Hydrogeological processes in fractured and porous media: insights from geophysical case studies

Conference (2013, January 18)

This presentation focuses on geophysical case studies with the aim to highlight the possibilities to study and monitor hydrogeological processes in the subsurface, including transport processes in ... [more ▼]

This presentation focuses on geophysical case studies with the aim to highlight the possibilities to study and monitor hydrogeological processes in the subsurface, including transport processes in fractured or in porous media. The presentation emphasizes two geoelectrical methods, namely electrical resistivity tomography (ERT) which images the electrical resistivity distribution of the subsurface and self-potential (SP) whose measured signal is directly sensitive to groundwater fluxes. The first case study concerns the geophysical identification and characterization of large hydraulically-active fractured areas in calcareous synclines and in particular the assessment of the joint use of ERT and SP to set up new piezometers in fractured limestone. This assessment shows that piezometers drilled inside less resistive areas and/or in negative SP anomalies presented high hydraulic capacities. Inversely, piezometers drilled inside more resistive zones and/or outside an SP anomaly presented low hydraulic capacities. The SP anomaly related to preferential flow in fractures was thus demonstrated for the first time. All these fractures information, obtained with geophysics, improved the conceptualization and calibration of the groundwater flow model of the calcareous valley. A seasonal monitoring of SP signals proved to be a successful methodology to better understand the hydrodynamics of calcareous aquifers and in particular to follow the seasonal drawdown of the water table in the calcareous valley. Different methodologies to delineate the main groundwater flow direction were also tested. The latter can be achieved for example by drawing an SP map showing the main hydraulic gradients or by monitoring a salt tracer test with ERT to highlight preferential flow in fractures. The second case study concerns the ERT monitoring of a shallow geothermal test conducted in a porous medium (sand). The main objective of this study was to derive temperature from a series of electrical resistivity images since the electrical resistivity is directly sensitive to temperature changes. This field work demonstrates that surface electric resistivity tomography can monitor heat injection and storage experiments in shallow aquifers providing a number of practical applications, such as the monitoring or the design of shallow geothermal systems or the use of heated water to replace salt water in tracer tests. Through these two different case studies, this presentation also emphasizes in a practical way on the importance of data inversion and image appraisal since these issues are crucial to quantitatively study hydrogeological processes. [less ▲]

Inversion of multi-temporal geoelectrical data sets: insights from several case studies

in Berichte der Geologischen Bundesanstalt (2012, September), 93

Projet MINERVE - Convention n°6600 - Revue critique de la littérature et choix des techniques géophysiques potentielles

Report (2012)

Geophysical identification, characterization, and monitoring of preferential groundwater flow paths in fractured media

Doctoral thesis (2012)

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to ... [more ▼]

This thesis investigates fractured zones leading to preferential groundwater flow paths. In this context, we used the electrical resistivity tomography (ERT) and the self-potential (SP) methods to identify, characterize, monitor, and finally model preferential flow in hydraulically-active fractured zones at a scale representative of real world applications. From an experimental point of view, we first identified the magnitude of self-potential signature, a -15 mV anomaly that could be associated with preferential groundwater flow in a shallow quartzite aquitard whereas the streaming potential theory was originally developed for porous media. This signature was confirmed experimentally in limestone aquifers at greater depths. The joint use of surface ERT and SP allowed the identification of water-bearing fractured areas which were electrically more conductive, presenting contrasts from 1 to 10 and which were hydraulically-active presenting negative SP anomalies ranging from -10 to -30 mV. We were also able to correlate hydraulic heads and SP gradients during a low and a high groundwater level period leading to interesting perspectives in understanding the dynamics of complex groundwater flow systems. Finally, a preferential flow and rapid transport path, over 10 m/h, was highlighted in a 20 m deep fractured and karstified limestone valley by monitoring a salt tracer test with only surface ERT. This methodology was being mostly used for relatively shallow and homogeneous aquifers up to now. Such information is crucial to set up new monitoring wells or to define the sampling rates of classic tracer test. From a methodological point of view, we quantitatively assessed the efficiency of blocky and minimum-gradient-support regularizations in electrical imaging to recover sharp interfaces on numerical benchmarks and with field data. The usefulness of resolution indicators such as the cumulative sensitivity matrix and the resolution matrix were also assessed in this context. We demonstrated that noise characterization is crucial in time-lapse inversion and may supplant the choice of the time-lapse inversion scheme, calling for a systematic analysis of reciprocal measurements (or a subset of them). We also showed that, when using data differences in an inversion scheme, the data error, as estimated by time-lapse reciprocal measurements, depends on the mean measured resistance. These error characterization studies should always be performed if one wants to avoid wrong interpretations about the hydrodynamics. We further showed that focused inversion techniques (blocky inversion, minimum-gradient-support) may offer great perspectives when recovering model changes in time-lapse inversion. Finally, ERT and SP were jointly used to conceptualize a physically-based and spatially distributed hydrogeological model, in particular to characterize the preferential flow paths. Predicted hydraulic heads and SP-derived hydraulic heads using the water table model showed a clear correlation, leading to perspectives in terms of hydrogeological model calibration. Further experiments are however needed to fully estimate the streaming potential apparent coupling coefficient, but the use of the full SP signals for hydrogeological model calibration is a clear perspective to this work. [less ▲]

