Showing papers in "Near Surface Geophysics in 2009"

A review of helicopter‐borne electromagnetic methods for groundwater exploration

Abstract: For about three decades, airborne electromagnetic (AEM) systems have been used for groundwater exploration purposes. Airborne systems are appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralization of the water, electromagnetic systems are suitable for providing information about the aquifer structures and water quality, respectively. More helicopter than fixed-wing systems are used in airborne groundwater surveys. Helicopter-borne frequency-domain electromagnetic (HEM) systems use a towed rigid-boom. Helicopter-borne time-domain (HTEM) systems, which use a large transmitter loop and a small receiver within or above the transmitter, are generally designed for mineral exploration purposes but recent developments have made some of these systems usable for groundwater purposes as well. The quantity measured, the secondary magnetic field, depends on the subsurface conductivity distribution. Due to the skin-effect, the penetration depths of the AEM fields depend on the system characteristics used: high-frequency data/early-time channels describe the shallower parts of the conducting subsurface and the low-frequency data/late-time channels the deeper parts. Typical investigation depths range from some ten metres (conductive grounds) to several hundred metres (resistive grounds), where the HEM systems are appropriate for shallow to medium deep (about 1–100 m) and the HTEM systems for medium deep to deep (about 10–400 m) investigations. Generally, the secondary field values are inverted into resistivities and depths using homogeneous or layered half-space models. As the footprint of AEM systems is rather small, one-dimensional interpretation of AEM data is sufficient in most cases and single-site inversion procedures are widely used. Laterally constrained inversion of AEM data often improves the stability of the inversion models, particularly for noisy data. Higher dimensional inversion is still not possible for standard-size surveys. Based on typical field examples the advantages as well as the limitations of AEM surveys compared to long-established ground-based geophysical methods used in groundwater surveys are discussed. In a case history from a German island an airborne frequency-domain system is used to successfully locate freshwater lenses on top of saltwater. An example from Denmark shows how a time-domain system is used to locate large-scale buried structures forming ideal groundwater aquifers.

Automated monitoring of coastal aquifers with electrical resistivity tomography

Abstract: An Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the long-term monitoring of coastal aquifers. This ALERT system has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry riverbed with electrode take-outs at regular intervals of 10 m. The maximum depth of investigation is about 160 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Volumetric geoelectric images of the subsurface can be obtained ‘on demand’ or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is fully automated and seamless. The ALERT technology can provide early warning of potential threats to vulnerable water systems such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.

Characterization of seawater intrusion using 2D electrical imaging

Abstract: We 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.

Preferential fluid flow pathways in embankment dams imaged by self-potential tomography

Abstract: We invert self-potential data in order to locate anomalous water flow pathways in dams and embankments and to estimate the seepage velocity. The inversion of the self-potential data is performed using the modified singular value decomposition for the inverse problem using a linear formulation of the forward problem. The kernel is solved numerically accounting for the topography of the system and the resistivity distribution, which is independently obtained through electrical resistance tomography. A prior constraint based on finite element modelling of ground water flow can also be used to provide a prior source current density model if needed. This self-potential tomography approach is first validated with a synthetic case study showing how the position of a preferential fluid flow pathway can be retrieved from self-potential and resistivity data and how the seepage velocity can be obtained inside one order of magnitude. This methodology is then applied to a test site corresponding to a portion of an embankment dam along the Rhone River in France. Two self-potential maps (with 1169 and 2076 measurements, respectively) and four resistivity tomograms are used to locate a leak. One self-potential profile and one resistivity profile are used together to perform the 2D inversion of the self-potential data to locate the anomalous leakage at depth and to estimate the flow rate. The depth at which the preferential fluid flow pathway is located, according to self-potential tomography, agrees with an independent geotechnical test using the Permeafor. This demonstrates the usefulness of this methodology to detect preferential water channels inside the body of a dam.

