Ground-penetrating Radar Observations Research Articles

Ground penetrating radar for moisture assessment in railway tracks: An experimental investigation

The health monitoring of railway networks offers a means to ensuring high-quality service, avoiding safety risks, and optimally planning maintenance actions to minimize life-cycle costs. Monitoring of the substructure is particularly linked to the tracking of degradation, which often stems from water infiltration, causing moisture accumulation in the underlying ballast layers. Such deterioration incurs substantial expenses in the order of several million per annum, while reducing the useful life of the track infrastructure. In optimizing management, it is important to improve detection schemes via affordable and non-invasive procedures. Such a solution is found in use of Ground Penetrating Radar (GPR) technology, which employs non-invasive radar pulses to map the subsurface, and possibly detect water infiltration. Train-based GPR systems thus offer tremendous potential for the development of preventive and automated monitoring of railway network infrastructure. Nevertheless, despite previous efforts in this direction, the technology remains relatively under-explored and lacks comprehensive studies to establish its suitability for this task. Moreover, there is no consensus on a standardized procedure for automatic inference of reliable indicators of railway health from GPR observations. In this work, we report on an extensive experimental analysis conducted on a controlled railway track, built by the Swiss Federal Railways, for this campaign. The humidity condition of the railway track was artificially altered to reach different levels of water content and GPR measurements were gathered under the varying conditions, with ground truth assessed through lab tests on collected samples. GPR data with complete ground truth labels deliver a rare benchmark, which can enhance understanding of this technology. Our findings show that GPR systems can effectively detect moisture infiltration in railway tracks, although several challenges are to be addressed for the development of accurate, automated procedures.

A Time Series of Snow Density and Snow Water Equivalent Observations Derived From the Integration of GPR and UAV SfM Observations

Snow depth can be mapped from airborne platforms and measured in situ rapidly, but manual snow density and snow water equivalent (SWE) measurements are time consuming to obtain using traditional survey methods. As a result, the limited number of point observations are likely insufficient to capture the true spatial complexity of snow density and SWE in many settings, highlighting the value of distributed observations. Here, we combine measured two-way travel time from repeat ground-penetrating radar (GPR) surveys along a ∼150 m transect with snow depth estimates from UAV-based Structure from Motion Multi-View Stereo (SfM-MVS) surveys to estimate snow density and SWE. These estimates were successfully calculated on eleven dates between January and May during the NASA SnowEx21 campaign at Cameron Pass, CO. GPR measurements were made with a surface-coupled Sensors and Software PulseEkko Pro 1 GHz system, while UAV flights were completed using a DJI Mavic 2 Pro platform and consisted of two orthogonal flights at ∼60 m elevation above ground level. SfM-MVS derived dense point clouds (DPCs) were georeferenced using eight ground control points and evaluated using three checkpoints, which were distributed across the ∼3.5 ha study plot containing the GPR transect. The DPCs were classified to identify the snow surface and then rasterized to produce snow-on digital surface models (DSMs) at 1 m resolution. Snow depths on each survey date were calculated by differencing these snow-on DSMs from a nearly snow-off DSM collected near the end of the melt season. SfM-derived snow depths were evaluated with independent snow depth measurements from manual probing (mean r2 = 0.67, NMAD = 0.11 m and RMSE = 0.12 m). The GPR-SfM derived snow densities were compared to snow density measurements made in snowpits (r2 = 0.42, NMAD = 39 kg m−3 and RMSE = 68 kg m−3). The integration of SfM and GPR observations provides an accurate, efficient, and a relatively non-destructive approach for measuring snow density and SWE at intermediate spatial scales and over seasonal timescales. Ongoing developments in snow depth retrieval technologies could be leveraged in the future to extend the spatial extent of this method.