Improving groundwater flow model conceptualisation and calibration with electrical resistivity tomography and self-potential methods

Conference (2011, September 19)

Developing a conceptual model for groundwater flow requires knowledge on the distribution of geological materials, which generally comes from geological observations on outcrops and boreholes, from the ... [more ▼]

Developing a conceptual model for groundwater flow requires knowledge on the distribution of geological materials, which generally comes from geological observations on outcrops and boreholes, from the interpretation of hydraulic tests or from geophysical surveys. The identification of spatial structures in the subsurface, such as preferential flow paths created by fractured zones, is also critical in developing a reliable conceptual model but it is difficult to achieve. Geophysical methods have been widely used to map the subsurface distribution of geological materials. Recent developments in geophysics, such as the increased use of joint inversion of geophysical and hydrogeological data, have further allowed to quantify the hydraulic conductivity of geological materials. The objective of our work is to demonstrate that the electrical resistivity tomography (ERT) and the self-potential (SP) methods can improve both the conceptual model developed for groundwater flow systems and the calibration of the corresponding groundwater flow model. The use of the two geophysical methods, combined with a groundwater flow model, is presented for a fractured limestone aquifer. The self-potential method relies on passive measurements of the ambient electrical potential at ground surface or in boreholes. One of the mechanisms responsible for the measured signal measured is the transport of dissolved ions with groundwater flow. When this electrokinetic effect is the dominant contribution, the resulting signal is called the streaming potential and it contains information about groundwater fluxes that can be useful to calibrate groundwater flow models. The solution to the SP forward problem was added to the HydroGeoSphere model, which simulates 3D groundwater flow and solute transport in porous media, including fractured geological formations. With this addition, the model can calculate the self-potential signal associated with groundwater flow, given the distribution of Darcy fluxes resulting from the forward flow solution and the electrical resistivity that is, for example, outputted by ERT data inversion. Darcy fluxes are transformed into sources of electrical current by using the streaming potential coupling coefficient. This parameter can be measured either in the laboratory or in-situ from the self-potential signal between two locations where the depth of the water table is known, such as observation wells. We used here both ERT and SP to develop a conceptual model for groundwater flow in a typical carboniferous limestone syncline in South Belgium. The rolling topography in the investigated area results from a succession of calcareous valleys (synclines) and sandstone crests (anticlines). The calcareous synclines form aquifers that are very complex since they are highly fractured and even karstified. A typical calcareous syncline has a width of about 800 m and, using ERT, we could subdivide the syncline into zones of different hydraulic conductivity, based on the degree of fracturation. The zones are oriented along the axis of the syncline and their width ranges between 10 and 40 m. The ERT profiles showed that there is a highly conductive zone, in terms of electrical conductivity, near the syncline fold axis. That zone is interpreted as being highly fractured. Other conductive zones are located symmetrically along both flanks of the calcareous syncline, with respect to the syncline fold axis. The main flow direction is along the axis of the syncline, towards a nearby river. The SP raw signals also showed that, locally, there is a second flow component perpendicular to the axis of the syncline, with groundwater flowing from the flanks of the syncline towards the axis. The conceptual groundwater flow model developed here includes the zones identified with ERT, which were then incorporated into the numerical model. The SP signals were inverted with PEST to calibrate the hydraulic conductivity value of the different zones. HydroGeoSphere was therefore used to simulate first groundwater flow and then the associated self-potential signals in an iterative process. At the start of an iteration, HydroGeoSphere solves the groundwater flow equation given one particular set of hydraulic conductivities and calculates the resulting Darcy fluxes. These fluxes are transformed into sources of electrical current assuming that the electrokinetic effect is the dominant contribution of the SP signals. HydroGeoSphere then calculates the distribution of self-potential given the sources of electrical current and the distribution of electrical resistivity. The hydraulic conductivity values of the zones are then modified and the iteration continues until the model reproduces the measured self-potential signal. [less ▲]

