Computing Localized Breakthrough Curves and Velocities of Saline Tracer from Ground Penetrating Radar Monitoring Experiments in Fractured Rock

Solute tracer tests are an established method for the characterization of flow and transport processes in fractured rock. Such tests are often monitored with borehole sensors which offer high temporal sampling and signal to noise ratio, but only limited spatial deployment possibilities. Ground penetrating radar (GPR) is sensitive to electromagnetic properties, and can thus be used to monitor the transport behavior of electrically conductive tracers. Since GPR waves can sample large volumes that are practically inaccessible by traditional borehole sensors, they are expected to increase the spatial resolution of tracer experiments. In this manuscript, we describe two approaches to infer quantitative hydrological data from time-lapse borehole reflection GPR experiments with saline tracers in fractured rock. An important prerequisite of our method includes the generation of GPR data difference images. We show how the calculation of difference radar breakthrough curves (DRBTC) allows to retrieve relative electrical conductivity breakthrough curves for theoretically arbitrary locations in the subsurface. For sufficiently small fracture apertures we found the relation between the DRBTC values and the electrical conductivity in the fracture to be quasi-linear. Additionally, we describe a flow path reconstruction procedure that allows computing approximate flow path distances using reflection GPR data from at least two boreholes. From the temporal information during the time-lapse GPR surveys, we are finally able to calculate flow-path averaged tracer velocities. Our new methods were applied to a field data set that was acquired at the Grimsel Test Site in Switzerland. DRBTCs were successfully calculated for previously inaccessible locations in the experimental rock volume and the flow path averaged velocity field was found to be in good accordance with previous studies at the Grimsel Test Site.

Download Full-text

Time-lapse ground penetrating radar difference reflection imaging of saline tracer flow in fractured rock

Geophysics ◽

10.1190/geo2019-0481.1 ◽

2020 ◽

Vol 85 (3) ◽

pp. H25-H37 ◽

Cited By ~ 2

Author(s):

Peter-Lasse Giertzuch ◽

Joseph Doetsch ◽

Mohammadreza Jalali ◽

Alexis Shakas ◽

Cédric Schmelzbach ◽

...

Keyword(s):

Ground Penetrating Radar ◽

Fractured Rock ◽

Sampling Rate ◽

Time Lapse ◽

Transport Processes ◽

Local Data ◽

Tracer Injection ◽

Flow Through ◽

Reflection Imaging ◽

Ground Penetrating

The characterization of flow and transport processes in fractured rock is challenging because they cannot be observed directly and hydrologic tests can only provide sparse and local data. Time-lapse ground penetrating radar (GPR) can be a valuable tool to monitor such processes in the subsurface, but it requires highly reproducible data. As part of a tracer injection experiment at the Grimsel Test Site (GTS) in Switzerland, borehole reflection GPR data were acquired in a time-lapse survey to monitor saline tracer flow through a fracture network in crystalline rock. Because the reflections from the tracer in the sub-mm fractures appear extremely weak, a differencing approach has been necessary to identify the tracer signal. Furthermore, several processing steps and corrections had to be applied to meet the reproducibility requirements. These steps include (1) single-trace preprocessing, (2) temporal trace alignment, (3) correction of sampling rate fluctuations, (4) spatial trace alignment, (5) spike removal, and (6) postprocessing procedures applied to the difference images. This allowed successful tracer propagation monitoring with a clear signal that revealed two separate tracer flow paths. The GPR results are confirmed by conductivity meters that were placed in boreholes in the GTS. If sufficient data processing is applied, GPR is shown to be capable of resolving tracer flow through sub-mm aperture fractures by difference reflection imaging even in challenging surroundings where many reflectors are present.