Comparison of regolith physical and chemical characteristics with geophysical data along a climate and ecological gradient, Chilean Coastal Cordillera (26 to 38° S)

Abstract. We combine geophysical observations from ground-penetrating radar (GPR) with regolith physical and chemical properties from pedons excavated in four study areas spanning 1300 km of the climate and ecological gradient in the Chilean Coastal Cordillera. Our aims are the following: (1) to relate GPR observations to depth-varying regolith physical and weathering-related chemical properties in adjacent pedons and (2) to evaluate the lateral extent to which these properties can be extrapolated along a hillslope using GPR observations. Physical observations considered include regolith bulk density and grain size distribution, whereas chemical observations are based on major and trace element analysis. Results indicate that visually determined pedolith thickness and the transition from the B to C horizons generally correlate with maximums in the 500 and 1000 MHz GPR envelope profiles. To a lesser degree, these maximums in the GPR envelope profiles agree with maximums in weathering-related indices such as the chemical index of alteration (CIA) and the chemical index of mass transfer (τ) for Na. Finally, we find that upscaling from the pedon to hillslope scale is possible with geophysical methods for certain pedon properties. Taken together, these findings suggest that the GPR profiles down hillslopes can be used to infer lateral thickness variations in pedolith horizons in different ecologic and climate settings, and to some degree the physical and chemical variations with depth.

Detection of Leakage Areas in an Earth Embankment from GPR Measurements and Permeability Logging

Ground penetrating radar (GPR) is a nondestructive method allowing the improvement of our knowledge of civil engineering structures. In particular, this method may be a nondestructive efficient tool for dike diagnosis and complete classical geotechnical methods. In this paper, we present GPR observations obtained on an earth embankment (crest and sloped paved revetment) in bad condition and located on the lateral canal of the Loire river (Saint Firmin, 80 km South East of Orléans). These measurements are combined with corings, visual inspection, and permeability logging performed with an updated drilling system, the Perméafor. This survey leads (i) to the detection of decompressed zones associated with leakage areas visible at the foot of the downstream slope and (ii) to the location of potentials voids underneath the paved revetment. This multidisciplinary approach complied with the dike inspection methodology proves its efficiency for the assessment of earth embankments.

Glacier meltwater flow paths and storage in a geomorphologically complex glacial foreland: The case of the Tapado glacier, dry Andes of Chile (30°S)

The Tapado catchment is located in the upper Elqui river basin (4000–5550 m) in northern Chile. It comprises the Tapado glacial complex, which is an assemblage of the Tapado glacier and the glacial foreland (debris-covered glacier, rock glacier, and moraines). Although the hydrological functioning of this catchment is poorly known, it is assumed to actively supply water to the lower semi-arid areas of the Elqui river basin. To improve our knowledge of the interactions and water transfers between the cryospheric compartment (glacier, debris-covered glacier, and rock glacier) and the hydrological compartment (aquifers, streams), the results of monitoring of meteorological conditions, as well as discharge, conductivity and temperature of streams and springs located in the Tapado catchment were analyzed. The hydrological results are compared to results inferred from a ground penetrating radar (GPR) survey of the underground structure of the glacial foreland. Water production from the Tapado glacier was shown to be highly correlated with daily and monthly weather conditions, particularly solar radiation and temperature. The resulting daily and monthly streamflow cycles were buffered by the glacial foreland, where underground transfers took place through complex flow paths. However, the development of a thermokarst drainage network in a portion of the glacial foreland enabled rapid concentrated water transfers that reduced the buffer effect. The glacial foreland was shown to act as a reservoir, storing water during high melt periods and supplying water to downstream compartments during low melt periods. GPR observations revealed the heterogeneity of the internal structure of the glacial foreland, which is composed of a mixture of ice and rock debris mixture, with variable spatial ice content, including massive ice lenses. This heterogeneity may explain the abovementioned hydrological behaviors. Finally, calculation of a partial hydrological budget confirmed the importance of the Tapado catchment in supplying water to lower areas of the Elqui river basin. Water production from, and transfer through, cryospheric compartments, and its subsequent interactions with hydrological compartments are key processes driving the summer water supply from the Tapado catchment.

