Improving geological and process model integration through TIN to 3D grid conversion

The ability to extract properties from 3D geological framework models for use in the construction of conceptual and mathematical models is seen as increasingly important, however, tools and techniques are needed to support such information flows. Developing such methodologies will maximize the opportunity for information use and re-use, this is particularly important as the true value of such assets is not always known when they are first acquired. This paper briefly describes the cultural and technical challenges associated with the application of information derived from 3D geological framework models by hydrogeological process models. We examine how these issues are being addressed and present a tool, SurfGrid, which allows a user to generate 3D grids (voxels) of parameterized data from a series of geological surfaces. The procedures and tools described offer the ability to re-use expensively created assets by providing user friendly techniques that enable multidisciplinary scientists to extrapolate property distributions from geological models

A geological model of London and the Thames Valley, southeast England

Many geological survey organisations have started delivering digital geological models as part of their role. This article describes the British Geological Survey (BGS) model for London and the Thames Valley in southeast England. The model covers 4800 km2 and extends to several hundred metres depth. It includes extensive spreads of Quaternary river terraces and alluvium of the Thames drainage system resting on faulted and folded Palaeogene and Cretaceous bedrock strata. The model extends to the base of the Jurassic sedimentary rocks. The baseline datasets used and the uses and limitations of the model are given. The model has been used to generate grids for the elevation of the base of the Quaternary, the thickness of Quaternary deposits, and enabled a reassessment of the subcrop distribution and faulting of the Palaeogene and Cretaceous bedrock units especially beneath the Quaternary deposits. Digital outputs from the model include representations of geological surfaces, which can be used in GIS, CAD and geological modelling software, and also graphic depictions such as a fence diagram of cross-sections through the model. The model can be viewed as a whole, and be dissected, in the BGS Lithoframe Viewer. Spatial queries of this and other BGS models, at specific points, along defined lines or at a specified depth, can be performed with the new BGS Groundhog application, which delivers template-based reports. The model should be viewed as a first version that should be improved further, and kept up to date, as new data and understanding emerges

Improving Aquifer Characterization through Integration of Airborne Electromagnetics (AEM) and Well Hydrographs

The objective of this study is to evaluate methods of hydrostratigraphic modeling using geophysics and well hydrographs at the eastern edge of the High Plains aquifer (HPA) in Platte and Colfax counties within Nebraska, USA. The HPA is very heterogeneous in the study area, being hosted by architecturally complex glacial sediments and having many irregular hydraulic boundaries. Further, the HPA exhibits local variations between unconfined and confined conditions. Pumping in such bounded aquifers can be unsustainable because of cost increases and lost agricultural productivity. Moreover, the large drawdowns typical of confined aquifers can contribute to well interference during heavy pumping. Mapping the HPA accurately at small (10’s of km2 ) to medium (100’s of km2 ) scales is vital to sustainable management. AEM modeling and well hydrograph interpretation methods were used to characterize the aquifer in the study area. A 2016 airborne electromagnetic (AEM) survey mapped the electrical resistivity of subsurface strata to depths of 300 m. This data was used in the present study to create 3D hydrostratigraphic models using cognitive-layer modeling and voxel-based geostatistical modeling approaches, both with their own advantages and disadvantages. Water-level hydrographs from piezometers near irrigated fields provide the basis for aquifer characterization at each site and for assessing the accuracy of the two AEM modeling approaches, which are applied commonly in Nebraska and elsewhere. The temporal pattern of water-level drawdown indicated possible boundaries and confinement. The existence of background displacement, size of displacement, and responses of nearby wells led to aquifer interpretations. Little correlation existed between the hydrograph interpretations and both of the modeling approaches, but the voxel model did show boundaries near many of the irrigation wells with bounded hydrograph signatures. Overall, the simple modeling approaches failed to adequately convert resistivity to accurate interpretations of subsurface stratigraphy, rendering both types of hydrostratigraphic models largely invalid here. Nevertheless, the results of this study lead to important future work recommendations: (1) modeling and quantifying uncertainty using more sophisticated methods, (2) applying different modeling approaches in different areas to fit hydrologic data, and (3) using hydrograph data and pumping tests to validate the results of hydrostratigraphic modeling. Advisor: Jesse Koru

Under-representation of faults on geological maps of the London region: reasons, consequences and solutions