Download Full-text

Ground-penetrating-radar response to fracture-fluid salinity: Why lower frequencies are favorable for resolving salinity changes

Geophysics ◽

10.1190/1.2957893 ◽

2008 ◽

Vol 73 (5) ◽

pp. J25-J30 ◽

Cited By ~ 42

Author(s):

Georgios P. Tsoflias ◽

Matthew W. Becker

Keyword(s):

Electrical Conductivity ◽

Ground Penetrating Radar ◽

Field Experiments ◽

Fractured Rock ◽

Low Frequency ◽

Signal Amplitude ◽

Time Lapse ◽

Fracture Aperture ◽

Fracture Fluid ◽

Ground Penetrating

Time-lapse ground-penetrating-radar (GPR) surveys exploit signal-amplitude changes to monitor saline tracers in fractures and to identify groundwater flow paths. However, the relationships between GPR signal amplitude, phase, and frequency with fracture aperture and fluid electrical conductivity are not well understood. We used analytical modeling, numerical simulations, and field experiments of multifrequency GPR to investigate these relationships for a millimeter-scale-aperture fracture saturated with water of varying salinity. We found that the response of lower-frequency radar signals detects changes in fluid salinity better than the response of higher-frequency signals. Increasing fluid electrical conductivity decreases low-frequency GPR signal wavelength, which improves its thin-layer resolution capability. We concluded that lower signal frequencies, such as [Formula: see text], and saline tracers of up to [Formula: see text] conductivity are preferable when using GPR to monitor flow in fractured rock. Furthermore, we found that GPR amplitude and phase responses are detectable in the field and predictable by EM theory and modeling; therefore, they can be related to fracture aperture and fluid salinity for hydrologic investigations of fractured-rock flow and transport properties.

Download Full-text

Four-dimensional tracer flow reconstruction in fractured rock through borehole ground-penetrating radar (GPR) monitoring

Solid Earth ◽

10.5194/se-12-1497-2021 ◽

2021 ◽

Vol 12 (7) ◽

pp. 1497-1513

Author(s):

Peter-Lasse Giertzuch ◽

Joseph Doetsch ◽

Alexis Shakas ◽

Mohammadreza Jalali ◽

Bernard Brixel ◽

...

Keyword(s):

Ground Penetrating Radar ◽

Fractured Rock ◽

Time Lapse ◽

Test Site ◽

Crystalline Rock ◽

Flow Paths ◽

Tracer Injection ◽

Flow Reconstruction ◽

Fracture Apertures ◽

Ground Penetrating

Abstract. Two borehole ground-penetrating radar (GPR) surveys were conducted during saline tracer injection experiments in fully saturated crystalline rock at the Grimsel Test Site in Switzerland. The saline tracer is characterized by an increased electrical conductivity in comparison to formation water. It was injected under steady-state flow conditions into the rock mass that features sub-millimeter fracture apertures. The GPR surveys were designed as time-lapse reflection GPR from separate boreholes and a time-lapse transmission survey between the two boreholes. The local increase in conductivity, introduced by the injected tracer, was captured by GPR in terms of reflectivity increase for the reflection surveys, and attenuation increase for the transmission survey. Data processing and difference imaging was used to extract the tracer signal in the reflection surveys, despite the presence of multiple static reflectors that could shadow the tracer reflection. The transmission survey was analyzed by a difference attenuation inversion scheme, targeting conductivity changes in the tomography plane. By combining the time-lapse difference reflection images, it was possible to reconstruct and visualize the tracer propagation in 3D. This was achieved by calculating the potential radially symmetric tracer reflection locations in each survey and determining their intersections, to delineate the possible tracer locations. Localization ambiguity imposed by the lack of a third borehole for a full triangulation was reduced by including the attenuation tomography results in the analysis. The resulting tracer flow reconstruction was found to be in good agreement with data from conductivity sensors in multiple observation locations in the experiment volume and gave a realistic visualization of the hydrological processes during the tracer experiments. Our methodology was demonstrated to be applicable for monitoring tracer flow and transport and characterizing flow paths related to geothermal reservoirs in crystalline rocks, but it can be transferred in a straightforward manner to other applications, such as radioactive repository monitoring or civil engineering projects.