Periodic mapping of reinforcement corrosion in intrusive chloride contaminated concrete with GPR

This paper presents an experimental effort for developing a novel reinforcement corrosion monitoring technique based on Ground Penetrating Radar (GPR). Experiments were carried out to periodically monitor the accelerated reinforcement corrosion process with GPR. The data were processed in both time and time–frequency domains to investigate the tendencies of GPR signal attribute changes related with corrosion, moisture and chloride contamination. Data processing methods were proposed to visualize the reinforcement corrosion and chloride distribution with target specified signal attribute mapping. Half-Cell Potential (HCP) and Laser-Induced Breakdown Spectroscopy (LIBS) methods were employed to verify the GPR observations.

Glacier meltwater flow paths and storage in a Geomorphologically complex glacial foreland : the case of the Tapado glacier, dry Andes of Chile (30°S)

Summary The Tapado catchment is located in the upper Elqui river basin (4000–5550 m) in northern Chile. It comprises the Tapado glacial complex, which is an assemblage of the Tapado glacier and the glacial foreland (debris-covered glacier, rock glacier, and moraines). Although the hydrological functioning of this catchment is poorly known, it is assumed to actively supply water to the lower semi-arid areas of the Elqui river basin. To improve our knowledge of the interactions and water transfers between the cryospheric compartment (glacier, debris-covered glacier, and rock glacier) and the hydrological compartment (aquifers, streams), the results of monitoring of meteorological conditions, as well as discharge, conductivity and temperature of streams and springs located in the Tapado catchment were analyzed. The hydrological results are compared to results inferred from a ground penetrating radar (GPR) survey of the underground structure of the glacial foreland. Water production from the Tapado glacier was shown to be highly correlated with daily and monthly weather conditions, particularly solar radiation and temperature. The resulting daily and monthly streamflow cycles were buffered by the glacial foreland, where underground transfers took place through complex flow paths. However, the development of a thermokarst drainage network in a portion of the glacial foreland enabled rapid concentrated water transfers that reduced the buffer effect. The glacial foreland was shown to act as a reservoir, storing water during high melt periods and supplying water to downstream compartments during low melt periods. GPR observations revealed the heterogeneity of the internal structure of the glacial foreland, which is composed of a mixture of ice and rock debris mixture, with variable spatial ice content, including massive ice lenses. This heterogeneity may explain the abovementioned hydrological behaviors. Finally, calculation of a partial hydrological budget confirmed the importance of the Tapado catchment in supplying water to lower areas of the Elqui river basin. Water production from, and transfer through, cryospheric compartments, and its subsequent interactions with hydrological compartments are key processes driving the summer water supply from the Tapado catchment.

Pavement structure segmentation method based on results derived from ground-penetrating radar data

This paper presents a pavement structure segmentation method based on the results derived from ground-penetrating radar (GPR) data. Surface type, layer thickness, base/sub-base type, shortest segment and outlier point criteria are developed to perform the segmentation. A boundary comparing method using t-tests is developed to perform layer thickness criteria. The value t = 4 is suggested for the HMA (hot mix asphalt) or aggregate layer; and t = 6 is suggested for Portland cement concrete and cement-treated base. During segmentation, segments that are shorter than the minimum length of 100 m are merged into adjacent segments. Furthermore, the flow charts of pavement structure segmentation as well as a procedure for automated segmentation are detailed. Proposed rules for fixing segment boundaries between GPR observations are also presented. Several segmentation examples are calculated by the automated procedure using the data from a pilot project in California. The results show that the method presented in this paper is reasonable for pavement structure segmentation based on GPR data.