Comparison of instruments for geoelectrical soil mapping at the field scale

Abstract: In precision agriculture geoelectrical methods have shown their capability to detect spatial variation of important physico-chemical soil parameters in an efficient way. Nevertheless, relationships between the electrical parameters (electrical conductivity or resistivity) and other soil properties are not always consistent over different fields. This can, to some extent, be due to the characteristics of instruments used for soil mapping. However, a limited amount of research has addressed this issue. In this study, seven instruments for mobile mapping (continuous geoelectrical measurements) available on the market were tested (ARP 03, CM-138, EM38, EM38-DD, EM38-MK2, OhmMapper and Veris 3100). Instruments were employed on a sandy site in north-east Germany. Measurements were compared to a profile, which has been investigated with a high accuracy reference. Additional investigations were conducted concerning the influences of temperature drift, seasonal variations and soil properties on soil EC. Marked differences between the instruments were found with respect to depth of investigation, accuracy and handling that have to be taken into account when geoelectrical surveys are planned or interpreted. Regarding depth of investigation and robustness of the measurements, ARP 03 and Veris 3100 seem to be the most suitable instruments for precision agriculture.

Monitoring the hydrologic behaviour of a mountain slope via time-lapse electrical resistivity tomography

Abstract: Catchment and hillslope hydrology is a major research area in geoscience and the understanding of its underlying processes is still poor. Direct investigation of steep hillslopes via drilling is often infeasible. In this paper, we present the results of non-invasive time-lapse monitoring of a controlled infiltration test at a site in the Italian Central Alps. The hillslope considered is steep (30–35°), covered with grass and a soil layer 1–1.5 m thick above a variably fractured metamorphic bedrock. The key hydrologic question is whether rainfall infiltrates mainly into the underlying fractured bedrock, thus recharging a deeper hydraulic system, or flows in the soil layer as interflow towards the stream channel a few hundred metres downhill. In order to respond to this question, we applied 2200 mm of artificial rain on a 2 m × 2 m slope box over about 18 hours. We estimated the effective infiltration by subtracting the measured runoff (7% of total). Due to the limited irrigation time and the climate conditions, the evapotranspiration was considered as negligible. The soil moisture variation and the underlying bedrock were monitored via a combination of electrical resistivity tomography (ERT), TDR probes and tensiometers. A small-scale 3D cross-hole ERT experiment was performed using 2 m deep boreholes purposely drilled and completed with electrodes in the irrigated plot. A larger scale (35 m long) 2D surface ERT survey was also continuously acquired across the irrigated area. Monitoring continued up to 10 days after the experiment. As a result, we observed a very fast vertical infiltration through the soil cover, also favoured by preferential flow patterns, immediately followed by infiltration into the fractured bedrock. The surface layer showed a fast recovery of initial moisture condition nearly completed in the first 12 hours after the end of irrigation. The lateral transmission of infiltrating water and runoff were negligible as compared to the vertical infiltration. These experiment results confirm that the fractured bedrock has a key role in controlling the fast hydrological dynamics of the small catchment system under study. We concluded that deep water circulation is the key pathway to hillslope processes at this site.important

Spectral induced polarization for the characterization of free-phase hydrocarbon contamination of sediments with low clay content

