Capability Of Ground Penetrating Radar Research Articles

Nondestructive evaluation of wooden logs using ground penetrating radar

This paper addresses the crucial need for a non-invasive method for precisely detecting the subsurface defects in wooden logs. The method utilises ground penetrating radar (GPR) technology which has the potential to be implemented in real time in saw mills in the near future. Internal defects such as knots, decays and embedded metals are of great concern for lumber production. Nondestructive scanning of logs using GPR to detect defects in logs prior to sawing can greatly increase the productivity and yield of high value defect-free lumber by optimal orientation of the saw blade, and prevent damage to saw blade from embedded metals. Also, GPR offers high speed scanning capability which is needed for future online implementation in saw mills. In this research, the scanned GPR data have been analysed to investigate the capability of GPR to detect subsurface defects such as metals, decays and knots. This paper presents the comprehensive results from GPR scanning of several wooden logs and shows comparisons with actual condition inside the log observed after sawing the log. The advantages of the GPR technique, experimental setup and data processing used for detection of subsurface defects in logs are explained in detail. This study has shown that GPR can be used to accurately identify the subsurface defects such as knots, decays and metallic nails inside the logs which are not visible from outside observations. The results show that GPR can be a very promising technique for future online implementation in saw mills.

Mapping subsurface conditions within the near-shore zone of an Arctic delta using ground penetrating radar

Abstract This paper demonstrates the capabilities of ground penetrating radar (GPR) to map and resolve shallow subsurface features in the near-shore zone: (i) ice conditions (floating or bottom fast); (ii) water bathymetry; (iii) sedimentary structures; and (iv) interfaces between frozen and unfrozen sediment. These features were resolved in the near-shore zone of the Mackenzie Delta, N.W.T., Canada, using multi-frequency (50, 100 and 250 MHz) GPR data collected in winter (2005 and 2006). The capability of GPR to resolve subsurface features in the near-shore zone was strongly controlled by bottom-fast and floating ice conditions. The latter were discriminated using a novel approach involving the energy return from the base of ice and the presence of ice-bottom multiples. Beneath zones of bottom-fast ice, sedimentary structures and interfaces between frozen and unfrozen sediment were discriminated by reflection geometry and amplitude. Beneath floating ice, water depths were measured to depths greater than 5 m using a multi-layer depth calculation. Overall, this research demonstrates that baseline information for geotechnical investigations and climate change research in Arctic coastal zones can be greatly enhanced with GPR data.

Improved concrete bridge deck evaluation using GPR by accounting for signal depth–amplitude effects

Ground-penetrating radar (GPR) surveys have tended to provide unpredictable accuracy in estimating areas of corrosion-induced damage in reinforced concrete bridge decks. Six in-service reinforced concrete bridge decks with exposed (unpaved) concrete wearing surfaces were surveyed using GPR to evaluate internal corrosion-induced damage for comparison against the chain drag and half-cell potential survey methods. A normalization procedure for removing GPR signal losses resulting from depth-dependent two-way travel time variations in the data was developed and thresholds for predicting chain-drag and half-cell potential-based estimates of deterioration quantity and location were established. Thresholds established using the amplitude–time correction provided significant improvements in the spatial and quantitative predictive capabilities of GPR for delineating corrosion-induced damage.

Long-term ground penetrating radar monitoring of a small volume DNAPL release in a natural groundwater flow field