London lies mainly within an area of long-term tectonic stability known as the London Platform. This is characterised by relatively thin Cretaceous and Palaeogene sequences overlying Palaeozoic basement at shallow depths, less seismic activity than surrounding areas and, according to published geological maps, little faulting. However, observations of temporary exposures and borehole records, and other studies, show that in reality faults are numerous and widespread in the London region. Their relative absence on the geological maps is a consequence of past mapping methods, coupled with the relative uniformity of extensive bedrock units such as the London Clay Formation and the Chalk Group, and the widespread presence of Quaternary and anthropogenic deposits, and of urban development. However, complementary approaches to geological surveying, including the use of geophysical data and satellite-based radar interferometry, together with geological modelling in three dimensions using subsurface information, provide the means to accurately survey fault systems even in the most densely urbanised areas. Such work shows that earth movements in the London area, apparently including near-surface fault displacements, have taken place during the late Quaternary and continue at the present. These findings are important to civil engineering projects and hydrogeological studies in the London area and to understanding local tectonic development

Improving Aquifer Characterization through Integration of Airborne Electromagnetics (AEM) and Well Hydrographs

The objective of this study is to evaluate methods of hydrostratigraphic modeling using geophysics and well hydrographs at the eastern edge of the High Plains aquifer (HPA) in Platte and Colfax counties within Nebraska, USA. The HPA is very heterogeneous in the study area, being hosted by architecturally complex glacial sediments and having many irregular hydraulic boundaries. Further, the HPA exhibits local variations between unconfined and confined conditions. Pumping in such bounded aquifers can be unsustainable because of cost increases and lost agricultural productivity. Moreover, the large drawdowns typical of confined aquifers can contribute to well interference during heavy pumping. Mapping the HPA accurately at small (10’s of km2 ) to medium (100’s of km2 ) scales is vital to sustainable management. AEM modeling and well hydrograph interpretation methods were used to characterize the aquifer in the study area. A 2016 airborne electromagnetic (AEM) survey mapped the electrical resistivity of subsurface strata to depths of 300 m. This data was used in the present study to create 3D hydrostratigraphic models using cognitive-layer modeling and voxel-based geostatistical modeling approaches, both with their own advantages and disadvantages. Water-level hydrographs from piezometers near irrigated fields provide the basis for aquifer characterization at each site and for assessing the accuracy of the two AEM modeling approaches, which are applied commonly in Nebraska and elsewhere. The temporal pattern of water-level drawdown indicated possible boundaries and confinement. The existence of background displacement, size of displacement, and responses of nearby wells led to aquifer interpretations. Little correlation existed between the hydrograph interpretations and both of the modeling approaches, but the voxel model did show boundaries near many of the irrigation wells with bounded hydrograph signatures. Overall, the simple modeling approaches failed to adequately convert resistivity to accurate interpretations of subsurface stratigraphy, rendering both types of hydrostratigraphic models largely invalid here. Nevertheless, the results of this study lead to important future work recommendations: (1) modeling and quantifying uncertainty using more sophisticated methods, (2) applying different modeling approaches in different areas to fit hydrologic data, and (3) using hydrograph data and pumping tests to validate the results of hydrostratigraphic modeling. Advisor: Jesse Koru

Integrating deterministic lithostratigraphic models in stochastic realizations of subsurface heterogeneity. Impact on predictions of lithology, hydraulic heads and groundwater fluxes

Realistic representations of geological complexity are important to address several engineering and environmental challenges. The spatial distribution of properties controlling physical and geochemical processes can be effectively described by the geological structure of the subsurface. In this work, we present an approach to account for geological structure in geostatistical simulations of categorical variables. The approach is based on the extraction of information from a deterministic conceptualization of the subsurface, which is then used in the geostatistical analysis for the development of models of spatial correlation and as soft conditioning data. The approach was tested to simulate the distribution of four lithofacies in highly heterolithic Quaternary deposits. A transition probability-based stochastic model was implemented using hard borehole data and soft data extracted from a 3-D deterministic lithostratigraphic model. Simulated lithofacies distributions were also used as input in a flow model for numerical simulation of hydraulic head and groundwater flux. The outputs from these models were compared to corresponding values from models based exclusively on borehole data. Results show that soft lithostratigraphic information increases the accuracy and reduces the uncertainty of these predictions. The representation of the geological structure also allows a more precise definition of the spatial distribution of prediction uncertainty, here quantified with a metric based on Shannon information entropy. Correlations between prediction uncertainties for lithofacies, hydraulic heads and groundwater fluxes were also investigated. The results from this analysis provide useful insights about the incorporation of soft geological data into stochastic realizations of subsurface heterogeneity, and emphasize the critical importance of this type of information for reducing the uncertainty of simulations considering flux-dependent processes

