Ingelise Møller

Integrated management and utilization of hydrogeophysical data on a national scale

Development of more time-efficient and airborne geophysical data acquisition systems during the past decades have made large-scale mapping attractive and affordable in the planning and administration of e.g., groundwater resources or raw material deposits. The handling and optimized use of large geophysical data sets covering large geographic areas requires a system that allows data to be easily stored, extracted, interpreted, combined and used one time after another with different purposes. Such an integrated system for management and utilization of hydrogeophysical data on a national scale has been developed during the past decade in Denmark. This data handling system includes a comprehensive national geophysical data base (the GERDA data base), a national data base for borehole information (the Jupiter data base), a program package for processing, interpretation and visualization of electrical and electromagnetic data as well as preparation of these data for upload to the geophysical data base (the Aarhus Workbench) and finally a 3D visualization and modelling tool used for geological modelling and data quality control. The Aarhus Workbench program package allows visualization and analysis of subsets of data from the geophysical data base, which may include data from many individual mapping campaigns. The 3D visualization and modelling tool uses data from the geophysical and the borehole data bases directly; moreover, it handles maps and grids produced in the Aarhus Workbench. The integrated system for management of hydrogeophysical data allows management of large amounts of data collected over several years in different mapping campaigns, of different consultant companies and with different geophysical methods and instrumentation. It is now used by all partners involved in the groundwater mapping in Denmark. The system promotes reuse of geophysical data and models in future mapping projects, as well as easing and promoting the use of geophysical data in the geological modelling. The integrated system secures transfer of documentation all the way from data acquisition over processing and inversion of the geophysical data to geological modelling through storage of data acquisition parameters, data processing parameters, inversion parameters and uncertainties on data and models in the geophysical data base. The benefits of the large amount of geophysical data gathered in the national geophysical data base and utilized by the two program packages are invaluable for all future groundwater planning and administration.

Multichannel deconvolution, MCD, in geophysics and helioseismology

Abstract Whenever data sampling is regular along a coordinate in space and/or time, it is relevant to look for approximate shift invariance which casts the forward problem into a convolution formulation. The resulting computations may be speeded up significantly through the Fourier transform. For nonlinear problems the Born approximation in horizontally stratified media leads to just such a result. In many cases the noise is well approximated by a stationary process, and it turns out that the resulting inverse solution is then a multi-channel deconvolution. This formulation allows very fast inversion in the periodic approximation of densely sampled high volume data sets. New applications within geophysical well logging, continuous geoelectrical sounding/profiling, and 3D helioseismic tomography demonstrate the wide applicability of this method.

Rapid inversion of 2-D geoelectrical data by multichannel deconvolution

Modern geoelectrical data acquisition systems can record more than 100 000 data values per field day. Despite the growth in computer power and the development of more efficient numerical algorithms, interpreting such data volumes remains a nontrivial computational task. We present a 2-D one-pass inversion procedure formulated as a multichannel deconvolution. It is based on the equation for the electrical potential linearized under the Born approximation, and it makes use of the 2-D form of the Fr ´ echet derivatives evaluated for the homogeneous half-space. The inversion is formulated in the wavenumber domain so that the 2-D spatial problem decouples into many small 1-D problems. The resulting multichannel deconvolution algorithm is very fast and memory efficient. The inversion scheme is stabilized through covariance matrices representing the stochastic properties of the earth resistivity and data errors. The earth resistivity distribution is assumed to have the statistical characteristics of a two-parameter, selfaffine fractal. The local apparent amplitude and fractal dimension of the earth resistivity are estimated directly from geoelectrical observations. A nonlinearity error covariance matrix is added to the conventional measurement error covariance matrix. The stochastic model for the dependence of nonlinearity error on electrode configuration as well as resistivity amplitude and fractal dimension is determined pragmatically through nonlinear simulation experiments. Tests on synthetic examples and field cases including well control support the conclusion that for long data profiles this method automatically produces linearized resistivity estimates which faithfully resolve the main model features.

Architecture of an Upper Jurassic barrier island sandstone reservoir, Danish Central Graben: implications of a Holocene-Recent analogue from the Wadden Sea

Abstract An unusually thick ( c . 88 m), transgressive barrier island and shoreface sandstone succession characterizes the Upper Jurassic Heno Formation reservoir of the Freja oil field situated on the boundary of Denmark and Norway. The development and preservation of such thick transgressive barrier island sands is puzzling since a barrier island typically migrates landwards during transgression and only a thin succession of back-barrier and shoreface sands is preserved. Investigation of the development and geometry of the Freja reservoir sandstones is problematic since the reservoir is buried c. 5 km and seismic resolution is inadequate for architectural analysis. Description of the reservoir sandstone bodies is thus based on sedimentological interpretation and correlation of seven wells, of which five were cored. Palaeotopography played a major role in the position and preservation of the thick reservoir sandstones. Using the nearest maximum flooding surface above the reservoir as a datum for well-log correlations, the base of the barrier island succession in the wells is reconstructed as a surface with steep, seaward-dipping palaeotopography. The relief is c . 270 m over a distance of c . 8 km and dips WNW. As a complementary approach to investigation of the reservoir architecture, a Holocene–Recent barrier island system in the Danish part of the NW European Wadden Sea has been studied and used as an analogue. The barrier island of Romo developed during a relative sea-level rise of c. 15 m during the last c. 8000 years and is up to 20 m thick. To unravel the internal 3D facies architecture of the island, an extensive ground penetrating radar (GPR) survey of 35 km line length and seven cores, c. 25 m long, was obtained. Although the barrier island experienced a rapid relative sea-level rise, sedimentation kept pace such that the island aggraded and even prograded seawards and became wider and longer due to the large surplus of sand.