An earlier field experiment at Canadian Forces Base Borden by Brewster and Annan [Geophysics 59 (1994) 1211] clearly demonstrated the capability of ground penetrating radar (GPR) reflection profiling to detect and monitor the formation of DNAPL layers in the subsurface. Their experiment involved a large volume release (770 L) of tetrachloroethylene into a portion of the sand aquifer that was hydraulically isolated from groundwater flow by sheet pile walls. In this study, we evaluated the ability of GPR profiling to detect and monitor much smaller volume releases (50 L). No subsurface confining structure was used in this experiment; hence, the DNAPL impacted zone was subjected to the natural groundwater flow regime. This condition allowed us to geophysically monitor the DNAPL mass loss over a 66 month period. Reflectivity variations on the GPR profiles were used to infer the presence and evolution of the solvent layers. GPR imaging found significant reflectivity increases due to solvent layer formation during the two week period immediately after the release. These results demonstrated the capacity of GPR profiling for the detection and monitoring of lesser volume DNAPL releases that are more representative of small-scale industrial spills. The GPR imaged solvent layers subsequently reduced in both areal extent and reflectivity after 29 months and almost completely disappeared by the end of the 66 month monitoring period. Total DNAPL mass estimates based on GPR profiling data indicated that the solvent mass was reduced to 34%-36% of its maximum value after 29 months; only 4%-9% of the solvent mass remained in the study area after 66 months. These results are consistent with independent hydrogeological estimates of remaining DNAPL mass based on the downgradient monitoring of the dissolved solvent phase. Hence, we have concluded that the long-term GPR reflectivity changes of the DNAPL layers are likely the result from the dissolution of chlorinated solvents residing in those layers. The long-term monitoring results demonstrated that GPR profiling is a promising non-invasive method for use at DNAPL contaminated sites in sandy aquifers where temporal information about immiscible contaminant mass depletion due to either natural flow or remediation is needed. However, our results also indicated that the GPR signature of older DNAPL impacted zones may not differ greatly from the uncontaminated background if significant mass reduction due to dissolution has occurred.

Rapid lake infill following major rockfall (bergsturz) events revealed by ground-penetrating radar (GPR) measurements, Reintal, German Alps

The capability of GPR (ground-penetrating radar) measurements to decipher the sedimentary history of young alluvial plains is investigated. The sediment basins result from the infill of two lakes, which were dammed by high-magnitude rockfalls 400—600 and 200 years ago in an Alpine trough valley. The thickness and structure of the dammed sediments was derived from a series of longitudinal and cross profiles. In order to take advantage of both high penetration and high resolution measurements, we applied 25, 50, 100 and 200 MHz antennas. The radargrams allow for a detailed analysis of the history of the alluvial fill. The pre-existing valley bottom is clearly visible under one of the basins. The profiles display the former clayey lake bottom and illustrate several stages of lake infill. Small-scale sedimentary structures, such as topset beds or foreset beds, are discernible and provide proxy information on water level, sediment supply and phases of infill activity. At the second basin, discordant structures in the adjacent sediment cones indicate the abrupt rise of the base level resulting from the rockfall event. Radar-derived information and historical records of lake retreat were used to reconstruct the sediment budget of the 200 year lifespan of the younger alluvial plane. Therefore, it can be concluded that the sedimentation rates of the twentieth century (489 ± 87 m3/yr) are several times larger than those of the nineteenth century (103 ± 7 m3/yr). A 130-yr rainstorm record suggests that the overall increase in sediment supply in the twentieth century corresponds to increased rainstorm frequencies during warm periods.

FDTD Simulation for Ground Penetrating Radar Wave in Dispersive Medium

AbstractThe earth material probed by Ground Penetrating Radar (GPR) is often dispersive. To investigate the detecting capability of GPR in dispersive medium, we introduced a Finite Difference Time Domain (FDTD) method to solve Maxwell's equations. We give an iterative method for calculation of electrical displacement and magnetic field in Debye medium, and a method to compute electric field from electrical displacement. The physical parameters are used only in calculating electric field. A new absorbing boundary algorithm is introduced by adding fictitious dielectric constant and magnetic permeability, therefore the wave can be attenuated without reflection in absorbing layer. This method overcomes the troubles caused by field split in Berenger's Perfectly Matched Layer (PML), so that the computing efficiency is increased. Examples demonstrate that the electromagnetic wave in dispersive medium attenuates very fast and the measured signals are very weak.

