bluuebunny/crossref_metadata_2025_split huggingface dataset:
Spatial mapping of submerged cave systems by means of airborne electromagnetics: an emerging technology to support protection of endangered karst aquifers
Authors: Supper Robert , Motschka Klaus , Ahl Andreas , Bauer‐Gottwein Peter , Gondwe Bibi , Alonso Gonzalo Merediz , Römer Alexander , Ottowitz David , Kinzelbach Wolfgang 
Published: September 2009

ABSTRACTKarst aquifers represent important but very vulnerable sources for water supply to a significant part of the Earth’s population. For sustainable use of these resources, development of integrated management tools based on numerical groundwater models is required. In principle karst aquifers are characterized by the presence of two distinct flow domains: the limestone matrix fractures and the conduits. A flow model of karst aquifers requires detailed, spatially distributed information on the hydrologic characteristics of the aquifer and flow paths. Geophysical methods determining the distribution of the electrical resistivities within the subsurface could provide such information. An international scientific research project was initiated to explore the potential of airborne electromagnetic mapping for providing such innovative information for improving groundwater modelling of karst aquifers. The project study area is located in the Sian Ka’an Biosphere Reserve located in Yucatán, Mexico, a coastal wetland of international importance. As a first step ground geoelectric and ground electromagnetic measurements were performed in March 2006 to determine the electrical properties of the Sian Ka’an Biosphere Reserve subsurface environment. These results were used for 3D forward modelling to calculate the expected airborne electromagnetic response. Based on these promising results, an airborne pilot survey was performed in 2007 to evaluate the applicability of airborne electromagnetic methodology. This survey covers an area of 40 square kilometres above the well‐mapped Ox Bel Ha cave system. The results showed that the signature of the cave system could be clearly detected. The pilot survey offered as well the chance to define the limits of current state‐of‐the‐art airborne data acquisition and inversion. The study helped to define the needs for further developments and improvements to establish the frequency domain electromagnetic method as a practical karst exploration method.
Effective medium theories for modelling the relationships between electromagnetic properties and hydrological variables in geomaterials: a review
Authors: Cosenza P. , Ghorbani A. , Camerlynck C. , Rejiba F. , Guérin R. , Tabbagh A. 
Published: September 2009

ABSTRACTThe paper reviews the effective medium theories used for modelling the relationships between electromagnetic properties (low‐frequency conductivity and high‐frequency permittivity) and hydrological variables (water content, salinity, suction, permeability) in soils and rocks. It aims a) to provide a simple presentation of these theoretical approaches, b) to present their theoretical and practical limitations and c) to establish some connections with empirical equations usually used in hydrogeophysics (i.e., Archie relationships, Topp equation and complex refractive index model). This review demonstrates that two groups of effective medium theories can be clearly identified. The first group constituted by the Maxwell‐Wagner and Symmetric‐Bruggeman rules is characterized by easy‐to‐use models. When the volumetric water content and the texture are known, they allow to obtain fast estimates of effective electromagnetic properties. In the second group, the differential effective medium schemes that are more complex from a mathematical point of view are preferred when frequency‐dependent properties are studied. In particular, differential effective medium schemes are used for providing some insights into the physical basis of spectral induced polarization measurements in hydrogeophysical applications.
Simultaneous inversion of magnetic resonance sounding in terms of water content, resistivity and decay times
Authors: Braun Martina , Kamm Jochen , Yaramanci Ugur 
Published: September 2009

ABSTRACTMagnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for direct groundwater exploration and for an improved aquifer characterization. Currently, it is the only geophysical method that is capable of directly determining the free water content and estimating the pore sizes of the aquifer in the subsurface. However, MRS is basically an electromagnetic method. Therefore, it is sensitive to the resistivity of the subsurface. The water content is the main target of investigation, therefore first inversion routines focused on the water content. Later on, inversion routines determining water content and decay times became available. Very recently, MRS inversion for water content and resistivity has been realized.We present here a simultaneous inversion of MRS in terms of determining the three inversion parameters – water content, resistivity and decay time – within one single inversion routine. Within the iterative inversion scheme, the extrapolated initial values are determined on the basis of the physical effective decay times in the subsurface, which are estimated within the inversion scheme.Due to an instrumental dead time, the initial values for amplitude and phase, which are related to water content and resistivity, cannot be measured directly. Therefore, the initial amplitude must be extrapolated using the decay time of the signal. The standard approach is a mono‐exponential decay curve; implicitly, the phase is assumed to be time‐invariant. However, multi‐exponential signals are natural when considering relaxation behaviour in the underground. It originates from multi‐modal pore size distributions or simply a number of differently relaxing signal contributions from the various lithological units.
Characterization of a coastal aquifer using seismic and geoelectric borehole methods
Authors: Maurer Hansraedi , Friedel Sven , Jaeggi David 
Published: September 2009

ABSTRACTTo characterize a coastal aquifer in southern Mallorca (Spain), we have acquired geoelectric and seismic data. The area of investigation lies within a Miocene reefal carbonate platform consisting of lagoon sediments, reef cores and talus sediments. Since the groundwater table was at about 40 m depth , the delineation of small‐scale structural features (1–2 m) within the aquifer required the use of borehole methods. Geoelectric single‐hole tomography was performed in six deep boreholes. This fast and inexpensive technique provided horizontal resolution well beyond the limits of traditional logging techniques. Furthermore, the tomographic inversion results remained virtually unaffected by the insertion of slotted PVC casing tubes and comparisons with electromagnetic induction logs showed an excellent match of resistivities in regions near to the boreholes. In addition to the single‐hole measurements, geoelectric and seismic cross‐hole data were acquired. Combined analyses of all data sets showed the reef core to be particularly susceptible to karstification and recrystallization. These two processes have caused substantial variations in the porosity. Comparison of fluid logs and electrical resistivity versus depth and seismic velocity versus depth functions along all boreholes revealed a surprisingly high correspondence. This unexpected correlation of electrical resistivity (governed by the fluid salinity) and seismic velocities can be explained by geochemical processes occurring in the saltwater‐freshwater mixing zone and/or near the groundwater table. Such processes need to be considered for establishing reliable ground‐water flow models.
Ultra‐shallow imaging using 3D seismic‐reflection methods
Authors: Sloan Steven D. , Steeples Don W. , Tsoflias Georgios P. 
Published: September 2009

ABSTRACTThree‐dimensional ultra‐shallow seismic reflection methods were used to image multiple reflectors less than 20 m deep, including the top of saturated zone, a paleo‐channel feature and the top of bedrock at a field site located near Lawrence, Kansas. A small 3D demonstration survey was designed and acquired using source and receiver intervals of 0.5 m and source line and receiver line intervals of 2 m. The survey had a nominal fold of 48 and covered an area of ~15.5 m × 35.5 m. Large variations in velocity were present, ranging from ~300–600 m/s laterally in the shallowest layer and ~300–1600 m/s vertically, a degree of variability that is not uncommon in the ultra‐shallow subsurface. Normal moveout corrections cannot account for intersecting reflection hyperbolae such as those caused by large vertical velocity gradients, so a novel processing scheme was applied by extracting offset‐dependent subsets based on the optimum window for each reflection. The subsets were normal moveout corrected independently and then stacked together using conventional 3D processing techniques. Despite the large lateral and vertical velocity variations, we were successful in imaging the top of the saturated zone, paleo‐channel features and the overburden‐bedrock interface located at depths of ~5 m, 8 m and 14 m, respectively. Results of the 3D survey are in agreement with previous studies conducted at the site. The methods presented here could be applied to situations where 3D imaging of the ultra‐shallow subsurface is necessary and may help to identify possible contaminant sinks or flowpaths.
Photometric flowmeter for measuring slow vertical water flow in wells
Authors: Urík J. , Mareš S. , Kobr M. , Lukeš J. , Řezanina S. 
Published: September 2009

