Multi-layer detection/tracking for monostatic ground penetrating radar

Monostatic ground penetrating radar (GPR) has proven to be a useful technique in pavement profiling. In road and highway pavements, layer thickness and permittivity of asphalt and concrete can be estimated by using an inverse scattering approach. Layer-stripping inversion refers to the iterative estimation of layer properties from amplitude and time of delay (TOD) of echoes after their detection. This method is attractive for real-time implementation, in that accuracy is improved by reducing false alarms. To make layer stripping useful, a multitarget detection/tracking (D/T) algorithm is proposed. It exploits the lateral continuity of echoes arising from a multilayered medium. Interface D/T means that both detection and tracking are employed simultaneously (not sequentially). For each scan, both detection of the target and tracking of the corresponding TOD of the backscattered echoes are based on the evaluated a posteriori probability density. The TOD is then estimated by using the maximum a posteriori (MAP) or the minimum mean square error (MMSE) criterion. The statistical properties of a scan are related to those of the neighboring ones by assuming, for the interface, a first-order Markov model.

Characterization of the porosity distribution in the upper part of the karst Biscayne aquifer using common offset ground penetrating radar, Everglades National Park, Florida

Time-delay estimation (TDE) using ground penetrating radar (GPR) is of great importance in roadway surveys. The conventional GPR methods apply a uniform sampling strategy for TDE, which requires numerous frequency sampling points, leading to lengthy data acquisition time and large data storage, especially for ultra-wideband (UWB) radar. Moreover, detecting the overlapped backscattered echoes from the thin layer of roadways remains a challenge in TDE, due to the limited resolution of GPR and the characteristics of GPR signals. To address these issues, we derive a co-prime sampling strategy-based TDE for thin layers in roadway survey by exploiting off-grid sparse Bayesian learning (OGSBL), referred to co-prime-OGSBL. In our scheme, the sampling rate of GPR signals with a co-prime sampling strategy is greatly reduced compared with the uniform sampling, which therefore reduces the data acquisition burden and computational complexity. The estimation performance of time delays and thickness is also enhanced with OGSBL by utilizing radar pulse, co-prime sampling, and noncircularity of GPR signals. Both simulation and experimental results demonstrate the efficiency and accuracy of the proposed method in the estimation of time delays and thickness.

This paper proposes a study of a time-domain ground penetrating radar (GPR) signal by using the time- domain uniform geometrical theory of diffraction (UTD) method. The monostatic GPR system is employed in this study. Rays tracing method is employed to find the reflected ray and diffracted ray paths between the transmitting antenna (Tx) and the receiving antenna (Rx). Moreover, the conventional UTD method is adapted to calculate the frequency-domain transfer function of the electric field between Tx antenna and the Rx antenna. The medium including air, ground and underground objects are taken into account. Finally, the received time-domain GPR signal is obtained by using the inverse fast Fourier transform (iFFT) method. The calculated results found that the time-domain GPR signal can be calculated by using the time-domain UTD method. The ground reflection, object reflection and object diffraction time-domain signal can be separately calculated. The reflected signal from the ground and an underground curved object can be illustrated.

The blind regions for single-borehole bistatic and monostatic pulse radars are considered in this paper. The blind region of a single-borehole radar may restrict its capability on detecting anomalies adjacent to the antenna. We explain the causes of the blind regions in bistatic and monostatic radars. In bistatic radar, the blind region is caused by direct signal along the borehole. In contrast, the blind region of monostatic radar is complexly generated by direct signal radiated from the circuit, feed reflection and switching noise. And we investigate how much such critical signals decrease using switch time delay. The blind region of a single-borehole pulse radar is measured using practical experiments. And the considerations of the blind region according to the switching time for monostatic radar system are also analyzed. Finally, we demonstrate B-scan data of actual adjacent target with optimum blind region.

As the transmitter and receiver (Tx and Rx, respectively) are located in close proximity during a typical ground‐penetrating radar (GPR) survey, the powerful signal generated by the Tx and which is then recorded by the Rx at various time delays, can be saturated at early times (i.e., this is the direct wave (DW) signal reaching the Rx). This often causes the masking of shallow targets, complicating data interpretation. In this study, our aim is to examine the spatial distribution of the electromagnetic signals around the Tx, attempting to locate areas where the DW becomes minimum, whereas the signal strength from subsurface targets (i.e., reflected wave – RW) remains ideally unchanged. The position of these local minima in the DW signal could give rise to advantageous Tx–Rx configurations, where clear reflections from subsurface targets lying at shallow depths can be obtained with the least possible involvement of the DW. To perform such a study, we carried out static field measurements over a flat lying reflector as well as numerical simulations in a reflection, common‐offset mode around a transmitting antenna. In the field, we also collected wide‐angle reflection–refraction data to determine the GPR wave velocity in the uppermost layer. GPR signals were recorded by the Rx around the Tx in three concentric circles of various radii (i.e., varying the Tx/Rx separation), using a specific angular step and varying the Tx/Rx polarization each time. The synthetic data were produced using a three‐dimensional finite‐difference time‐domain modelling tool. Field and numerically simulated data were analysed and compared to study the behaviour of both the DW and RW events around the Tx when changing the Tx/Rx distance, their respective angular position, as well as their relative polarization/orientation.