Abstract: The identification of organic pollutants in the soil and the subsurface is a goal of primary importance in the management of contaminated sites. However, only a few non-invasive techniques can be useful towards this goal. One such technique is spectral induced polarization. In this study, we investigate the spectral induced polarization effect of changing fluid saturation in a well-characterized porous medium, analysing the difference between air and hydrocarbons, at different degrees of water saturation. The experiments were conducted on fine eolic sand samples coming from an experimental site near Turin, Italy. Octanol and benzene were used as non-aqueous phase liquids. Samples were initially saturated with water having controlled electrical conductivity and subsequently de-saturated stepwise with injection of air at known pressure. The eolic sand samples were then re-saturated with the same water contaminated with hydrocarbons and then a non-aqueous phase liquid phase (either octanol or benzene) was injected in volumetric steps, in order to compare the effects of air and non-aqueous phase liquid invasion. At each saturation step, spectral induced polarization measurements were conducted in the 0.01 Hz to 1 kHz range using the ZEL-SIP04 impedance meter developed at the Forschungszentrum Juelich. The measurement setup guaranteed a 1 mrad phase precision for the entire frequency measurement range. Measurements were conducted under temperature controlled conditions at 20 (±0.5)° C. All spectral induced polarization curves show a peak in the range 0.01–1 Hz that changes in intensity and frequency with varying saturation and a high-frequency phase shift increase dominated by capacitive coupling effects of the measuring system. A multiple Cole-Cole model was fitted to the data. The effects of de-saturation on the low-frequency Cole-Cole parameters are that a) resistivity increases with decreasing water saturation but increases less with non-aqueous phase liquid than with the same volume of air; b) chargeability increases with decreasing water saturation but in presence of non-aqueous phase liquids its value is sometimes lower, sometimes higher and sometimes similar to the one observed in presence of air; c) the time constant τ increases with decreasing water saturation and is consistently larger with non-aqueous phase liquid than with air. These differences between air and non-aqueous phase liquid injection can be explained in terms of differences in non-aqueous phase distribution within the porous medium, as observed by X-ray micro-CT: while air is homogeneously distributed, non-aqueous phase liquids segregate under density effects. In summary, all spectral induced polarization effects of air and non-aqueous phase liquid injection in the considered porous medium are volumetric, i.e., are not due to interaction with grain surfaces or other electrical-chemical effects but are caused by pore obstruction by the electrically non-conductive phase.

Effective medium theories for modelling the relationships between electromagnetic properties and hydrological variables in geomaterials: a review

Abstract: The 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.

Hydrogeological prospecting using P- and S-wave landstreamer seismic reflection methods

Abstract: We present two case histories from different areas and geological settings in Canada where we have used a vibrating seismic source coupled to a landstreamer receiver array in hydrogeological investigations related to aquifers in glacial sediments. In Manitoba, our P-wave seismic reflection profiles are used to provide an assessment of the subsurface architecture of buried valleys, estimate the thickness and properties of both the channel fill and the overlying sediments to depths of ~100 m and locate optimum sites for groundwater well placements. In eastern Ontario, we collected P- and S-wave seismic reflection as well as electrical resistivity data to investigate buried esker aquifers. The geophysical data provide detailed high-resolution information (to ~30 m depth) on the structure of the esker core and its overlying sand cover and on the thickness and variability of the overlying fine-grained aquitard. The data presented in this paper demonstrate that shallow seismic reflection methods are very effective tools to explore, assess and evaluate groundwater reservoirs and resources. The recent advent of landstreamer receiver arrays, especially when coupled to a vibratory seismic source, makes these methods significantly more cost-effective and efficient. We now routinely collect ~1000 records/day, or 1.5-6 line-km/day, using our Minivib/landstreamer data acquisition system. With this type of efficient data collection, it is anticipated that the use of shallow seismic reflection methods in hydrogeological prospecting will increase as groundwater and its protection become more valued by society.

Electrical conductivity mapping for precision farming

Abstract: Precision farming overcomes the paradigm of uniform field treatment by site-specific data acquisition and treatment to cope with within-field variability. Precision farming heavily relies on spatially dense information about soil and crop status. While it is often difficult and expensive to obtain precise soil information by traditional soil sampling and laboratory analysis some geophysical methods offer means to obtain subsidiary data in an efficient way. In particular, geoelectrical soil mapping has become widely accepted in precision farming. At present it is the most successful geophysical method providing the spatial distribution of relevant agronomic information that enables us to determine management zones for precision farming. Much work has been done to test the applicability of existing geoelectrical methods and to develop measurement systems applicable in the context of precision farming. Therefore, the aim of this paper was to introduce the basic ideas of precision farming, to discuss current precision farming applied geoelectrical methods and instruments and to give an overview about our corresponding activities during recent years. Different experiments were performed both in the laboratory and in the field to estimate first, electrical conductivity affecting factors, second, relationships between direct push and surface measurements, third, the seasonal stability of electrical conductivity patterns and fourth, the relationship between plant yield and electrical conductivity. From the results of these experiments, we concluded that soil texture is a very dominant factor in electrical conductivity mapping. Soil moisture affects both the level and the dynamic range of electrical conductivity readings. Nevertheless, electrical conductivity measurements can be principally performed independent of season. However, electrical conductivity field mapping does not produce reliable maps of spatial particle size distribution of soils, e.g., necessary to generate input parameters for water and nutrient transport models. The missing step to achieve this aim may be to develop multi-sensor systems that allow adjusting the electrical conductivity measurement from the influence of different soil water contents.