Outcrop analog for Trenton–Black River hydrothermal dolomite reservoirs, Mohawk Valley, New York

Geochemical analysis and field relations of linear dolomite bodies occurring in outcrop in the Mohawk Valley of New York suggest that the area has undergone a significant fault-related hydrothermal alteration. The dolomite occurs in the Lower Ordovician Tribes Hill Formation, which is regionally a Lower Ordovician shaley limestone with patchy dolomitization. The outcrop has an en echelon fault, fracture, and fold pattern. A three-dimensional (3-D) ground-penetrating radar (GPR) survey of the quarry floor has helped to map out faults, fractures, anticlines, synclines, and the extent of dolomitization. Most of the dolomitization occurs in fault-bounded synclines or sags flanked by anticlines. The dolomite structures are highly localized, occurring around faults, and are absent away from the faults and fractures. Trenches cut across the outcrop help relate offset along faults to the overall geometry of the dolomitized bodies. Geochemical analysis, although helpful in characterizing the conditions of dolomitization, does not define its origin absolutely. This study uses fluid inclusions, stable isotopes, 3-D GPR, core analysis, and surficial observations, which all show a link between faulting, dolomitization, and other hydrothermal alteration. Although the outcrop is much too small and shallow to act as a producing gas field, it serves as a scaled analog for the Trenton–Black River hydrothermal dolomite reservoirs of eastern United States. It may therefore be studied to help petroleum geologists characterize existing gas plays and prospect future areas of exploration.

Field GPR monitoring of biostimulation in saturated porous media

Abstract Time-lapse borehole radar was used to monitor a sandy aquifer with dielectric property changes hypothesized to result from biomass growth and biodegradation of hydrocarbons. A blend of gasoline and ethanol was released below the water table at Canadian Forces Base (CFB) Borden, Canada, and a source of oxygen was used for cyclic stimulation of microbial activity over a period of two years. A dense grid of fourteen borehole ground-penetrating radar (GPR) pairs monitoring the bioactive region showed 200 MHz radar wave velocity changes of ± 4 % and signal attenuation changes of ± 25 % during three cycles of variable biostimulation. GPR signal changes correlated spatially and temporally to independent measurements of groundwater velocity flow changes and geochemical variations that occurred in response to microbial activity. Greater relative changes in radar wave velocity of propagation were observed in the region of enhanced bacterial stimulation, adjacent to the oxygen release wells, and along the path of the hydrocarbon plume where biomass growth was the greatest. In the zone of maximum biological activity, radar wave velocity decreased during two cycles of enhanced biostimulation, whereas it increased during the intervening period of low-level biostimulation. Monitored regions at the fringe of the plume and farther down-gradient of the oxygen source, where biostimulation was less or absent, exhibited GPR signal changes of lesser magnitude and consistency. Spatial and temporal variabilities of GPR observations indicate a high degree of transient heterogeneity within the relatively homogeneous Borden aquifer as a result of hydrocarbon biodegradation. Mechanisms postulated to cause the dielectric property changes observed in the radar data are biogenic gas formation, mineral dissolution/precipitation, and direct biomass growth. Geochemical data, in-situ groundwater velocity measurements and biomass quantification suggest biomass formation in the pore space as a plausible mechanism for the changes observed in the aquifer. Therefore, at 200 MHz frequency, biomass formation is observed to cause a decrease of EM wave velocity of propagation suggesting an increase of the bulk dielectric constant of the water saturated medium. This finding is in agreement with published laboratory-scale GPR monitoring of microbial growth in porous media. This study shows that time-lapse GPR along with supporting biogeochemical observations can be used for monitoring of biological activity at the field scale.

Spatial relationship between clay content and geophysical data

AbstractAn important attribute for soil use is clay content, because it affects the water-holding capacity and hydraulic properties of a soil. Soil surveys are time-consuming, labour-intensive and costly, while geophysical methods, such as Electromagnetic Induction (EMI) and Ground Penetrating Radar (GPR), offer a non-invasive and non-destructive approach to mapping soil features. The objective of this paper is to assess the spatial relationship between clay content and geophysical data. The EMI and GPR data and soil cores were collected and analysed in a field of about 3 ha size in Rutigliano, Bari, in SE Italy. The EMI data and clay contents were interpolated using ordinary cokriging. The GPR data were pre-processed and the envelope of the filtered GPR data was used to produce 3Dmaps of the kriged estimates. The correlation with clay content was large and positive for EMI, whereas it was negative for the GPR measurement. In this work, a combination of geostatistical and statistical analysis has shown a significant correlation between the EMI and GPR observations. Estimation of the mathematical function relating these two groups of variables requires a multivariate approach of non-stationary geostatistics.

