M. Karaoulis

Review: Some low-frequency electrical methods for subsurface characterization and monitoring in hydrogeology

Low-frequency geoelectrical methods include mainly self-potential, resistivity, and induced polarization techniques, which have potential in many environmental and hydrogeological applications. They provide complementary information to each other and to in-situ measurements. The self-potential method is a passive measurement of the electrical response associated with the in-situ generation of electrical current due to the flow of pore water in porous media, a salinity gradient, and/or the concentration of redox-active species. Under some conditions, this method can be used to visualize groundwater flow, to determine permeability, and to detect preferential flow paths. Electrical resistivity is dependent on the water content, the temperature, the salinity of the pore water, and the clay content and mineralogy. Time-lapse resistivity can be used to assess the permeability and dispersivity distributions and to monitor contaminant plumes. Induced polarization characterizes the ability of rocks to reversibly store electrical energy. It can be used to image permeability and to monitor chemistry of the pore water–minerals interface. These geophysical methods, reviewed in this paper, should always be used in concert with additional in-situ measurements (e.g. in-situ pumping tests, chemical measurements of the pore water), for instance through joint inversion schemes, which is an area of fertile on-going research.RésuméLes méthodes géoélectriques basses-fréquences incluent principalement la méthode de potentiel spontané, la résistivité, et la polarisation provoquée. Ces méthodes ont de nombreuses applications environnementales et hydrogéologiques. Elles fournissent des informations complémentaires les unes vis-à-vis des autres ainsi que vis-à-vis des mesures in situ. La méthode de potentiel spontané est une mesure passive de la réponse électrique associée à l’existence d’un courant in situ, lequel est lié soit à l’écoulement de l’eau dans un milieu poreux, à un gradient de salinité, et/ou à un gradient de concentrations d’espèces redox ioniques. Sous certaines conditions, cette méthode peut être utilisée pour visualiser l’écoulement de l’eau dans le sous-sol, pour déterminer la perméabilité, ou pour détecter des chemins d’écoulement préférentiels. La résistivité électrique dépend du contenu en eau des roches, de la température, de la salinité de l’eau porale, du contenu en argile ainsi que de la minéralogie de la phase argileuse. Le monitoring de la résistivité électrique peut être utilisé pour avoir accès à la perméabilité ainsi qu’à la dispersivité hydrodynamique et peut être utilisé pour le suivi de panaches de contamination. La polarisation provoquée caractérise la capacité des roches à stocker de manière réversible de l’énergie électrique. Cette méthode peut être utilisée pour déterminer la perméabilité et pour suivre dans le temps la chimie de l’interface eau/minéraux. Ces méthodes géophysiques, dont la synthèse est faite dans cet article, devraient toujours être associées à des mesures in situ (e.g., tests de pompage, mesures chimiques de l’eau porale), à travers des schémas d’inversion jointe, ce qui correspond à un domaine de recherche très actif.ResumenLos métodos geoeléctricos de baja frecuencia incluyen principalmente potencial espontáneo, resistividad, y técnicas de polarización inducida, las cuales que tienen potencialidad en muchas aplicaciones hidrogeológicas y ambientales. Ellos proporcionan información complementaria recíprocamente y a las mediciones in situ. El método de potencial espontáneo es una medida pasiva de la respuesta eléctrica asociada con la generación in situ de corrientes eléctricas debido a flujo de agua poral en un medio poroso, un gradiente de salinidad, y/o la concentración de especies redox activas. Bajo ciertas condiciones, este método puede ser usado para visualizar el flujo de agua subterránea, determinar la permeabilidad, y detectar trayectorias preferenciales en el flujo. La resistividad eléctrica depende del contenido de agua, de la temperatura, de la salinidad de agua poral, y el contenido y mineralogía de las arcillas. La resistividad en función del tiempo puede ser usada para evaluar a las distribuciones de permeabilidad y dispersividad y monitorear plumas de contaminación. La polarización inducida caracteriza la habilidad de las rocas para almacenar energía eléctrica reversiblemente. Ello puede ser usado para dar una imagen de la permeabilidad y para el monitoreo químico de la interfase agua poral - minerales. Estos métodos geofísicos, revisados en este trabajo, deben ser siempre usados en combinación con otras mediciones in situ (por ejemplo, ensayos de bombeo in situ, mediciones químicas del agua poral), por ejemplo a través de esquemas de inversión conjunta, que es un área de investigación fértil y activa.摘要低频地电方法主要包括自然电位、电阻率以及激发极化技术,它们在许多环境和水文地质的应用中均有潜力。这些方法互相间提供补充信息,同样也给原位测试提供信息。自然电位方法是电反应的一个消极测量方法,该电反应与电流的原位生成相关,而电流的原位生成是因为多孔介质中水的流动,盐度梯度的存在,以及(或)氧化还原活性物种的聚集。在某些情况下,该方法可用于可视化地下水流动,确定渗透率和查明优先流路径。电阻率取决于含水率、温度、孔隙水的盐度以及粘土含量和矿物。延时电阻可以用于估算渗透率和分散性分布,以及检测污染晕。激发极化法刻画了延时可逆的储存电能的能力。它可以用于描绘渗透率以及检测孔隙水-矿物分界面的化学特征。本文综述了这些地球物理学方法,且应该一直用于与原位测试(如原位抽水试验、孔隙水的化学测试)的协同作用,例如通过联合转换方案,这是一个正在进行的内容丰富的研究领域。ResumoMétodos geoelétricos de baixa-frequência incluem principalmente o potencial espontâneo, a resistividade e métodos de polarização induzida, os quais apresentam potencial em muitas aplicações ambientais e hidrogeológicas. Esses métodos providenciam informação complementar uns em relação aos outros e em relação a medições in situ. O método do potencial espontâneo é uma medição passiva da resposta elétrica associada com a geração in situ de corrente elétrica devido ao fluxo de água nos poros em meios porosos, ao gradiente de salinidade e/ou à concentração de espécies redox activas. Sob certas condições, este método pode ser usado para visualizar o fluxo de água subterrânea, para determinar a permeabilidade e para detetar caminhos de fluxo preferencial. A resistividade elétrica está dependente do conteúdo em água, da temperatura, da salinidade da água nos poros, do conteúdo em argila e da mineralogia. A resistividade de lapso de tempo pode ser usada para avaliar a distribuição da permeabilidade e dispersividade e para monitorizar plumas contaminantes. A polarização induzida carateriza a capacidade das rochas para armazenar energia elétrica de forma reversível. Pode ser usada para criar uma imagem da permeabilidade e para monitorizar o quimismo da interface água-minerais nos poros. Os métodos geofísicos, revistos neste documento, devem sempre ser usados em conjunto com medições adicionais in situ (eg. ensaios de caudal in situ, dados químicos da água dos poros), por exemplo através de esquemas de inversão conjunta, que é uma área de pesquisa atual muito fértil.