Embankment dam seepage evaluation from resistivity monitoring data

Abstract: Methods 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 non-destructive 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 Sadva 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 Sadva 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 Sadva 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 Sadva dam constitute an interesting initial approach. However, many assumptions and simplifications are made and more work on refining the method is needed.

Integrated management and utilization of hydrogeophysical data on a national scale

Abstract: 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.

Estimating the depth of fresh and brackish groundwater in a predominantly saline region using geophysical and hydrological methods, Zeeland, the Netherlands

Abstract: The province of Zeeland is situated in the coastal zone of the Netherlands. The ground surface level is around or below mean sea level. Therefore seepage of brackish to saline groundwater is very common. Sea level rise as a result of climate change will very likely increase the pressure on the coastal groundwater system, leading to an increased salinization of the groundwater and surface water system. Still, freshwater agriculture is being practiced in large parts of the province. The vegetation extracts its fresh water from the unsaturated zone and thin rainwater lenses that 'float' on top of brackish and saline groundwater. Geophysical and hydrogeological data have been com-bined on two spatial scales to obtain a better insight into this fresh-brackish-saline groundwater distribution. This information is being used to: assess groundwater abstractions, plan landuse and improve the input of variable-density groundwater flow and coupled solute transport models. For over 6000 locations various types of data have been used to estimate the depth of the brackish-saline groundwater interface of 1000 mg [Cl -]/L. These types of data are both of geophysi-cal (vertical electrical soundings, EM34, geoelectrical well logs and electrical cone penetration tests) and of hydrological origin (water samples and abstraction wells). These data have been inter-preted and combined with knowledge on the distribution of geological units to make an estimation of the depth of the brackish-saline water transition for the whole province (~66 km × 63 km). In addition to the regional brackish-saline interface map, continuous vertical electrical soundings (CVES) have been executed to map the fresh-brackish-saline distribution on a local scale. The CVES profiles were made at eight different plots where brackish-saline water is occurring at shal-low depths (<5 m below the surface) according to the regional map and where freshwater agriculture is still being practised. Six of the eight sites have thin (0–3 m) brackish to slightly saline water lenses. At two sites up to 15 m thick brackish water lenses have been observed with CVES. The thickness of the brackish water lenses varies laterally over short distances. Sandy sediment and a higher topography are favourable factors for the development of such lenses.

Case histories of hydraulic conductivity estimation with induced polarization at the field scale