Investigating multi‐polarization GPR wave transmission through thin layers: Implications for vertical fracture characterization

We investigate the controls governing the response of ground‐penetrating radar (GPR) wave transmission through thin layers in order to explore the use of variable polarization GPR signals for remote characterization of fracture aperture and fluid‐fill. We employ an experimental setting that provides controlled observations of the effects of thin‐layer properties to the transmitted GPR wavefield. GPR signals of variable polarization, variable angle of incidence, and variable frequency are transmitted through an air‐ and water‐filled layer of variable thickness. We observe that at high angles of incidence, variable polarization GPR signals display characteristic and quantifiable phase and amplitude responses that are related to thin‐layer properties. The GPR data are in agreement to analytical solutions of plane‐wave oblique‐incidence transmission through layered media. We conclude that multi‐polarization GPR observations can be exploited to determine fracture properties. This work has implications in the remote determination of fractured formation anisotropic properties, such as fluid‐flow.

Ground-penetrating radar imaging of carbonate mound structures and implications for interpretation of marine seismic data

Images of carbonate mound structures in Denmark and Sweden are obtained from ground-penetrating radar (GPR) reflections and outcrop analysis. The GPR data are collected in limestone quarries and provide a depth penetration of about 10 m (33 ft) and a vertical resolution of about 0.5 m (1.6 ft). The mounds are part of the northwest European Upper Cretaceous–Danian (lower Paleocene) Chalk Group, and they are similar to each other in terms of architecture, spatial distribution, and size, with widths and lengths of 30–60 m (100–200 ft) and heights of 5–10 m (16–33 ft).Seismic reflection images from the sea of Kattegat between Sweden and Denmark and the North Sea show large (500–1000-m [1640–3280-ft]-wide and 50–100-m [160–330-ft]-high) moundlike carbonate structures. Interpretations of such large mounds conflict with the GPR and outcrop observations. We address these conflicting observations. We construct realistic reference models of carbonate mound complex geometries based on the results of the GPR measurements and outcrop analysis and calculate synthetic seismic sections for the models. The modeling results show that the size of individual mounds is below the seismic resolution of 10–25 m (33–82 ft), and that interference effects caused by stacks of mounds may explain the observed large moundlike structures.Our findings are important for the interpretation of seismic images of carbonate mound structures. Carbonate mound buildups may form traps, and correct seismic interpretation of mound complex geometry may be essential for evaluation of the nature and reservoir potential of such structures.

Ground penetrating radar for determining volumetric soil water content; results of comparative measurements at two test sites

Ground penetrating radar (GPR) can provide information on the soil water content of the unsaturated zone in sandy deposits via measurements from the surface, and so avoids drilling. Proof of this was found from measurements of radar wave velocities carried out ten times over 13 months at two test sites in the Netherlands. At these same locations and on the same dates, soil water content was measured in access tubes using a capacitance probe. Comparison of GPR and capacitance probe observations revealed that: 1. the inferred absolute values of soil water content agree well. This is remarkable because the soil water content is deduced entirely differently for the two methods. 2. Seasonal fluctuations in soil water content established for different (general) depth zones of radar waves correlate well with the fluctuations observed in the access tubes. 3. The various methods to determine the propagation velocities of radar waves are complementary; together they produce a realistic and reasonably complete image of the vertical distribution of the soil water content of the entire unsaturated zone. High-frequency (200 MHz) direct groundwaves and refracted waves constitute a particularly attractive complementary combination, which provides information on consecutive shallow zones in the underground, i.e. the zones of major soil water content fluctuations. 4. Lateral variations established at one of the test sites where several access tubes have been placed in a transect also follow from the GPR measurements along that profile; this non-destructive determination of soil water content in practically continuous and detailed sections is one of the great assets of GPR, opening the way to mapping preferential soil water flow paths.