Characterization of groundwater and surface water mixing in a semiconfined karst aquifer using time‐lapse electrical resistivity tomography

Groundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with end-member mixing analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and base flow conditions. To our knowledge, this is the first time EMMA and synthetic geophysical simulations have been combined. Here we interpret an 8 week time-lapse electrical resistivity data set to assess groundwater-surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time-lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m3/d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions, it was determined that conduit water composition ranged from 75% groundwater during base flow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time-lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water-rock reactions, in a semiconfined, sink-rise system.

4D time-lapse ERT inversion: introducing combined time and space constraints

Time-lapse Electrical Resistivity Tomography (ERT) can be used to characterize dynamic processes occurring in the subsurface of the Earth. It involves the installation of a permanent array of electrodes to monitor changes in resistivity associated with changes in pore-water properties (salinity, temperature, water content) or porosity (compaction or dilation). The interpretation of time-lapse data is complicated by both the presence of noise in the data and the influence of low sensitivity in parts of the model. A uniform space and time constraint is not able to address this problem. In this work, we propose a new approach to distinguish noise-related artefacts to true changes in resistivity, while at the same time addressing the problem of the lack of sensitivity of electrical resistivity tomography with depth. We propose transforming the space and time constraints to be active, meaning that the regularization parameters are distributed rather than being uniform for the entire model. This way, both time-related noise (assumed to be random) in the data and the lack of sensitivity are addressed and we can incorporate prior information in a natural way into the inversion scheme. Using this strategy, the inversion scheme is able to favour areas where the expected changes are likely to occur while filtering out areas where no changes should occur. The favoured areas can be either selected from a preliminary analysis of the data, or by incorporating other types of prior information into the system based on the process that is monitored.