Abstract: We have carried out spectral induced polarization (IP) measurements at three different hydrogeological test sites (Hasloh, Ludingworth and Kappelen) and estimated hydraulic conductivity using empirical equations previously derived from laboratory measurements. We also reviewed previously published data from another site (Krauthausen). The intention was to explore the potential and practical limitations when applying the method at the field scale. The test sites cover a lithological spectrum from gravel to silt, with a variation in hydraulic conductivity ( K ) over three orders of magnitude. At each site, hydraulic conductivity was estimated from the real and imaginary conductivity resulting from 2D inversion. We applied the constant phase angle model, where only one frequency, typically around 1 Hz is being used. The uncertainty in K -estimates arising from inversion ambiguity was assessed by exploring the model space with a control parameter that permits a transition from smooth to blocky models and by using different starting models. At the Kappelen site, this uncertainty is larger than four orders of magnitude but a reasonable lower limit for K can be obtained. At the other three sites, the uncertainties are typically one order of magnitude. The IP-based hydraulic conductivity estimates were compared with K -values obtained from grain size analyses and pumping tests. At the Hasloh and Ludingworth sites the results agree within one order of magnitude and at the Kappelen site the derived lower boundary for K is consistent with grain size information. At the Krauthausen site, the difference between IP-based data and the values derived from grain size and pumping tests is significantly larger than the estimated uncertainties, which is probably due to the non-uniform grain size distribution. The overall results indicate that order of magnitude K -estimates from IP data at the field scale are realistic targets. However, sites with significant deviations from the empirical equations can exist, emphasizing the recommendation to use a priori information whenever possible.

Integrated analysis and interpretation of cross-hole P- and S-wave tomograms: a case study

Abstract: We present cross-hole P- and S-wave seismic experiments that have been performed along a ~100 m long transect for the detailed characterization of a contaminated sedimentary site (Bitterfeld research test site, Germany). We invert the corresponding first break arrival times for the P- and S-wave velocity structure and compare two different strategies to interpret these models in terms of pertinent lithological and geotechnical parameter variations. The first (common) approach is based on directly translating the tomographic velocity models into the parameters of interest (e.g., elastic moduli). The second (zonal) approach first reduces the tomographic parameter information to a limited number of characteristic velocity combinations via k-means cluster analysis. Then, for each zone (cluster) further parameters including uncertainties can be estimated. In the presented case study, our results indicate that the zonal approach provides an effective means for the integrated interpretation of different co-located data.

Inversion of airborne time-domain electromagnetic data to a 1D structure using lateral constraints

Abstract: We invert time-domain airborne electromagnetic data in such a way as to obtain a model that varies slowly along the profile. This is achieved by modifying a typical one-dimensional inversion algorithm to include lateral constraints. The lateral constraints are included as a roughness matrix that is solved simultaneously with the Jacobian matrix in an iterative eigenparameter inversion. In this case, multiple soundings along a line are all solved simultaneously. The lateral constraints can be applied to the resistivities and the thicknesses, both separately and together. We apply these techniques in two situations where airborne geophysical data are applied to near-surface exploration. The first case is in a resistive environment where we are interested in quantifying a superficial conductive overburden. In this case, lateral constraints improve the geological image compared with those images obtained using unconstrained layered-earth inversion. In the second case, we want to map the thickness of a resistive aquifer covering a saline layer. In this case, we show how varying the weights on the lateral constraints can change the image of the thickness of the aquifer. The presence of numerous cultural artefacts makes the inversion problematic. Application of a first-difference constraint did the best job at removing culture but resulted in oversmoothing of the hydrogeology. The use of a second-difference lateral constraint gave a good rendition of the hydrogeology but did not suppress the culture as well as the first-difference constraint.

Monitoring freshwater‐seawater interface dynamics with audiomagnetotelluric data

Abstract: Saltwater intrusion is one of the main environmental concerns within coastal aquifers. In this study we test the audiomagnetotelluric (AMT) method as a technique that can detect changes in electrical resistivity as a result of seasonal groundwater salinity changes. AMT is a frequency domain electromagnetic induction technique ideally suited for hydrogeophysical investigations at the basin scale, specifically in low resistivity environments such as saltwater encroachments areas. We present numerical seawater intrusion models to explore the effects of saline content variability on the model resolution. Survey data were also acquired during a long-term AMT monitoring experiment in a natural condition aquifer system and these results were compared to the numerical modelling results. The aquifer system is located in the deltaic zone of the Tordera River (north-eastern of Iberian Peninsula), where a main paleochannel that works as a seawater intrusion path was already identified in previous studies. Every four months, between 2004 and 2006, seven AMT soundings were recorded along a 1700 m long profile over the paleochannel. The final models reveal dynamic changes in the seawater-freshwater interface that correspond directly with the hydrologic state of the aquifer system.