Download Full-text

Quantitative high-resolution observations of soil water dynamics in a complicated architecture using time-lapse ground-penetrating radar

Hydrology and Earth System Sciences ◽

10.5194/hess-19-1125-2015 ◽

2015 ◽

Vol 19 (3) ◽

pp. 1125-1139 ◽

Cited By ~ 22

Author(s):

P. Klenk ◽

S. Jaumann ◽

K. Roth

Keyword(s):

High Resolution ◽

Water Content ◽

Soil Water ◽

Ground Penetrating Radar ◽

Time Lapse ◽

Test Site ◽

Water Dynamics ◽

Soil Water Dynamics ◽

Capillary Fringe ◽

Ground Penetrating

Abstract. High-resolution time-lapse ground-penetrating radar (GPR) observations of advancing and retreating water tables can yield a wealth of information about near-surface water content dynamics. In this study, we present and analyze a series of imbibition, drainage and infiltration experiments that have been carried out at our artificial ASSESS test site and observed with surface-based GPR. The test site features a complicated but known subsurface architecture constructed with three different kinds of sand. It allows the study of soil water dynamics with GPR under a wide range of different conditions. Here, we assess in particular (i) the feasibility of monitoring the dynamic shape of the capillary fringe reflection and (ii) the relative precision of monitoring soil water dynamics averaged over the whole vertical extent by evaluating the bottom reflection. The phenomenology of the GPR response of a dynamically changing capillary fringe is developed from a soil physical point of view. We then explain experimentally observed phenomena based on numerical simulations of both the water content dynamics and the expected GPR response.

Download Full-text

3D aquifer characterization of the Hermalle-sous-Argenteau test site using crosshole ground-penetrating radar amplitude analysis and full-waveform inversion

Geophysics ◽

10.1190/geo2020-0067.1 ◽

2020 ◽

Vol 85 (6) ◽

pp. H133-H148

Author(s):

Zhen Zhou ◽

Anja Klotzsche ◽

Thomas Hermans ◽

Frédéric Nguyen ◽

Jessica Schmäck ◽

...

Keyword(s):

Electrical Conductivity ◽

Waveform Inversion ◽

Test Site ◽

Tracer Experiment ◽

Transport Processes ◽

High Porosity ◽

Low Velocity ◽

Amplitude Analysis ◽

Ground Penetrating ◽

Clay Lenses

To improve the understanding of flow and transport processes in the critical zone, high-resolution and accurate estimation of the small-scale heterogeneity is essential. Preferential flow paths related to high-porosity layers and clay lenses in gravel aquifers greatly affect flow and transport processes in the subsurface, and their high electrical contrast to their surrounding matrix and limited extent can act as low-velocity electromagnetic waveguides. In the past decade, time-domain full-waveform inversion (FWI) of crosshole ground-penetrating radar (GPR) data has shown to provide 2D decimeter-scale resolution images of relative permittivity and electrical conductivity of the subsurface, which can be related to porosity and soil texture. Most studies using crosshole GPR FWI resolved high-porosity zones that were identified by an amplitude analysis approach. But clay lenses or zones with higher electrical conductivity that act as low-velocity waveguides are hard to distinguish in the measured data and amplitude analysis because of the absence of characteristic wave-propagation features. We have investigated a set of nine crosshole GPR data sets from a test site in Hermalle-sous-Argenteau near the Meuse River in Belgium to characterize the aquifer within a decimeter-scale resolution and to improve the understanding of a previously performed heat tracer experiment. Thereby, we extend the amplitude analysis to identify two different types of low-velocity waveguides either caused by an increased porosity or a higher electrical conductivity (and higher porosity). Combining the GPR amplitude analysis for low-velocity waveguide zones with the standard FWI results provided information on waveguide zones, which modified the starting models and further improved the FWI results. Moreover, an updated effective source wavelet is estimated based on the updated permittivity starting models. In comparison with the traditional FWI results, the updated FWI results present smaller gradient of the medium properties and smaller root-mean-squared error values in the final inversion results. The nine crosshole sections are used to generate a 3D image of the aquifer and allowed a detailed analysis of the porosity distribution along the different sections. Consistent structures of the permittivity and electrical conductivity show the robustness of the updated FWI results. The aquifer structures obtained by the FWI results agree with those results of the heat tracer experiment.

Download Full-text

Hydrogeophysical characterization of transport processes in fractured rock by combining push-pull and single-hole ground penetrating radar experiments

Water Resources Research ◽

10.1002/2015wr017837 ◽

2016 ◽

Vol 52 (2) ◽

pp. 938-953 ◽

Cited By ~ 22

Author(s):

Alexis Shakas ◽

Niklas Linde ◽

Ludovic Baron ◽

Olivier Bochet ◽

Olivier Bour ◽

...