Case Study: Application of GPR to Detection of Hidden Dangers to Underwater Hydraulic Structures

After long-term operation, a variety of hidden defects will occur in hydraulic structures, which will endanger the safety of a water conservancy project. In particular, hidden defects located below the water surface cannot be easily recognized by conventional methods and thus constitute the main reason for occurrence of dangerous situations in such hydraulic structures. It is therefore necessary to carry out periodic checks on the underwater structures. Ground-penetrating radar (GPR) is a high-resolution, nondestructive technology for detection of underground objects using a high-frequency electromagnetic pulse, and has been extensively applied to a variety of fields such as engineering/geologic exploration, underground archaeological survey, detection of scour holes around bridge piers, etc. In order to investigate the capability of GPR in detecting hidden dangers to underwater hydraulic structures, the authors have conducted GPR application-related experiments for more than 20 hydraulic projects since 1...

Monitoring Temporal Development of Spatial Soil Water Content Variation: Comparison of Ground Penetrating Radar and Time Domain Reflectometry

We compare the capability of ground penetrating radar (GPR) and time domain reflectometry (TDR) to assess the temporal development of spatial variation of surface volumetric water content. In the case of GPR, we measured surface water content with the ground wave, which is a direct wave between the sender and receiver through the upper centimeters of the soil. Spatial water content variation was measured on 18 d with GPR and TDR during a 30‐d monitoring period. To ensure large fluctuations in the spatial water content variation, we created a heterogeneous pattern of water content by irrigation on 2 d. The temporal development of the spatial variation was studied by means of the variogram and interpolated water content maps. To compare GPR and TDR variograms, we estimated confidence intervals of the experimental variograms and the variogram model parameters with a jackknife approach and a first‐order approximation of model parameter uncertainty. The results showed that the 95% confidence intervals of the GPR experimental variogram were one to two orders of magnitude smaller than the 95% confidence intervals of the TDR experimental variogram because of the larger number of GPR measurements. Consequently, the uncertainty in the variogram model parameters was also much lower for GPR, which meant that the temporal development of the fitted GPR variogram model parameters was easier to interpret. Furthermore, the larger GPR measurement volume resulted in a low spatial nugget variance of 1 × 10−6to 1 × 10−9(m3m−3)2because short distance variation was averaged. This meant that GPR accurately measured spatial correlation lengths, even in the case of low water content variation. Interpolated maps showing the increase of water content due to irrigation and the subsequent gradual drying of the soil were more accurate and reproducible for GPR. It was concluded that the noninvasive GPR measurements provide the means to accurately and consistently monitor the development of spatial water content variation in time.

Monitoring Temporal Development of Spatial Soil Water Content Variation: Comparison of Ground Penetrating Radar and Time Domain Reflectometry

We compare the capability of ground penetrating radar (GPR) and time domain reflectometry (TDR) to assess the temporal development of spatial variation of surface volumetric water content. In the case of GPR, we measured surface water content with the ground wave, which is a direct wave between the sender and receiver through the upper centimeters of the soil. Spatial water content variation was measured on 18 d with GPR and TDR during a 30-d monitoring period. To ensure large fluctuations in the spatial water content variation, we created a heterogeneous pattern of water content by irrigation on 2 d. The temporal development of the spatial variation was studied by means of the variogram and interpolated water content maps. To compare GPR and TDR variograms, we estimated confidence intervals of the experimental variograms and the variogram model parameters with a jackknife approach and a first-order approximation of model parameter uncertainty. The results showed that the 95% confidence intervals of the GPR experimental variogram were one to two orders of magnitude smaller than the 95% confidence intervals of the TDR experimental variogram because of the larger number of GPR measurements. Consequently, the uncertainty in the variogram model parameters was also much lower for GPR, which meant that the temporal development of the fitted GPR variogram model parameters was easier to interpret. Furthermore, the larger GPR measurement volume resulted in a low spatial nugget variance of 1 × 10−6 to 1 × 10−9 (m3 m−3)2 because short distance variation was averaged. This meant that GPR accurately measured spatial correlation lengths, even in the case of low water content variation. Interpolated maps showing the increase of water content due to irrigation and the subsequent gradual drying of the soil were more accurate and reproducible for GPR. It was concluded that the noninvasive GPR measurements provide the means to accurately and consistently monitor the development of spatial water content variation in time.