Fast, laterally smooth inversion of airborne time-domain electromagnetic data

Abstract: We present a fully developed, fast approximate method for 1D inversion of time-domain electromagnetic data. The method is applied to a helicopterborne transient electromagnetic data set from the Toolibin Lake area of Western Australia using the lateral parameter correlation method to ensure lateral smoothness of the inverted models. The method is based on fast approximate forward computation of transient electromagnetic step responses and their derivatives with respect to the model parameters of a 1D model. The inversion is carried out with multi-layer models in an iterative, constrained least-squares inversion formulation including explicit formulation of the model regularization through a model covariance matrix. The method is 50 times faster than conventional inversion for a layered earth model and produces model sections of concatenated 1D models and contoured maps of mean conductivity in depth intervals almost indistinguishable from those of conventional inversion. To ensure lateral smoothness of the model sections and to avoid spurious artefacts in the mean conductivity maps, the inversion is integrated with the lateral parameter correlation method. In this way, well determined parameters are allowed to influence the more poorly determined parameters in the survey area. Applied to the Toolibin data set, the inversion produces model sections and conductivity maps that reveal the distribution of conductivity in the area and thereby the distribution of salinity. This information is crucial for any remediation effort aimed at alleviating the salinization problems.

Spatial mapping of submerged cave systems by means of airborne electromagnetics: an emerging technology to support protection of endangered karst aquifers

Abstract: Karst 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 Yucatan, 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.

Correlation of electrical resistivity, electrical conductivity and soil parameters at a long-term fertilization experiment

Abstract: Geophysical methods are progressive, non-destructive but indirect techniques for characterization of soil properties and mapping of soil heterogeneities. Geophysical surveys for soil mapping lead generally to ambiguous results since geophysical parameters are influenced by several soil properties, e.g., organic content, clay content and bulk density. The investigations presented here focus on the effect of different stages of organic content on DC-geoelectrics and electromagnetic induction (EMI) at a long-term fertilization experiment. This experiment gives, after 105 years running, an excellent opportunity to study the correlations between electrical resistivity, apparent electrical conductivity and soil parameters. Results from DC-geoelectrical measurements (profile length 80–160 m, electrode distance 0.5 m) twice, in August after harvest and in January during black fallow period, are presented. Additionally electromagnetical investigations were conducted in January. Correlations of resistivity and carbon input into the soil are significant and very strong; especially in January with r = –0.89 but contradictive in summer and winter. The analysis of resistivity and apparent electrical conductivity is critical since bulk density and water storage capacity is influenced by fertilization and plant growth. Interpretation of a combination of DC-geoelectrics and electromagnetical techniques on agricultural areas has to be done with respect to management aspects.

Numerical modelling of the IP effect at the pore scale

Abstract: Geophysicists 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.

Aquifer investigations in the Leon-Chinandega plains, Nicaragua, using electromagnetic and electrical methods

Abstract: The Leon-Chinandega plains are located in the north-western part of Nicaragua and represent the most important groundwater reservoir of the country. The aquifer is comprised of three hydrogeological units. The top unit is an unconfined alluvial and below a more consolidated volcanic aquifer is found. These aquifers rest on an ignimbrite unit that acts as the basement of the aquifer. The plains are mainly used for agricultural purposes and most of the irrigation comes from upper aquifer wells (not deeper than 70 m) allocated within the shallow unconfined aquifer. Transient electromagnetic soundings (TEM) and continuous vertical electrical soundings (CVES) were carried out in order to obtain information regarding the geology of the study area. Two TEM profiles extending from the Pacific coast towards the volcanic chain were carried out with an approximate separation between soundings of 250 metres. A grid of TEM soundings was also performed in an area located between the towns of Posoltega and Quezalguaque. CVES were carried out in different areas of the plains in order to obtain detailed information on the geology and distribution of the shallow aquifer. This information was later correlated with information available from wells in the area. One of the objectives of this investigation was to test and evaluate the applicability of the TEMfast48 equipment in the study area. In the plains area the TEM method shows very good results where a general geophysical model of mostly a three-layered earth can be obtained. In the coastal areas the models are more irregular and complex. The results from the CVES generally agree with the TEM models but present more details, especially in areas of complex geology. The penetration depth for the TEM soundings reached almost 100 m at most, depending on the depth to a low resistivity layer. In some areas the depth to the basement was estimated with long-layout CVES. In general, it was possible to obtain consistent geoelectrical models of the area and the methods complement each other well. The geoelectrical models are an excellent addition to other investigation methods as they provide an overview of the aquifer system and can serve as a basis for refining the conceptual and numerical models of the aquifer system. This information is expected to become very valuable for the exploration, management and protection of the groundwater resources in the Leon-Chinandega plains to ensure sustainability of the resource.

