Ground Penetrating Radar Signal Attenuation Research Articles

GPR and EMI Characterization of the Hyperarid Study Site of Yungay, Chile: Implications of Applying Geophysical Methods on Mars

AbstractTo unequivocally discover the actual presence of life or even of near surface liquid water on extraterrestrial planetary bodies would be a key scientific breakthrough for humankind. For this reason, studying similar environments on Earth is essential to understanding the processes shaping such extraterrestrial objects. The Yungay area in the Chilean part of the Atacama Desert is deemed to be particularly suitable as a terrestrial analog of Mars (TAM). In this study, we deployed multi‐frequency ground penetrating radar (GPR) and a six‐coil electromagnetic induction (EMI) system with a maximum depth of investigation of 1.8 m over an area of 0.66 hectares (110 x 60 m). By applying a LOWESS algorithm to the GPR envelope data, we aimed to extrapolate the strongest amplitudes indicating physical contrasts to 3D. The results were constrained with two existing pits 100 m apart. Whereas clay content was mostly responsible for GPR signal attenuation, changes in texture and stratigraphy were linked with strong amplitude reflections. EMI showed very low apparent electrical conductivity (ECa) values between 0 and 5 mS/m. The ECa variability could be linked to changes in clay content with depth. This agreed with the surface obtained from the LOWESS algorithm. Although soil samples are still necessary to constrain the measured signals, we showed the benefits of applying geophysics for large‐scale characterization and can conclude that these two methods are suitable for such hyperarid TAM environments. A similar routine if applied on the surface of Mars could deliver promising results for similar characteristics.

Different Characteristics of Radar Signal Attenuation Depending on Concrete Condition of Bare Bridge Deck

This paper examined the attenuation method, which is used abroad, to derive a method to evaluate the condition of bare concrete bridge decks using ground penetrating radar (GPR) technology. For this purpose, 12 GPR surveys of 10 bridge decks in public service from the beginning of its service to 25 years of service were carried out to examine the attenuation characteristics of GPR signals under various concrete conditions. The survey revealed the signal below the top rebar of concrete bridge deck under the condition of using de-icing chlorides for snow removal was not clear. Therefore, using the receiving wave signal from top rebar was reasonable when attenuation of GPR signal was to be used for bridge deck evaluation. Examining the signal attenuation by the condition of concrete, dispersion of the attenuated signal was overall large in the initial performance period. However, since this is not due to deterioration, exclusion of the large dispersion from the evaluation of bridge deck condition was desirable. The attenuation size was linearly proportional to its two way travel time (signal transmission time), i.e., the depth of the top rebar, in a sound bridge deck. Also, the dispersion of the attenuation was small and symmetric to the linear regression line. If the bridge deck was maintained adequately, the linear regression correlation was also maintained similarly for the next several years. However, if deterioration occurred to be accompanied by increased attenuation and dispersion, correlation coefficient of the linear regression line declined. Nonetheless, the upper value of the attenuation representing sound concrete condition was still tended to be linearly proportional to two way travel time. Since the attenuation quantity could result in considerable debilitation just by the construction error, it is necessary to deal with the depth-error of top rebar for evaluating the concrete condition by using the GPR signal attenuation. Calibration was carried out by deriving a linear regression line for the signal two way travel time and the upper 90th percentile values of the attenuation obtained from the top bar position of the bridge and then removing it from the total attenuation.

Nondestructive Evaluation of Concrete Bridge Decks with Automated Acoustic Scanning System and Ground Penetrating Radar.