How to incorporate prior information in geophysical inverse problems: deterministic and geostatistical approaches.

in EarthDoc - Near Surface 2011 - 17th European Meeting of Environmental and Engineering Geophysics (2011, September 13)

Many geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by ... [more ▼]

Many geophysical inverse problems are ill-posed leading to non-uniqueness of the solution. It is thus important to reduce the amount of mathematical solutions to more geologically plausible models by regularizing the inverse problem and incorporating all available prior information in the inversion process. We compare three different ways to go beyond standard Occam’s inversion for electrical resistivity tomography (ERT) using electromagnetic logging data in the context of salt water infiltration: a simple reference model, a structural constraint and a geostatistical constraint based on a vertical correlation length. Results with the traditional smoothness constraint yield small contrasts of resistivity, far from the reality revealed by borehole measurements. Incorporating prior information from boreholes clearly improves the misfit with logging data. If a good reference model can always be used, it can lead to misinterpretation if its weight is too strong. When the computation of the correlation length is possible, the geostatistical inversion gives satisfactory results everywhere in the section. In this specific case, the geostatistical approach seems to be a more robust way to incorporate prior information. The structural constraint seems to be more indicated when integrating information from other geophysical methods such as GPR or seismic. [less ▲]

Improved automatic calibration of groundwater flow models using self-potential measurements

Conference (2011, April 11)

A Saline Tracer Test Monitored with ERT to Detect Preferential Flow/Transport Paths in Limestones

in EarthDoc - Near Surface 2010 – 16th European Meeting of Environmental and Engineering Geophysics (2010, September 07)

The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test ... [more ▼]

The success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test to recover information such as the local groundwater flow direction and an estimate of the transport velocities. In this work, we used electrical resistivity tomography (ERT) to monitor a saline tracer test. This experiment was performed in fractured limestones where high transport velocities and strong dilution effects were expected. This required a continuous injection and fast ERT acquisition. Two different salt concentrations (40 and 160 g/l) were injected to deal with dilution effects. We also tested the resolution and the depth of investigation of our dipole-dipole sequence by changing the electrode spacing. Two transversal (and a longitudinal) profiles were placed every 20 m from the injection well. During the first (second) test, a maximum of -8 % (-16 %) change of electrical resistivity was observed in the nearest ERT profile while no change occurred in the other ones. We were then able to estimate the transport velocities in addition to the local groundwater flow direction even if the dilution effects were important. [less ▲]

Electrical resistivity tomography and self-potential methods as a tool for fractured aquifer characterization and monitoring

Poster (2010, January 12)

Electrical resistivity tomography (ERT) and self-potential (SP) investigations are currently conducted in carboniferous limestones in Belgium. The aims of this study are (1) to characterize and (2) to ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations are currently conducted in carboniferous limestones in Belgium. The aims of this study are (1) to characterize and (2) to monitor the groundwater flow in fractured or karstic area and (3) to use these geophysical data as a help for groundwater flow model conceptualisation and inverse calibration. Large ERT profiles (320 meters) allow us to image the electrical resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) some fractured and/or karstic zones expected to be less resistive. Data errors as well as indicators of resolution (resolution matrix, sensitivity matrix and DOI index) are analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements are performed along the electrical profiles and permit us to find some negative anomalies possibly related with groundwater preferential flow pathways. Since SP signals are related with the groundwater flow when taking into account the electrokinetic effect, we are able to estimate a first distribution of the water table along our profiles. These geophysical data concurrently with ‘ground truth’ geological and hydrogeological data give us a way to better understand the groundwater flow in limestone synclines of the Dinant Synclinorium geological structure. Still, further efforts are needed (1) to fully cover the syncline area and (2) to couple the SP, ERT and hydrogeological data through a more complex forward model of the electrokinetic effect of the self-potential method. [less ▲]

Using Electrical Resistivity Tomography and Self-potential Methods for Wells Implementations in Fractured Limestones

in EarthDoc - Near Surface 2009 – 15th European Meeting of Environmental and Engineering Geophysics (2009, September 09)

Electrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water ... [more ▼]

Electrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water wells. Large ERT profiles (640 meters) allowed us to image the resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, DOI indexes and sensitivity models were analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to narrow the possible locations given by the electrical images. Some negative anomalies possibly related to preferential flow were detected. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling programme. Wells implemented in low resistivity zones associated with SP anomalies have very high yields. Inversely, wells drilled in resistive zones or outside SP anomalies have poorer capacities. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table. [less ▲]