Ground penetrating radar: 2-D and 3-D subsurface imaging of a coastal barrier spit, Long Beach, WA, USA

Abstract The ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar (GPR). The GPR method, which is based on the propagation and reflection of pulsed high frequency electromagnetic energy, provides high resolution (cm to m scale) and shallow subsurface (0–60 m), near continuous profiles of many coarser-grained deposits (sediments of low electrical conductivity). This paper presents 2-D and 3-D GPR results from an experiment on a regressive modern barrier spit at Willapa Bay, WA, USA. The medium-grained sand spit is 38 km long, up to 2–3.5 km wide, and is influenced by a 3.7-m tidal range (spring) as well as high energy longshore transport and high wave energy depositional processes. The spit has a freshwater aquifer recharged by rainfall. The GPR acquisition system used for the test was a portable, digital pulseEKKO™ system with antennae frequency ranging from 25 to 200 MHz and transmitter voltages ranging from 400 to 1000 V. Step sizes and antennae separation varied depending on the test requirements. In addition, 100-MHz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data (50×50 m sampled every meter). The 2-D digital profiles were processed and plotted using pulseEKKO™ software. The 3-D datasets, after initial processing, were entered into a LANDMARK™ workstation that allowed for unique 3-D perspectives of the subsurface. To provide depth, near-surface velocity measurements were calculated from common midpoint (CMP) surveys. Results from the present study demonstrate higher resolution from the 200-MHz antennae for the top 5–6 m, whereas the 25- and 50-MHz antennae show deeper penetration to >10 m. For the study site, 100-MHz antennae provided acceptable resolution, continuity of reflections, and penetration. The dip profiles show a shingle-like accretionary depositional pattern, whereas strike profiles show a horizontal and subhorizontal, nearly continuous reflection pattern. Results from the GPR experiment reveal upper shoreface reflections with dip towards the ocean at about 1–2°. The loss of signal from below a depth of 6–8 m indicates a lithofacies change because of the storm wave base. The parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy, continuity, and depth of penetration. The cross-polarization test exhibits reduced continuity of reflections and less depth of penetration, but dipping reflections are apparent. The grid pattern data provided a detailed view of 3-D geometry of individual reflections. High quality data were obtained, processed, and directly exported into a LANDMARK™ workstation for interpretation. The resulting interpretations of the upper shoreface beds from the test cube (50×50 m; total 2600 traces) are shown as vertical sections (slices), horizontal sections (time slices), contour maps, 3-D representations of individual beds, and an isopach map. The 3-D depositional framework allows a more detailed interpretation than widely spaced 2-D profiles.

Monitoring Temporal Development of Spatial Soil Water Content Variation

We compare the capability of ground penetrating radar (GPR) and time domain reflectometry (TDR) to assess the temporal development of spatial variation of surface volumetric water content. In the case of GPR, we measured surface water content with the ground wave, which is a direct wave between the sender and receiver through the upper centimeters of the soil. Spatial water content variation was measured on 18 d with GPR and TDR during a 30-d monitoring period. To ensure large fluctuations in the spatial water content variation, we created a heterogeneous pattern of water content by irrigation on 2 d. The temporal development of the spatial variation was studied by means of the variogram and interpolated water content maps. To compare GPR and TDR variograms, we estimated confidence intervals of the experimental variograms and the variogram model parameters with a jackknife approach and a first-order approximation of model parameter uncertainty. The results showed that the 95% confidence intervals of the GPR experimental variogram were one to two orders of magnitude smaller than the 95% confidence intervals of the TDR experimental variogram because of the larger number of GPR measurements. Consequently, the uncertainty in the variogram model parameters was also much lower for GPR, which meant that the temporal development of the fitted GPR variogram model parameters was easier to interpret. Furthermore, the larger GPR measurement volume resulted in a low spatial nugget variance of 1 × 10−6 to 1 × 10−9 (m3 m−3)2 because short distance variation was averaged. This meant that GPR accurately measured spatial correlation lengths, even in the case of low water content variation. Interpolated maps showing the increase of water content due to irrigation and the subsequent gradual drying of the soil were more accurate and reproducible for GPR. It was concluded that the noninvasive GPR measurements provide the means to accurately and consistently monitor the development of spatial water content variation in time.