IP4DI: A software for time-lapse 2D/3D DC-resistivity and induced polarization tomography

We propose a 2D/3D forward modelling and inversion package to invert direct current (DC)-resistivity, time-domain induced polarization (TDIP), and frequency-domain induced polarization (FDIP) data. Each cell used for the discretization of the 2D/3D problems is characterized by a DC-resistivity value and a chargeability or complex conductivity for TDIP/FDIP problems, respectively. The governing elliptic partial differential equations are solved with the finite element method, which can be applied for both real and complex numbers. The inversion can be performed either for a single snapshot of data or for a sequence of snapshots in order to monitor a dynamic process such as a salt tracer test. For the time-lapse inversion, we have developed an active time constrained (ATC) approach that is very efficient in filtering out noise in the data that is not correlated over time. The forward algorithm is benchmarked with simple analytical solutions. The inversion package IP4DI is benchmarked with three tests, two including simple geometries. The last one corresponds to a time-lapse resistivity problem for cross-well tomography during enhanced oil recovery. The algorithms are based on MATLAB^(R) code package and a graphical user interface (GUI).

Evaluating four-dimensional time-lapse electrical resistivity tomography for monitoring DNAPL source zone remediation.

Practical, non-invasive tools do not currently exist for mapping the remediation of dense non-aqueous phase liquids (DNAPLs). Electrical resistivity tomography (ERT) exhibits significant potential but has not yet become a practitioners tool due to challenges in interpreting the survey results at real sites. This study explores the effectiveness of recently developed four-dimensional (4D, i.e., 3D space plus time) time-lapse surface ERT to monitor DNAPL source zone remediation. A laboratory experiment demonstrated the approach for mapping a changing NAPL distribution over time. A recently developed DNAPL-ERT numerical model was then employed to independently simulate the experiment, providing confidence that the DNAPL-ERT model is a reliable tool for simulating real systems. The numerical model was then used to evaluate the potential for this approach at the field scale. Four DNAPL source zones, exhibiting a range of complexity, were initially simulated, followed by modeled time-lapse ERT monitoring of complete DNAPL remediation by enhanced dissolution. 4D ERT inversion provided estimates of the regions of the source zone experiencing mass reduction with time. Results show that 4D time-lapse ERT has significant potential to map both the outline and the center of mass of the evolving treated portion of the source zone to within a few meters in each direction. In addition, the technique can provide a reasonable, albeit conservative, estimate of the DNAPL volume remediated with time: 25% underestimation in the upper 2m and up to 50% underestimation at late time between 2 and 4m depth. The technique is less reliable for identifying cleanup of DNAPL stringers outside the main DNAPL body. Overall, this study demonstrates that 4D time-lapse ERT has potential for mapping where and how quickly DNAPL mass changes in real time during site remediation.

Complex conductivity of soils

The complex conductivity of soils remains poorly known despite the growing importance of this method in hydrogeophysics. In order to fill this gap of knowledge, we investigate the complex conductivity of 71 soils samples (including four peat samples) and one clean sand in the frequency range 0.1 Hz to 45 kHz. The soil samples are saturated with six different NaCl brines with conductivities (0.031, 0.53, 1.15, 5.7, 14.7, and 22 S m21, NaCl, 258C) in order to determine their intrinsic formation factor and surface conductivity. This data set is used to test the predictions of the dynamic Stern polarization model of porous media in terms of relationship between the quadrature conductivity and the surface conductivity. We also investigate the relationship between the normalized chargeability (the difference of in-phase conductivity between two frequencies) and the quadrature conductivity at the geometric mean frequency. This data set confirms the relationships between the surface conductivity, the quadrature conductivity, and the normalized chargeability. The normalized chargeability depends linearly on the cation exchange capacity and specific surface area while the chargeability shows no dependence on these parameters. These new data and the dynamic Stern layer polarization model are observed to be mutually consistent. Traditionally, in hydrogeophysics, surface conductivity is neglected in the analysis of resistivity data. The relationships we have developed can be used in field conditions to avoid neglecting surface conductivity in the interpretation of DC resistivity tomograms. We also investigate the effects of temperature and saturation and, here again, the dynamic Stern layer predictions and the experimental observations are mutually consistent.