Anomalous buried hollows in London: development of a hazard susceptibility map

Engineering works carried out in central London over many decades have revealed a number of buried hollows that exhibit curious characteristics. Some extend deep into the bedrock geology and are in-filled with disturbed superficial deposits and reworked bedrock. Others are contained within the superficial deposits. They can be up to 500 m wide and more than 60 m in depth. As the infill material often has different behavioural characteristics from the surrounding deposits failure to identify them during an initial site investigation can prove costly. This paper considers their common characteristics and describes the method used to develop a buried hollow hazard susceptibility map. This map provides planners with a broader awareness of the potential location of difficult ground conditions associated with them, thereby reducing the potential for unforeseen ground conditions through effective site investigation design. The paper continues with a discussion of some of the likely processes associated with their formation, which are attributed to cryogenic processes, and concludes with potential future research directions

Drift-filled hollows in the London basin: characteristics, origins and assessment of risk

This thesis was submitted for the award of Doctor of Philosophy and was awarded by Brunel University LondonAnomalous geology creates sub-surface risk to engineering projects. Within the London Basin, anomalous geological depressions, named drift-filled hollows (DFHs) have been identified and inducing sub-surface risk for the last century. DFHs are depressions identified at an anomalous depth to the local bedrock, infilled with largely unconsolidated sediment and occasionally local bedrock. The features range in size from 5-500 m wide and up to 75 m deep, narrowing with depth. Not visible from ground level, DFHs are often missed during site investigations which induces sub-surface risk on construction projects. Not addressing the features if and once encountered could lead to failure through differential subsidence, tunnel face collapse, impact on nationally-significant infrastructure, excessive and unplanned costs, damage to groundwater resources and loss of life. This thesis compiles information to create the largest DFH dataset of 89 features and their known characteristics. A multi-disciplinary approach is employed to further understanding of the physical characteristics of DFHs as well as to use an evidence-based approach to determine their origin and processes which have shaped their current form. The results illustrate the high level of variability between features and within a single feature. Data quality restrictions and methodological limitations are discussed in detail throughout. These limitations are then amplified with highly variable sediments, such as the infill of DFHs. Further outcomes of the research include furthering the understanding of London’s geology, not solely through identifying further anomalies, but through mapping the sub-surface of two differing areas and showing notable differences in depth to bedrock and presence and absence of strata within relatively short distances. A guide for the site investigation sector has also been created to readily identify and characterise DFHs and so reduce risk. All of the outcomes reduce sub-surface risk through an increased knowledge of the features and London’s geology as a whole. Finally, the enlarged dataset and the recognition of the high level of variability within a single feature and between features has enabled the existing and potential process hypotheses to be analysed and a new formation hypothesis is proposed for features with a diapir being formed through an increase of water and gas pressure

Major faulting in London: relating tectonic-scale processes to site-scale engineering geology

Critical infrastructure projects in London increasingly encounter unexpected ground conditions caused by major faults in the near surface geology. Their presence challenges the traditional interpretation of minimal faulting, which stems from limited exposure and structural information, and historical misinterpretation. These major faults are now recognised as both a geological unknown and a geotechnical risk since their origins, extents and architectures are poorly constrained. This research investigates major faults in London to characterise them structurally and determine their engineering geological impact. Direct observations and indirect evidence are coupled to overcome information deficiencies, provide fault analyses, generate geological models, map faults indirectly, and assess fault reactivation and inheritance mechanisms. This investigation shows that Alpine reactivation of basement Variscan and post-Variscan faults caused propagation into overlying Late Cretaceous-Palaeogene cover through a series of en échelon Riedel shears and reversed faults. Fault compartmentalisation offset the ground into blocks and subtly influenced certain sedimentological and ongoing hydrogeological processes. Significant transpressive and transtensive shear zones developed where major faults interacted in the cover through linkage and/or confined block shearing. This research demonstrates that the London Basin is a structurally complex product of Alpine intraplate tectonism. The region is redefined here to reflect the differing Alpine responses of underlying basement domains, with the Variscan Front repositioned northward in light of London’s Variscan fault network. The identified local tectonism and regional partitioning cause lithological, mechanical, and hydrogeological properties to vary at the site-scale. Consequently, major faults inconsistently affect the ground across London to generate often unique local engineering geological impacts. Both a ground investigation workflow and fault zone categorisation criterion are proposed to improve their identification and engineering geology characterisation. This research has revealed how Alpine-triggered widespread but locally complex fault propagation in London has contributed to subsurface geological complexity to the detriment of ground conditions.Open Acces