Continuous electrical imaging for mapping aquifer recharge along reaches of the Namoi River in Australia

Abstract: In this study the capacity of towed floating electrical imaging for mapping a known aquifer recharge zone in more detail is investigated. A 50 km reach of the Namoi River, Australia, was surveyed. The river is perched up to 16 m above an unconfined aquifer that extends to 30 m below the ground surface. The unconfined aquifer overlies a semi-confined aquifer system. The electrical array consists of two current electrodes near the boat, followed by nine potential electrodes. The sequence of 1D electrical sounding curves generated was automatically inverted to obtain a layered earth electrical model along the river to a depth of approximately 40 m. There was weak correlation between the water conductivity measured in boreholes and the layered earth electrical model. Lithological logs from boreholes near the river indicate that the major layers seen in the electrical conductivity cross-section correspond to the major sedimentary units. Groundwater mounding in the vicinity of the river has been monitored after flooding. The groundwater flood mounds coincide with the location of the predominantly low electrically conductive sediments mapped beneath the river. This suggests that aquifer recharging waters migrate via the sands and gravels, then pool at the water table before dissipating. This survey demonstrates that mapping intervals of low electrically conductive sediments beneath the river maps potential aquifer recharge pathways.

3D seismic reflection surveying at the CO2SINK project site, Ketzin, Germany: a study for extracting shallow subsurface information

Abstract: Three-dimensional (3D) reflection seismic data were recorded as part of a pilot scale carbon dioxide (CO2) geological storage project (CO2 SINK) with the aim of mapping the structural geometry of t

Imaging and monitoring tree-induced subsidence using electrical resistivity imaging

Abstract: Tree-induced subsidence is a natural ground movement phenomenon that arises when tree roots absorb water from clay-rich soils, causing them to shrink. Tree-induced subsidence is particularly problematic for buildings and structures in close proximity to deciduous trees where changes in seasonal climate can have a significant effect on the degree of relative surface movement. Unfortunately, there is a lack of in-depth understanding of tree-induced subsidence dynamics and limited access to affordable/reliable assessment technologies for the victims of such phenomena. This paper presents and discusses the use of 2D electrical resistivity imaging (ERI) as a tool for recognizing tree-related subsurface desiccation and its associated subsidence. The findings of a two year ERI study in a high-risk tree-induced subsidence environment are presented (mature Oak and Willow trees growing in expansive London Clays) where repeat (time-lapse) resistivity profiles are compared to conventional ground levelling and soil moisture content data. This study confirms that ERI is a valuable tool for the characterization of seasonally varying subsurface moisture distributions and that the technique is capable of defining the architecture and temporal-spatial variance of an active tree root system. Moreover, the method provides a critical insight into the visualization of tree-induced subsidence phenomena and reveals promising practical potential for use as a modern, affordable, non-invasive tree-induced subsidence assessment tool.