Keyword(s):

Ground Penetrating Radar ◽

Fractured Rock ◽

Transport Processes ◽

Single Hole ◽

Ground Penetrating

Download Full-text

Quantitative high-resolution observations of soil water dynamics in a complicated architecture with time-lapse Ground-Penetrating Radar

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-11-12365-2014 ◽

2014 ◽

Vol 11 (11) ◽

pp. 12365-12404

Author(s):

P. Klenk ◽

S. Jaumann ◽

K. Roth

Keyword(s):

High Resolution ◽

Water Content ◽

Soil Water ◽

Ground Penetrating Radar ◽

Time Lapse ◽

Test Site ◽

Water Dynamics ◽

Soil Water Dynamics ◽

Capillary Fringe ◽

Ground Penetrating

Abstract. High-resolution time-lapse Ground-Penetrating Radar (GPR) observations of advancing and retreating water tables can yield a wealth of information about near-surface water content dynamics. In this study, we present and analyze a series of imbibition, drainage and infiltration experiments which have been carried out at our artificial ASSESS test site and observed with surface based GPR. The test site features a complicated but known subsurface architecture constructed with three different kinds of sand. It allows studying soil water dynamics with GPR under a wide range of different conditions. Here, we assess in particular (i) the accurate determination of soil water dynamics averaged over the whole vertical extent by evaluating the bottom reflection and (ii) the feasibility of monitoring the dynamic shape of the capillary fringe reflection. The phenomenology of the GPR response of a dynamically changing capillary fringe is developed from a soil physical point of view. We then explain experimentally observed phenomena based on numerical simulations of both the water content dynamics and the expected GPR response.

Download Full-text

4D Tracer Flow Reconstruction in Fractured Rock through Borehole GPR Monitoring

10.5194/se-2020-215 ◽

2021 ◽

Author(s):

Peter-Lasse Giertzuch ◽

Joseph Doetsch ◽

Alexis Shakas ◽

Mohammadreza Jalali ◽

Bernard Brixel ◽

...

Keyword(s):

Fractured Rock ◽

Time Lapse ◽

Test Site ◽

Crystalline Rock ◽

Flow Paths ◽

Tracer Injection ◽

Flow Reconstruction ◽

Fracture Apertures ◽

Ground Penetrating ◽

Engineering Projects

Abstract. Two borehole ground penetrating radar (GPR) surveys were conducted during saline tracer injection experiments in fully-saturated crystalline rock at the Grimsel Test Site in Switzerland. The saline tracer is characterized by an increased electrical conductivity in comparison to formation water. It was injected under steady state flow conditions into the rock mass that features sub-mm fracture apertures. The GPR surveys were designed as time-lapse reflection GPR from separate boreholes and a time-lapse transmission survey between the two boreholes. The local increase in conductivity, introduced by the injected tracer, was captured by GPR in terms of reflectivity increase for the reflection surveys, and attenuation increase for the transmission survey. Data processing and difference imaging was used to extract the tracer signal in the reflection surveys, despite the presence of multiple static reflectors that could shadow the tracer reflection. The transmission survey was analyzed by a difference attenuation inversion scheme, targeting conductivity changes in the tomography plane. By combining the time-lapse difference reflection images, it was possible to reconstruct and visualize the tracer propagation in 3D. This was achieved by calculating the potential radially-symmetric tracer reflection locations in each survey and determining their intersections, to delineate the possible tracer locations. Localization ambiguity imposed by the lack of a third borehole for a full triangulation was reduced by including the attenuation tomography results into the analysis. The resulting tracer flow reconstruction was found to be in good agreement with data from conductivity sensors in multiple observation locations in the experiment volume and gave a realistic visualization of the hydrological processes during the tracer experiments. Our methodology proved to be successful for characterizing flow paths related with geothermal reservoirs in crystalline rocks, but it can be transferred in a straightforward manner to other applications, such as radioactive repository monitoring or civil engineering projects.