Delamanintions and reinforcement corrosion are two common problems in concrete bridge decks. No single nondestructive testing method (NDT) is able to provide comprehensive characterization of these defects. In this work, two NDT methods, acoustic scanning and Ground Penetrating Radar (GPR), were used to image a straight concrete bridge deck and a curved intersection ramp bridge. An acoustic scanning system has been developed for rapid delamination mapping. The system consists of metal-ball excitation sources, air-coupled sensors, and a GPS positioning system. The acoustic scanning results are presented as a two-dimensional image that is based on the energy map in the frequency range of 0.5–5 kHz. The GPR scanning results are expressed as the GPR signal attenuation map to characterize concrete deterioration and reinforcement corrosion. Signal processing algorithms for both methods are discussed. Delamination maps from the acoustic scanning are compared with deterioration maps from the GPR scanning on both bridges. The results demonstrate that combining the acoustic and GPR scanning results will provide a complementary and comprehensive evaluation of concrete bridge decks.

Method for Compensating Signal Attenuation Using Stepped-Frequency Ground Penetrating Radar.

Ground penetrating radar (GPR) is a subsurface remote sensor that allows the user to detect, classify, and identify the buried target and structures. The radar signals are rapidly attenuated as they propagate into the ground; therefore, attenuation compensation is necessary for the visualization of the buried targets from GPR data. In this work, we developed a novel attenuation compensation approach based on the recently developed stepped-frequency continuous wave (SFCW) GPR system, which is a frequency domain sampling system with improved performance in dynamic range, sensitivity, and anti-interference ability. Because the regularly used time-varying gain function for compensating the attenuation of impulse GPR data does not make full use of the advancement of the SFCW modulation, an alternative procedure is proposed herein. The new approach is based the SFCW mechanism, and aims at improving the visualization of deeper targets by compensating the SFCW GPR signal attenuation. We first present the attenuation mode of the SFCW GPR echo, from which an inverse attenuation function is derived to compensate the amplitude loss. For the field measurement where the theoretical inverse attenuation function is difficult to achieve, we introduced a pseudo time–frequency distribution for estimating the inverse attenuation function. A procedure for amplitude attenuation has also been developed. Testing with both synthetic and experimental data return a good reconstruction of the signal amplitude, subsequently improving the ability for visualizing and detecting deeper targets.

GPR signal attenuation through fragmented rock

This paper presents results on detecting large rock fragments in rock piles using ground penetrating radar (GPR). The authors are researching sensors to augment the image provided to the remote operator, assisting with detection of oversized fragments beneath the surface. The attenuation of the GPR signal was calculated over the band 0–3 GHz from the signal energy in the frequency domain. The results show that attenuation measurement allowed the samples which included a large fragment to be differentiated from samples only containing smaller rock fragments. By combining velocity analysis with attenuation the rock samples can be further differentiated from samples containing air pockets or irregular surfaces. The combination of attenuation and signal velocity allowed prediction rates to detect the presence of large rock fragments over 93% correctly in laboratory testing.

A review of ground-penetrating radar studies related to peatland stratigraphy with a case study on the determination of peat thickness in a northern boreal fen in Quebec, Canada

Ground-penetrating radar (GPR) is a non-intrusive geophysical observation method based on propagation and reflection of high-frequency electromagnetic waves in the shallow subsurface. The vertical cross-sectional images obtained allow the identification of thickness and lithologic horizons of different media, without destruction. Over the last decade, several studies have demonstrated the potential of GPR. This paper presents a review of recent GPR applications to peatlands, particularly to determine peat stratigraphy. An example study of acquisition and comparison of peatland soil thickness of a fen-dominated watershed located in the James Bay region of Quebec, using (1) a meter stick linked to a GPS RTK and (2) a GSSI GPR, is given. A coefficient of determination ( r2) of 56% was obtained between the ordinary krigings performed on data gathered using both techniques. Disparities occurred mainly in the vicinity of ponds which can be explained by the attenuation of GPR signal in open water. Despite these difficulties – the higher time required for analysis and the error margin – it seems more appropriate to use a GPR, instead of a graduated rod linked to a GPS, to measure the peat depths on a site like the one presented in this study. Manual measurements, which are user-dependent in the context of variable mineral substrate densities and with the presence of obstacles in the substrate, may be more subjective.