ABSTRACTNew construction of a high resolution borehole flowmeter is presented. The logging tool is equipped with an injector enabling to create in the streaming water a colour mark using the food dye Brilliant Blue (E133) as a tracer. A pair of photometric sensors is located in the vertical distance 0.05 m and 0.1 m above and below the special outlet nozzle of the injector. The probe is capable of measuring vertical flow velocities in the range 10−4–10−2 m/s or vertical flow rates in the range 0.001– 0.1 l/s. The proper function and the reliability of the logging tool were checked in the model of the well under laboratory conditions and in two boreholes with different flow rate patterns, one with a very low flow rate upwards and the other with a very low flow rate downwards.
Characterizing lakebed seepage and geologic heterogeneity using resistivity imaging and temperature measurements
Authors: Nyquist Jonathan E. , Heaney Matthew J. , Toran Laura 
Published: September 2009

ABSTRACTThe contribution of groundwater‐surface water exchange to lake budgets is poorly understood and depends in part on lakebed heterogeneities. These heterogeneities are difficult to characterize using traditional point sampling methods. The goal of this project was to use electrical resistivity to identify potential zones of groundwater discharge and recharge, providing focus for point measurements. Multiple resistivity surveys were conducted at Lake Lacawac, a small, glacially‐formed lake in northeastern Pennsylvania. Two types of resistivity surveys were conducted. In a continuous resistivity profile, a multi‐electrode cable was towed parallel to shore to look for spatial variability in resistivity around the lake. Two parallel to shore surveys were conducted, an inner and outer loop, to help characterize the lateral extent of sediment types. The results of these surveys suggested lithology changes both along the shoreline and with distance from shore. Follow‐up resistivity data were collected using cables laid along the lake bottom perpendicular to the shoreline to look for finer scale zonation that affects seepage as a function of distance from shore.Follow‐up seepage measurements showed that seepage rates are very low, which is consistent with the resistivity data from which we concluded that most of Lake Lacawac is lined with glacial clay and that the lake is essentially perched above the groundwater flow system, with just minor amounts of seepage in a few locations where fingers of sandy sediments extend a short distance from the shore. Discontinuities in these patches of transmissive sediments can result in reversals in the direction of seepage at nearby locations.We conclude that towed resistivity is useful as a rapid reconnaissance tool for mapping geologic heterogeneity. The results can be used to guide the more time‐consuming but higher‐resolution, lake bottom resistivity measurements, which in turn can guide the placement of seepage meters.
Embankment dam seepage evaluation from resistivity monitoring data
Authors: Sjödahl P. , Dahlin T. , Johansson S. 
Published: September 2009

ABSTRACTMethods for monitoring seepage are important for the safety of embankment dams. Increased seepage may be associated with internal erosion in the dam and internal erosion is one of the main reasons for dam failures. Internal erosion progresses inside the dam and is difficult to detect by conventional methods. Therefore there is a need for new or improved methods. The resistivity method is a nondestructive method that may accomplish this task. It has been tried in an on‐going research programme in Sweden. Daily resistivity measurements are carried out on permanent installations on two Swedish embankment dams. In this paper the installations on the Sädva embankment dam are described and selected parts of the results are presented. In addition, a method for evaluating the seepage from resistivity monitoring data is theoretically described and tested for four selected areas in the foundation of the Sädva dam. Seasonal resistivity variations are apparent in the reservoir as well as inside the dam. Most parts of the dam have a homogeneous resistivity distribution with consistent variations. The overall status of the dam is satisfactory. However part of the foundation demonstrates a slightly different behaviour pattern with regard to the seasonal variation. The four selected areas represent localities with low, intermediate and high variations in seasonal resistivity. The areas are compared qualitatively and thereby permeable zones within the dam may be identified. Quantitative assessment of the seepage flow is also carried out as an initial test of the described method. It is concluded that the experiences from the Sädva dam are valuable with regard to the use of the resistivity method on embankment dams. Resistivity monitoring data may be used to qualitatively assess the seepage through the dam. For quantitative assessment, the method is promising and the data from the Sädva dam constitute an interesting initial approach. However, many assumptions and simplifications are made and more work on refining the method is needed.
Characterization of seawater intrusion using 2D electrical imaging
Authors: Nguyen F. , Kemna A. , Antonsson A. , Engesgaard P. , Kuras O. , Ogilvy R. , Gisbert J. , Jorreto S. , Pulido‐Bosch A. 
Published: September 2009