Testing ground-penetrating radar for resolving facies architecture changes - a radar stratigraphic and sedimentological analysis along a 30 km profile on the Karup Outwash Plain, Denmark

Knowledge of the spatial distribution of lithofacies, obtained through detailed modelling of the movement of water and solutes, may improve precise predictions of an area’s vulnerability towards contaminants. This calls for information on lithofacies variability on several scales. The possibilities for upscaling lithofacies distribution from excavation scale to field scale or to an even larger scale with the use of ground-penetrating radar (GPR) is tested on one of the largest outwash plainsin Denmark: the Karup Outwash plain in the western part of Jutland. The testing involves GPR surveys at 16 locations along a 30 km long profile following the ancient mean flow direction of the braided-river system, as well as combined detailed sedimentological investigations at selected locations. The GPR testing is divided into several phases of which this paper reports on the first two. In the first phase, GPR surveys and sedimentary investigations at gravel-pits and excavation sites are carried out at the same location with the aim of determining whether lithofacies can be related to GPR reflection sequences either directly or through facies architecture models. Studies of outcropping glaciofluvial deposits at a gravel-pit and a shallow excavation in the proximal and intermediate parts of the outwash plain, respectively, show that the sand-dominated outwash plain typically contains lithofacies types with bed thicknesses of up to 0.5 m and successions of beds of thicknesses upto 1 m. GPR surveys carried out at the same locations show that the observed lithofacies types typically have dimensions that, in a vertical direction, are the same as or less than the resolution obtained by a 200 MHz GPR system. In such sand-dominated outwash deposits, the GPR reflections may more often be caused by the bedding structures in successions of beds and of larger erosion surfaces separating different lithofacies, rather than by the lithofacies boundaries themselves. In the second phase, tests are conducted to determine whether facies architecture changes along the ancient mean flow direction in the outwash plain can be recognized using radar stratigraphic analysis. Radar stratigraphic analysis, so far limited to two GPR sections orientated parallel and perpendicular to the ancient mean flow direction at 15 locations, reveals a systematic change in the pattern of radar sequence boundaries and the occurrence of radar facies. The changes in radar sequences seem to reflect changes in lithology and sedimentary structures related to the overall fall in the depositional energy from the proximal to the distal part of the outwash plain. The results clearly indicate that radar stratigraphic analysis reveals realistic facies architecture models for the outwash plain, provided densely spaced GPR data are acquired and locally calibrated against excavations and coreholes. It is concluded that GPR can be used for upscaling lithofacies distributions in sedimentological settings like the present Karup Outwash Plain.

Nitrate vulnerability assessment of aquifers

Protection of groundwater against nitrate has a high priority in a country like Denmark with intensive agricultural production and with drinking water production based on groundwater. This paper presents a Site-specific Concept for Aquifer Nitrate Vulnerability Assessment (SCANVA) aimed at protecting groundwater resources. SCANVA is a qualitative dynamic concept adjusted to the specific study area depending on the hydrogeological and geochemical conditions. It comprises a synthesis of very detailed site-specific geological, geophysical, hydrogeological and chemical data, and a definition of nitrate vulnerability founded on redox conditions in the ground. Data are used to interpret the geological architecture of the subsurface, the groundwater flow and the geochemical groundwater conditions. These interpretations are integrated to assess nitrate vulnerability of the aquifers in three dimensions and identifying nitrate-vulnerable zones. The paper demonstrates the practical use of the concept in a Danish intensive agricultural area with a glacially formed landscape. Glaciotectonic has led to a high geological heterogeneity and very complex composition of thrusted and folded layers, which to a large extent affect the nitrate vulnerability of the aquifers. SCANVA can be directly applied to specific hydrogeological conditions anywhere with intensive N loss from agriculture, a groundwater-based drinking water supply, nitrate reduction in the ground and glacially dominated landscapes and deposits. Potentially, the concept might also be adapted to other specific substances, dominating geochemical processes, and geological settings.