Time–frequency analysis of GPR data to investigate the damage of monumental buildings

The presence of particular microclimatic conditions inside monumental buildings is responsible for bio-deterioration processes. In many cases, efflorescence and moulds are visible on the facades of several monuments of historical importance. In many other cases, the effects of decay processes are not visible, thus making difficult the diagnosis and the consequent setup of effective rehabilitation and preservation interventions, especially in the presence of a complex geometry and/or a large variability of construction materials. In such cases, a valuable contribution could be provided by geophysical methods (such as electrical resistivity, electromagnetic conductivity, ground-penetrating radar (GPR), etc), which have been proved to be successful tools for sub-surface investigation and characterization of historical buildings. In old monumental buildings, the masonry structures frequently exhibit cracks, voids, detachments and high moisture contrasts that can give rise to reflection events in radar signals. However, the complexity of the geometry and the structural heterogeneity that characterize these old structures often make the GPR results difficult to analyse and interpret. In particular, the spatial variation in GPR signal attenuation can provide important information about the electrical properties of the investigated materials that, in turn, can be used to assess the physical parameters associated with damage. In this paper, we propose an approach that analyses the data in the form of ?frequency maps? to evidence absorption losses probably linked to higher moisture content. Two real case histories back up the proposed method.

Identification of Underground Karst Features using Ground‐Penetrating Radar in Northern Yucatán, México

Northern Yucatán is a dry tropical area where limestone karst terrain supports a subdeciduous forest that is critical for sustaining the local economy of Mayan people. The 5‐ to 10‐m‐deep vadose zone is characterized by shallow soils (<30 cm thick) with frequent rock outcrops overlying limestone bedrock, which contains the aquifer. This limestone has two important characteristics: (i) lithologic properties that change with depth and (ii) numerous dissolution cavities ranging from small pores to caves, some of them filled with soil (soil pockets). Due to soil shallowness, differences in rock properties are relevant because they restrict or favor root growth; in particular, caves, crevices, and soil pockets allow preferential movement of soil, water, and roots. These features may be important for water and nutrient availability, especially for tree species, but they have not been investigated within an ecological context. We studied the capability of ground‐penetrating radar (GPR) for identifying various limestone layers—the laja (0.3–2.5 m), the sascab (2.5–5 m), and the coquina (5–9 m)—and dissolution cavities. Research was conducted in a limestone quarry so that radar records could be compared with karst features revealed as blasting and rock extraction advanced. Radar records obtained using a 200‐MHz antenna were generally of good interpretative quality; observation depths ranged from 3 to 5 m. In areas that lacked a soil mantle, the interface between the laja and the sascab was clearly identified by GPR at about 2 m. However, the sascab–coquina, and coquina–aquifer boundaries could not be identified. Areas of deep soils (>1 m), microrelief (mounds and plains), and large soil pockets within the limestone matrix were also identified. Main sources of GPR signal attenuation were suspected to be the higher clay and water contents of soil material contained in underlying cavities. As a noninvasive tool, GPR can help to determine properties of the limestone and its dissolution features that are critical to vadose zone–forest interactions.

Complex dielectric permittivity measurements from ground‐penetrating radar data to estimate snow liquid water content in the pendular regime

Monitoring the snow water equivalent (SWE) is critical to effective management of water resources in many parts of the world that depend on the mountain snowpack for water storage. There are currently no methods to remotely sense SWE with accuracy over large lateral distances in the steep and often forested terrain of mountain basins. Previous studies have shown that measurements of ground‐penetrating radar (GPR) velocity can provide accurate estimates of SWE in dry snow. Introduction of liquid water into the snowpack results in a three‐phase system that cannot be accurately characterized with GPR velocity alone. We show that measuring the frequency‐dependent GPR signal attenuation and velocity provides a direct estimate of the complex dielectric permittivity. Because the imaginary component is a function only of liquid water content, we can utilize both the real and imaginary components of the permittivity to estimate liquid water content, snow density, and SWE using existing empirical relationships that are valid in the pendular regime. We tested this new method at two field sites and found that the estimates were accurate to within 12% of gravimetric methods in both a moist and a dry snowpack. GPR has the potential to provide SWE estimates across large lateral distances over a broad range of snow conditions.