The primary objectives of this study are to investigate the interference of ground penetrating radar (GPR) signals with air-cavities behind concrete linings subjected to three experimental parameters (GPR antenna frequency, thickness of concrete lining, and thickness of air-cavities), and their effects on the evaluation of important engineering parameters (thickness of concrete, and length and thickness of air-cavities) for tunnel structure inspection using GPR. For these purposes, first, a concrete model specimen is fabricated in the laboratory, which simulates air-cavity defects with various thicknesses (0 mm, 30 mm, 100 mm, 200 mm, and 300 mm) behind concrete with four different thicknesses (100 mm, 200 mm, 300 mm, and 400 mm) in tunnel structures. Second, a series of GPR survey is performed on the fabricated concrete model specimen using three ground-coupled GPR antennas with nominal center frequencies of 400 MHz, 900 MHz, and 1600 MHz. Third, a practical signal interpretation method is proposed to evaluate air-cavities in concrete tunnel linings subjected to the three experimental parameters. It is demonstrated that the cross-correlation analysis is effective for identifying air-cavities behind concrete, and for characterizing the length of the air-cavities. In addition, the thicknesses of air-cavities are estimated by a new approach in this study, measuring the apparent time delay of partially overlapped electromagnetic wave reflections within air-cavities. The results in this study provide a fundamental basis to improve GPR signal interpretation techniques for the evaluation of air-cavities behind concrete tunnel linings.

Investigating alluvial and tectonic features with ground-penetrating radar and analyzing diffractions patterns

International audience

Existing ground penetrating radar (GPR) and electromagnetic induction (EMI) sensors present some limitations related to their calibration procedure and the strong simplifying assumptions in modeling the antenna-subsurface system. We present a new EMI model in which a zero-offset, off-ground loop antenna is efficiently modeled using frequency-dependent, complex linear transfer functions. The air-subsurface is described by a Green’s function for wave propagation in three-dimensional multilayered media. We validated our model in laboratory conditions by performing measurements at different heights above a copper sheet as a perfect electrical conductor. Then joint inversion of full-waveform mono-static GPR and EMI data was carried out for the particular case of a one-layered medium. Analysis of the objective functions demonstrated the benefit of combining the two sources of information. Yet, due to the variations of the GPR and EMI model sensitivities with respect to the properties of the medium, the formulation of an optimal objective function is not straightforward.

Estimation of time delay and interface roughness by GPR using modified MUSIC

We have imaged the temporal and spatial response of an unconfined aquifer during a pumping test using ground penetrating radar (GPR) profiling. In particular, we have observed the transient behavior of the reflection generated by the water content variation occurring in the transition zone between the overlying residually saturated material and the water saturated capillary fringe below. The progressive arrival time delay of this reflection was measured and used to infer its drawdown that occurred during the pumping test. We also observed several other phenomena on the GPR profiles that are related to drainage: (1) the development of a series of diffractions indicating localized irregularities in water saturation and (2) the velocity pull‐up of stratigraphic reflections due to increased electromagnetic (EM) wave velocity in the overlying section. Comparing the GPR profiling data and piezometer measurements, we have observed that the drawdown of the transition zone reflection is smaller and delayed relative to the measured hydraulic head drawdown. From the combined GPR and piezometer data, we have inferred that a 0.20 m increase in the combined thickness of the transition zone and capillary fringe occurred before the drawdown of the GPR transition zone reflection commenced. Once achieved, this 0.20 m increased thickness remained for the duration of the pumping test. Using the distance‐drawdown relationship obtained from GPR profiling, we have estimated the drained water volume due to the downward movement of the transition zone. The results of our analysis account for only a fraction of the pumping well production, approximately 45% on the first day and about 25% on the second day. Of the various reasons examined to explain this result, we have concluded that it is likely that the behavior of the GPR reflection originating from the transition zone is representative of the shallower, less saturated portion of the transition zone and undetected drainage is occurring in the deeper, more saturated portion of the transition zone.

The utilization of continuously reinforced concrete pavement (CRCP) has been widely used throughout the United States during the construction of interstate highways since the 1930s. The essential design criterion of CRCP is to manage the crack type, size, and spacing that forms to reduce the distress; thus, identification of the factors affecting the location and extent of the cracks and distresses provides a prediction for the service life of CRCP pavements. In this paper, the influence that reinforcement placement and concrete cover has on the distresses of CRCPs is investigated using ground penetrating radar (GPR) and eddy current technology at six major interstate and state highway locations in Georgia. It was concluded that pavement having inefficient concrete cover depth and pavement thickness results in increased clusters of single transverse cracks and/or punchouts. A result of this study is the recommendation that longitudinal reinforcement be placed at one-third of the depth of the slab from the top surface of the pavement. Furthermore, a distress factor (DF) was established as a good indicator of the performances of pavements relative to each other. Ideal pavement with no crack acquires a DF of 0. In this study, pavements in good, fair, and poor conditions received DFs of “0.03 and 0.05,” “0.97 and 0.98,” and “3.23 and 3.33,” respectively.

Condition assessment of concrete pavements using both ground penetrating radar and stress-wave based techniques

This paper proposes a multi-target tracking and identity management method with multiple sensors: a primary sensor with a large detection range to provide the targets' state estimates, and multiple secondary sensors capable of recognizing the targets' identities. Each of the secondary sensors is assigned to a sector of the operation area; a secondary sensor decides which target in its assigned sector to be identified and controls itself to identify the target. We formulate the decision-making process as an optimization problem to minimize the uncertainty of the targets' identities subject to the sensor dynamic constraints. The proposed algorithm is decentralized since the secondary sensors only communicate with the primary sensor for the target information, and need not to synchronize with each other. By integrating the proposed algorithm with the existing multi-target tracking algorithms, we develop a closed-loop multi-target tracking and identity management algorithm. The effectiveness of the proposed algorithm is demonstrated with illustrative numerical examples.