Download Full-text

Laboratory test of snow wetness influence on electrical conductivity measured with ground penetrating radar

Hydrology Research ◽

10.2166/nh.2009.040 ◽

2009 ◽

Vol 40 (1) ◽

pp. 33-44 ◽

Cited By ~ 11

Author(s):

Nils Granlund ◽

Angela Lundberg ◽

James Feiccabrino ◽

David Gustafsson

Keyword(s):

Electrical Conductivity ◽

Electrical Properties ◽

Laboratory Test ◽

Liquid Water ◽

Ground Penetrating Radar ◽

Wave Attenuation ◽

Radar Measurements ◽

Ground Penetrating ◽

Radar Wave ◽

The Relationship

Ground penetrating radar operated from helicopters or snowmobiles is used to determine snow water equivalent (SWE) for annual snowpacks from radar wave two-way travel time. However, presence of liquid water in a snowpack is known to decrease the radar wave velocity, which for a typical snowpack with 5% (by volume) liquid water can lead to an overestimation of SWE by about 20%. It would therefore be beneficial if radar measurements could also be used to determine snow wetness. Our approach is to use radar wave attenuation in the snowpack, which depends on electrical properties of snow (permittivity and conductivity) which in turn depend on snow wetness. The relationship between radar wave attenuation and these electrical properties can be derived theoretically, while the relationship between electrical permittivity and snow wetness follows a known empirical formula, which also includes snow density. Snow wetness can therefore be determined from radar wave attenuation if the relationship between electrical conductivity and snow wetness is also known. In a laboratory test, three sets of measurements were made on initially dry 1 m thick snowpacks. Snow wetness was controlled by stepwise addition of water between radar measurements, and a linear relationship between electrical conductivity and snow wetness was established.

Download Full-text

Inferring saline tracer transport characteristics from time-lapse GPR experiments

10.5194/egusphere-egu21-7541 ◽

2021 ◽

Author(s):

Peter-Lasse Giertzuch ◽

Alexis Shakas ◽

Bernard Brixel ◽

Joseph Doetsch ◽

Mohammadreza Jalali ◽

...

Keyword(s):

Electrical Conductivity ◽

Velocity Field ◽

Nuclear Waste ◽

Time Lapse ◽

Transport Processes ◽

Nuclear Waste Repository ◽

Tracer Transport ◽

Tracer Injection ◽

Flow And Transport ◽

Conductivity Sensor

Monitoring and characterization of flow and transport processes in the subsurface has been a key focus of hydrogeological research for several decades. Such processes can be relevant for numerous applications, such as hydrocarbon and geothermal reservoir characterization and monitoring, risk assessment of soil contaminants, or nuclear waste disposal strategies.Monitoring of flow and transport processes in the subsurface is often challenging, as they are usually not directly observable. Here, we present an approach to monitor saline tracer migration through a weakly fractured crystalline rock mass by means of Ground Penetrating Radar (GPR), and we evaluate the data quantitatively in terms of a flow velocity field and localized difference GPR breakthrough curves (DRBTC).Two comparable and repeated tracer injection experiments were performed within saturated rock on the decameter scale. Time-lapse single-hole reflection data were acquired from two different boreholes during these experiments using unshielded and omnidirectional borehole antennas. The individual surveys were analyzed by difference imaging techniques, which allowed ultimately for tracer breakthrough monitoring at different locations in the subsurface. By combining the two complimentary GPR data sets, the 3D tracer velocity field could be reconstructed.Our DRBTCs agree well with measured BTCs of the saline tracer at different electrical conductivity monitoring positions. Additionally, we were able to calculate a DRBTC for a location not previously monitored with borehole sensors. The reconstructed velocity field is in good agreement with previous studies on dye tracer data at the same research locations. Furthermore, we were able to resolve separate flow paths towards different monitoring locations, which could not be inferred from the electrical conductivity sensor data alone. The GPR data thus helped to disentangle the complex flow field through the fractured rock.Out technique can be adapted to other use cases such as 3D monitoring of fluid migration (and thus permeability enhancement) during hydraulic stimulation and tracing fluid contaminants &#8211; e.g. for nuclear waste repository monitoring.

Download Full-text