GPR Attenuation and Scattering in a Mature Hydrocarbon Spill: A Modeling Study

Subsurface contamination by light nonaqueous‐phase liquids (LNAPLs) is a pressing environmental issue with many industrial sites having some degree of near‐surface pollution. Noninvasive, geophysical investigation methods, such as ground penetrating radar (GPR), have become increasingly popular, but their use has tended to be restricted to plume “mapping” and recent contamination events only. Mature LNAPL contamination, where the hydrocarbons undergo significant alteration and biodegradation, complicates the electrophysical properties of the subsurface, making GPR data interpretation more difficult. This is particularly true in shallow, coastal environments where LNAPLs form a disaggregated, smeared zone associated with the seasonally changing groundwaters. Therefore, any method that can improve data interpretation has a significant practical benefit in these complex environments. By using dielectric material property analysis, empirical mixing model evaluation, and three‐dimensional, finite‐difference time‐domain (FDTD) modeling, it has been possible to investigate the nature and spectral content of GPR signal attenuation and scattering within the vadose or smeared zone of a mature LNAPL‐contaminated site. Using FDTD models to simulate real subsurface conditions, a comparison between the GPR response of “clean” and contaminated environments has been made where the subsurface materials contain different LNAPL–porewater saturations and contaminant geometries. The results show that uniform mixtures, disaggregated scattering mixtures, and stratified pools of contaminated material all exhibit different, yet characterizable, temporal and spectral GPR responses and that the recorded data can provide new insights into the mode of contaminant distribution and saturation as long as the incident field of the GPR system can be determined accurately.

Evaluating LNAPL contamination using GPR signal attenuation analysis and dielectric property measurements: Practical implications for hydrological studies

Groundwater and sub-surface contamination by Light Non-Aqueous Phase Liquids (LNAPLs) is one of the industrial world's most pressing environmental issues and a thorough understanding of the hydrological, physical and bio-chemical properties of the sub-surface is key to determining the spatial and temporal development of any particular contamination event. Non-invasive geophysical techniques (such as electrical resistivity, electromagnetic conductivity, Ground-Penetrating Radar, etc.) have proved to be successful sub-surface investigation and characterisation tools with Ground-Penetrating Radar (GPR) being particularly popular. Recent studies have shown that the spatial/temporal variation in GPR signal attenuation can provide important information on the electrical properties of the sub-surface materials that, in turn, can be used to assess the physical and hydrological nature of the pore fluids and associated contaminants. Unfortunately, a high percentage of current LNAPL-related GPR studies focus on contaminant mapping only, with little emphasis being placed on characterising the hydrological properties (e.g., determining contaminant saturation index, etc.). By comparing laboratory-based, dielectric measurements of LNAPL contaminated materials with the GPR signal attenuation observed in both contaminated and ‘clean’ areas of an LNAPL contaminated site, new insights have been gained into the nature of contaminant distribution/saturation and the likely signal attenuation mechanisms. The results show that, despite some practical limitations of the analysis technique, meaningful hydrological interpretations can be obtained on the contaminant properties, saturation index and bio-degradation processes. A generalised attenuation/saturation model has been developed that describes the physical and attenuation enhancement characteristics of the contaminated areas and reveals that the most significant attenuation is related to smeared zone surrounding the seasonally changing water table interface. It is envisaged that the model will provide a basis for the interpretation of GPR data from analogous LNAPL contaminated sites and provide investigators with an appreciation of the merits and limitations of GPR-based, attenuation analysis techniques for hydrological applications.