Contributions to uncertainty related to hydrostratigraphic modeling using Multiple-Point Statistics

Forecasting the flow of groundwater requires a hydrostratigraphic model, which describes the architecture of the subsurface. State-of-the-art multiple-point statistical (MPS) tools are readily available for creating models depicting subsurface geology. We present a study of the impact of key parameters related to stochastic MPS simulation of a real-world hydrogeophysical dataset from Kasted, Denmark, using the snesim algorithm. The goal is to study how changes to the underlying datasets propagate into the hydrostratigraphic realizations when using MPS for stochastic modeling. This study focuses on the sensitivity of the MPS realizations to the geophysical soft data, borehole lithology logs, and the training image (TI). The modeling approach used in this paper utilizes a cognitive geological model as a TI to simulate ensemble hydrostratigraphic models. The target model contains three overall hydrostratigraphic categories, and the MPS realizations are compared visually as well as quantitatively using mathematical measures of similarity. The quantitative similarity analysis is carried out exhaustively, and realizations are compared with each other as well as with the cognitive geological model. The results underline the importance of geophysical data for constraining MPS simulations. Relying only on borehole data and the conceptual geology, or TI, results in a significant increase in realization uncertainty. The airborne transient electromagnetic SkyTEM data used in this study cover a large portion of the Kasted model area and are essential to the hydrostratigraphic architecture. On the other hand, the borehole lithology logs are sparser, and 410 boreholes were present in this study. The borehole lithology logs infer local changes in the immediate vicinity of the boreholes, thus, in areas with a high degree of geological heterogeneity, boreholes only provide limited large-scale structural information. Lithological information is, however, important for the interpretation of the geophysical responses. The importance of the TI was also studied. An example was presented where an alternative geological model from a neighboring area was used to simulate hydrostratigraphic models. It was shown that as long as the geological settings are similar in nature, the realizations, although different, still reflect the hydrostratigraphic architecture. If a TI containing a biased geological conceptualization is used, the resulting realizations will resemble the TI and contain less structure in particular areas, where the soft data show almost even probability to two or all three of the hydrostratigraphic units.

Compilation of a Resistivity Atlas of Danish lithologies based on direct resistivity measurements and wireline logging data

Electrical conductivity, or its inverse, the resistivity, is an important geophysical property within groundwater mapping. It is known to correlate empirically to lithology, primarily through clay minerals and pore water ions. Although, in Denmark, geoelectric and electromagnetic surveys have been carried out for decades, no systematic, nationwide study on the relationship between resistivity and lithology has been carried out. We present a procedure for generating a resistivity atlas based on resistivity measurements, which can be related directly to specific and well described soil samples. Data are obtained from archives, literature and the Danish national databases. The procedure implies a restricted use of wireline logging data in combination with direct measurements on samples, resulting in resistivity distributions for specific lithologies or geological formations. The use of documented high-quality data ensures reliable results, reflecting actual resistivity of a specific lithology. This procedure is illustrated on clay till. The resistivity variations obtained for this lithology seems to be related to real compositional variations, which reflect the process of forming the clay till. Our procedure is likely to provide equally reliable results for other main lithologies. Future detailed studies, in particular on sediments with low clay content, should consider resistivity differences related to the degree of saturation and variations in the formation water resistivity.

Geoelectrical mapping for improved performance of SUDS in clay tills

Many cities of the Northern Hemisphere are covered by low permeable clay tills, posing a challenge for stormwater infiltration practices. However, clay tills range amongst the most heterogeneous types of sediments and infiltration rates can vary by several orders of magnitude. This study evaluates if a 2D geoelectrical system can reveal such heterogeneity at field scale and thus be used to optimize the hydraulic performance of stormwater runoff infiltration systems. The assessment is based on a field study where data from non-invasive geoelectrical data are compared with data from invasive geological methods, including borehole soil sample descriptions, one excavation description and a near-surface spear auger-mapping project. The experiments returned a significant correlation of geoelectrical and spear auger-mapped surface sediments. Furthermore, a highly permeable oxidized fracture zone in greater depths could be revealed on the 2D geoelectrical profiles. The successful determination of highly permeable zones harbors potential for improved hydraulic performance of infiltration SUDS (sustainable urban drainage systems).

Geophysics‐Based Contaminant Mass Discharge Quantification Downgradient of a Landfill and a Former Pharmaceutical Factory

Contaminant mass discharge is a commonly applied tool to evaluate the environmental impact of contaminated sites on water resources. At large contaminated sites with heterogeneous sources, such as landfills, the number of wells available is often not sufficient, leading to a high uncertainty of mass discharge estimates. In this study, we tackle the uncertainty of the contaminant mass discharge due to low sampling densities by interpolating limited water-sample data with the support of surface direct current resistivity and induced polarization geophysical data. The method relies on finding a conceptual link between the bulk conductivity imaged from geophysics and the contaminant concentrations. We investigate the link between (1) imaged bulk and electrical water conductivity, (2) water conductivity and conservative ionic species, (3) water conductivity and redox-sensitive species, (4) water conductivity and semipersistent organic species, and (5) water conductivity and biodegradable organic compounds. The method successfully identify similarities between the distribution of the bulk conductivity and chloride and pharmaceutical compounds in a landfill leachate plume and between the bulk conductivity data and benzene and chlorinated ethenes for a contaminant plume from a former pharmaceutical factory. Contaminant concentrations were interpolated through regression kriging, using geophysical data as the dependent variable. The distribution of concentration determined with the novel method showed a lower mean relative estimation error than the traditional method of kriging only contaminant concentration data. At large sites, the method can improve contaminant mass discharge estimates, especially if surface geophysical measurements are integrated in the site investigation at an early stage.