Monitoring and simulation of salinity changes in response to tide and storm surges in a sandy coastal aquifer system

Tidal dynamics and especially storm surges can have an extensive impact on coastal fresh groundwater resources. Combined with the prospect of sea-level rise and the reliance of many people on these resources, this demonstrates the need to assess the vulnerability of coastal areas to these threats. In this study, we investigated the impact of tides and storm surges on coastal groundwater at a pilot location on the Dutch coast (viz., the Sand Engine). To monitor changes in groundwater salinity under a variety of conditions, we performed automated measurements with electrical resistivity tomography for a period of 2 months between November 2014 and January 2015. The obtained resistivity images were converted to salinity images, and these images served effectively as observations of the impact of tidal fluctuations, salt- water overwash during storm surges, and the recovery of the freshwater lens after land-surface inundations. Most of the observed changes in groundwater head and salinity could be reproduced with a two- dimensional variable-density groundwater flow and salt transport model. This shows that groundwater models can be used to make accurate predictions of the impact of tides and storm surges on fresh ground- water resources, given a thorough understanding of the (local) system. Comparisons of measurements and model simulations also showed that morphological changes and wave run-up can have a strong impact on the extent of land-surface inundations in (low-elevation) dynamic coastal environments, and can therefore substantially affect coastal fresh groundwater resources

Frequency-domain Helicopter-borne EM Survey for Delineation of the 3D Chloride Distribution in Zeeland, the Netherlands

In the project FRESHEM Zeeland the entire Province of Zeeland in the south-western part of the Netherlands was surveyed using frequency-domain helicopter-borne electromagnetics (FDHEM). The airborne survey of more than 9000 line-km was conducted in 2014-15 by the German Federal Institute for Geosciences and Natural Resources (BGR). Together with the Dutch partners Deltares and TNO, an approach has been developed to translate the FDHEM data into a full 3D mapping of the chloride concentration of an area of about 1800 km². Verification with an independent dataset showed that groundwater salinity can be accurately calculated based on FDHEM measurements and a stochastic lithological model. Using indicator kriging as interpolation method turns out to be suitable to construct a 3D voxel model, revealing high resolution spatial patterns of groundwater salinity.

Improved Time-lapse ERT Monitoring of Dense Non-aqueous phase Liquids (DNPLs) with Surface-to-horizontal Borehole Arrays

Electrical resistivity tomography (ERT) has long been recognized to exhibit the potential to significantly improve characterization of sites contaminated with dense non-aqueous phase liquids (DNAPLs). However, the technique has not become a common tool for mapping such contaminants due to the complexity of the DNAPL target coupled with the inherent limitations of the commonly used surface and cross-hole ERT configurations. The objective of this work is to evaluate the potential of surface-to-horizontal borehole (S2HB) ERT for mapping the spatio-temporal evolution of DNAPL mass during remediation. A coupled DNAPL-ERT model was employed to provide an initial, theoretical evaluation of S2HB ERT, relative to surface ERT, for monitoring the remediation of a realistic, field-scale DNAPL source zone. A laboratory experiment was then performed to demonstrate the S2HB ERT approach for a real system involving a changing NAPL distribution over time. Four-dimensional ERT inversion was employed for both numerical and experimental data to generate time-lapse resistivity images of the subsurface regions experiencing NAPL changes. Results demonstrate significantly improved resistivity imaging with S2HB ERT and its potential as a non-invasive DNAPL site tool for mapping DNAPL mass changes during remediation.

4D Active Time Constrained Resistivity Inversion

In this work an improved 4d technique for inverting monitoring resistivity data is examined and compared with the existing inversion schemes. Synthetic data tests illustrate that the time-lapse images of ERT data are affected by inversion artifacts.4D Active Time Constrained (4D-ATC) technique minimize those artifacts while producing realistic inversion models. Several comparative synthetic tests have been made between the techniques. Further, the presented algorithm is tested on field studies. The results demonstrate that this technique produces in most cases improved time-lapse images in relation with existing techniques.