Trends in waterborne electrical and EM induction methods for high resolution sub‐bottom imaging

Abstract: Shallow water applications of electrical and electromagnetic geophysical methods have grown in recent years with recognition of the information these methods can provide regarding groundwater‐surface water interaction, geotechnical engineering, exploration, marine geology and other fields. In many applications, spatial variations in resistivity are useful as a proxy for variations in another bulk material property such as pore water salinity, clay content, porosity, or temperature. Applications of galvanic resistivity methods have been buoyed by the development of marine configurations that are now commercially available. In contrast, with two notable exceptions, most applications of EM induction methods have involved experimental adaptations of instruments originally designed for use on land. Methods for shallow water resistivity and EM induction surveys are at an exciting stage of development where several promising applications have been demonstrated but the suite of tools and components commercially available and widely tested remains relatively small.

Multi-offset GPR methods for hyporheic zone investigations

Abstract: Porosity of stream sediments has a direct effect on hyporheic exchange patterns and rates. Improved estimates of porosity heterogeneity will yield enhanced simulation of hyporheic exchange processes. Ground-penetrating radar (GPR) velocity measurements are strongly controlled by water content thus accurate measures of GPR velocity in saturated sediments provides estimates of porosity beneath stream channels using petrophysical relationships. Imaging the substream system using surface based reflection measurements is particularly challenging due to large velocity gradients that occur at the transition from open water to saturated sediments. The continuous multi-offset method improves the quality of subsurface images through stacking and provides measurements of vertical and lateral velocity distributions. We applied the continuous multi-offset method to stream sites on the North Slope, Alaska and the Sawtooth Mountains near Boise, Idaho, USA. From the continuous multi-offset data, we measure velocity using reflection tomography then estimate water content and porosity using the Topp equation. These values provide detailed measurements for improved stream channel hydraulic and thermal modelling.

Integration of multidimensional archaeogeophysical data using supervised and unsupervised classification

Abstract: When multiple geophysical methods are used to survey an archaeological site, an integrated approach to interpreting the data is often pursued. The use of supervised and unsupervised classification methods are tested using ground-penetrating radar, magnetometry and magnetic susceptibility data sets from a site in the American Southwest. Pueblo Escondido was a large prehistoric village associated with the Mogollon culture in southern New Mexico, with peak occupation during the transition between pithouse and pueblo architectural periods (ca. 1280–1290 AD). Image classification has the benefit of producing unambiguous discrete maps and capitalizes on the multivariate relationships between data sets. Theoretically, unsupervised classification could identify new archaeological classes that were not anticipated but no such classes were identified. The K-means cluster analysis succeeded only in identifying weak, moderate and strong positive anomalies found in the original data sets. Supervised classification utilizing Mahalanobis distance produced much better results. Training sites based on archaeological excavations were used to classify all locations in the survey area, yielding a predictive model of archaeological features in three classes, plus a background class. The result shows features that were not easily identified in the original data sets but are made visible by the multivariate model. The model could be used for guiding future excavations and arguably leads to a better understanding of the site’s subsurface content and spatial organization.

Simultaneous inversion of magnetic resonance sounding in terms of water content, resistivity and decay times

Abstract: Magnetic 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.

A least-squares standard deviation method to interpret magnetic anomalies due to thin dikes

Abstract: We have developed a least-squares method to determine simultaneously the depth and the horizontal position (origin) of a buried thin dike that extends in both strike direction and down dip (2D) and in which the depth is much greater than the thickness from horizontal gradients obtained numerically from magnetic data using filters of successive window lengths. The method involves using a relationship between the depth and the horizontal position of the source and a combination of windowed observations. The method is based on computing the standard deviation of the depths determined from all horizontal gradient anomalies for each horizontal position. The standard deviation may generally be considered as a criterion for determining the correct depth and the horizontal position of the buried dike. When the correct horizontal position value is used, the standard deviation of the depths is less than the standard deviation using incorrect horizontal position values. This method can be applied to residuals as well as to the observed magnetic data. The method is applied to synthetic examples with and without random errors. The present method was able to provide both the depth and horizontal position of the source accurately. The practical utility of the method is tested on an outcropping dike in Canada.