ABSTRACTWe have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater‐freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in‐situ and laboratory‐derived petrophysical relationships. Surface‐to‐hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface‐to‐hole images are not easily interpretable in terms of larger‐scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non‐stationarity of the petrophysical relationship during the imaging process.
Evidence for composite hydraulic architecture in an active fault system based on 3D seismic reflection, time‐domain electromagnetics and temperature data
Authors: Hess Scott , Fairley Jerry P. , Bradford John , Lyle Mitch , Clement William 
Published: September 2009

ABSTRACTFault hydrology is a topic of scientific and practical importance but considerable uncertainty exists regarding the nature of structural controls on fluid flow. Here we use seismic reflection and time‐domain electromagnetic data to develop a three‐dimensional model of hydraulic architecture in a predominantly dip‐slip normal fault system and we predict the architectural elements based on subsurface fluid flow patterns inferred from near‐surface temperature measurements. Our observations indicate the presence of high‐permeability flow paths parallel to fault planes in poorly‐lithified sediments. These results are best explained using a combination of elements from commonly accepted conceptual models of fault architecture, a finding that exhibits the heterogeneous nature of the geologic materials comprising the site. These insights may be useful as a guide to future studies of active fault systems, where multiple‐mode investigations (geophysical, hydrologic, thermal, geochemical) will be required to better understand subsurface fluid/fault interactions.
Numerical modelling of the IP effect at the pore scale
Authors: Blaschek R. , Hördt A. 
Published: September 2009

ABSTRACTGeophysicists have tried for a long time to correlate induced polarization data with parameters of the pore geometry, often with the overall aim to estimate the important parameter of hydraulic conductivity. However, no correlation has been found so far that is applicable to more than just a few special cases. Using empirical relationships and equivalent circuits often neglects the description of the processes in the pore space. One reason is that the mechanisms controlling the low‐frequency polarization are still not completely understood. Only a few existing models try to explain the processes and derive relationships with geometry and most models need strong assumptions. Here, we aim at a deeper understanding by numerical modelling of the main processes responsible for the IP effect. Our approach is based on a 1D solution of the late 1950s that gives an expression for the maximum frequency effect for simplified geometries. The models depend on the lengths of active and passive zones and the corresponding ion mobilities in each zone for anions and cations respectively. The theory describes the ion diffusion along concentration gradients, the influence of an external electric field and the coupling between the two.We first verify our numerical results by comparison with analytical solutions and then extend the approach to flexible geometry, including higher dimension. This constitutes a considerable progress from 1D models restricted to two alternating media with fixed lengths. Apart from arbitrary geometry, we can also simulate the full spectral behaviour. A relatively simple model is able to explain frequency‐dependent magnitude and phase behaviour that is typically measured in the field. The Cole‐Cole model can be considered as the result of a network of pores of varying lengths. From our modelling studies, we derive scaling laws for mobility and geometry and suggest an empirical equation for the relationship between the length of passive pores and the time of maximum phase shift.
Integrated management and utilization of hydrogeophysical data on a national scale
Authors: Møller Ingelise , Søndergaard Verner H. , Jørgensen Flemming , Auken Esben , Christiansen Anders V. 
Published: September 2009

ABSTRACTDevelopment 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.