accuracy of the sediment distribution as compared to when borehole data are used alone. It is also shown that this use of AEM data can improve the predictive capability of a calibrated groundwater model that uses the geological structures as zones. However, such structural models will always contain errors because even with dense AEM data it is not possible to perfectly resolve the structures of a groundwater system. It is shown that when using such erroneous structures in a groundwater model, they can lead to biased parameter estimates and biased model predictions, therefore impairing the models predictive capability.

Climate change impacts on groundwater hydrology – where are the main uncertainties and can they be reduced?

ABSTRACT This paper assesses how various sources of uncertainty propagate through the uncertainty cascade from emission scenarios through climate models and hydrological models to impacts, with a particular focus on groundwater aspects from a number of coordinated studies in Denmark. Our results are similar to those from surface water studies showing that climate model uncertainty dominates the results for projections of climate change impacts on streamflow and groundwater heads. However, we found uncertainties related to geological conceptualization and hydrological model discretization to be dominant for projections of well field capture zones, while the climate model uncertainty here is of minor importance. How to reduce the uncertainties on climate change impact projections related to groundwater is discussed, with an emphasis on the potential for reducing climate model biases through the use of fully coupled climate–hydrology models. Editor D. Koutsoyiannis; Associate editor not assigned