Gated stepped-frequency ground penetrating radar

This paper describes a newly developed stepped-frequency ground penetrating radar (SFGPR) prototype operating across the 10–620 MHz frequency band. The SFGPR system has been developed to improve the maximum penetration depth capability of ground penetrating radar (GPR) applications without degradation of resolving power. The radar has the unique capability to employ a short gate at each frequency step that can suppress unwanted strong signals relative to weak reflections from deep targets. The system performance is approximately 175 dB in ungated mode and 170 dB in gated mode, with reference to a 10 ms integration time and a 10 dB signal-to-noise ratio. The current system is suitable for operation in a vehicle and incorporates a real time display similar to conventional impulse GPR. It is capable of recording 50 depth profiles per second. Subsurface images obtained with the gated SFGPR of a perched water table on a sand island, a culvert in high-loss ground, and a 500 lb bomb buried at 1.7 m deep are presented. A frequency compensation procedure is used to improve image clarity. The weakest confirmed underground target has a signal power 100 dB down on the transmitted power. Such a signal would be near the noise floor of a commercial impulse GPR.

A waveform-range performance diagram for ground-penetrating radar

The growing professional interest in ground-penetrating radar (GPR) is driving the need to formulate methods for characterizing the performance capabilities of GPR systems. Because a number of different factors, such as antenna ringing, noise, radio frequency interference and receiver dynamic range, contribute to limit a GPR's capability, no single performance measure gives an adequate system characterization. Instead capability can best be described by indicating the boundary of marginal performance for the various parameters that affect performance in the form a waveform-range performance diagram. The various limiting processes are used to define the performance boundaries. Within the performance boundary the radar has a capability, and outside the boundary the radar has no capability. Performance measures such as system performance, loop gain, and system dynamic range are interpreted using the waveform-range performance diagram.

A Laboratory Appraisal Of Ground·Penetrating Radar Over Water

The work presented here describes an experiment designed to determine the performance and capabilities of ground-penetrating radar (GPR) over water. The main factors that are addressed are: water conductivity and skin depth, accuracy of depth determination, effect of raising the antenna above the water surface plus the radiation pattern and its influence on imaging an inclined submerged interface. This study was undertaken for different frequency GPR antennas which were pulled across the wavetank at Edinburgh University. Two different targets, a metal bar and a plastic plate, were submerged in the tank and the reflections of the electromagnetic signals were recorded. The objective of this work is to clarify the interpretation of GPR images so that field data may be more accurately assessed. Practical applications which could benefit from this work include the investigation of scour around bridge piers and abutments and bathymetric surveys of ponds and lakes.

Using GPR for micro-analyses of soils and karst features on the Chiefland Limestone Plain in Florida

Karst and sand/limestone and sand/clay/limestone interfaces were investigated in two study plots on the Chiefland Limestone Plain in west-central Florida. The Chiefland Limestone Plain has many recently formed sinkholes and solution pipes that mark the landscape. Paleokarst features, such as relict sinkholes and solution pipes (with soils burying them), are not obvious due to the lack of surface expression. Prior to the development and use of ground-penetrating radar (GPR), accurate location of the paleokarst features, as well as determination of their configuration, was nearly impossible. Knowledge of the location of these features is important because the presence of these features indicates areas of subsurface instability. Additionally, in order to optimize the capability of GPR on karst, documentation of the effects of selected soil properties on the radar output, and the examination of the relationships of the clay layer, bedrock, and surface topography are imperative. The objectives of this study were: (1) to analyze selected soil properties and compare these properties to GPR performance in resoluting the sand/limestone (C/R horizons) and sand/clay/limestone (E/Bt/R horizons) interfaces; (2) to use GPR graphic data to identify paleo-sinkholes and solution pipes using computer-simulated diagrams of the limestone interface; and (3) to discuss implications for the association between the clay layer, bedrock, and surface topography in two study plots on the Chiefland Limestone Plain.