Yonghui Zhao

Maxwell curl equation datuming for GPR test of tunnel grouting based on Kirchhoff integral solution

In two-dimensional (2D) ground penetrating radar (GPR) data, the reflection from the detection targets in depth are severely obscured by the strong scattering generated from near-surface non-target structures. For using GPR as a geotechnical non-destructive testing device, how to eliminate the strong scattering caused by near-surface rebars in the tunnel liner to image and assess the grouting condition behind tunnel liner is still an unsolved problem. This study proposed a method for the reconstruction of deep GPR images, termed the Maxwell curl equation datuming. To eliminate the deleterious effect caused by near-surface diffractive scattering, we have redefined the reference surface to an actual geologic interface by using Maxwell curl equation datuming methodology based on Kirchhoff integral solution. Maxwell curl equation datuming procedure can redefine the reference surface into deeper horizon on which the GPR transmitters and receivers appear to be located. Case studies were presented for synthetic examples and real GPR data for assessments of tunnel grouting. The results show that the datuming technique based on Maxwell curl equation, is able to eliminate the strong scattering related to near-surface rebars in tunnel liners, and improve the quality of deeper images beneath the tunnel liners. The Maxwell curl equation datuming is also applicable to other GPR testing situations that depend on the elimination of scattering effects caused by near-surface structures.

Profiling experiment of a lake using ground penetrating radar

Great significance of underwater environment was gradually attracting attention and many traditional methods showed obvious shortcomings in underwater exploration, while the ground penetrating radar (GPR), as a new method in this field, performed well in identifying suspended solid and underwater relic. We conducted a GPR survey experiment on a drinking water reservoir to test the applicability of 100 MHz, 200 MHz and 400 MHz frequency antenna in underwater exploration. The experimental results showed that the electromagnetic waves of 100 MHz frequency antenna could penetrate to a greater depth below lakebed than two other antennas especially in water depth over 7 m where 200 MHz frequency antenna barely receipted any signal. But in shoal water less than 4 m, the profiles of 400 MHz frequency antenna presented higher vertical resolution than of 100 MHz. Before the GPR survey, a measurement of the dielectric constant and conductivity of the water was carried out to calculate the attenuation coefficient and velocity of electromagnetic waves in the water. Eventually, the effects of “half hyperbolas” and multiples of GPR were pointed in particular, and the profiling of the lake was given from GPR image. GPR can provide detailed information about underwater stratigraphy and obstructions.

Study of GPR image characteristics of dams' hidden weak points

For using GPR as a non-destructive testing to the hidden weak points in dam or seawall, how to reasonably analysis the GPR image and obtain the exact interpretations is still an unsolved problem. Combined with the dam engineering features and detection experiences, geophysical models of typical hidden weak points, such as horizontal non-homogeneous layer, slope non-homogeneous zones bearing coarse-grained or fine-grained material mezzanine and cavities filled with different materials in cohesive soil dam, were set up for numerical simulation. Numerical simulations of radar wave performance in these typical hidden weak point models were conducted by using the finite difference time domain (FDTD) solution over a 2D space, with a perfectly matched layer (PML) absorbing boundary conditions. Simulated results illustrated that spatial distribution of hidden weak points can be interpreted from the GPR image. On the condition of complicated environment, interpretation of local hidden weak point character relies upon careful analysis on dynamic characteristics including phase and frequency features of reflected wave, and diffracted wave character. Based on the wave field characteristics of the typical hidden weak points, some GPR images collected at a reservoir dam and seawall had been interpreted. Engineering applications indicated that appropriate numerical model construction and simulation interpretation could be a helpful guide to real GPR test for hidden weak points.

Ground Penetrating Radar Inspection Experiment on a Shield Tunnel Segment and the Backfill Grouting

Defects of the backfill grouting have a strong impact on the shield tunnel engineering and gotten more and more attention from the experts in recent years. To estimate the thickness of the grouting and the position of some objects behind a shield tunnel segment, we conducted a field experiment using Ground Penetrating Radar (GPR). In the paper, a segment was put vertically on the ground with half of its outer surface covered by a wooden box. We injected concrete grout into the box until 80% of the box was filled by it. SIR4000 pulse radar system made by GSSI (Geophysical Survey System Inc.) with 400 MHz center-frequency antenna was used in this experiment. First, static measurements show that the direct wave and the coupling wave were stacked together when the antenna was put on the internal surface of the segment. We, therefore, regard the peak point as the time zero of the signal. Then contrastive analysis reveals that Principal Component Analysis (PCA) performed better than Average Mean Removal (AMR) in eliminating the direct wave and highlighting the effective signals. From the processed radargram of dynamical measurement, we can see that it was easy to distinguish the inner structure and the interfaces of the segment, as well as the grouting and other objects in the back. After a week, we retested it again and found that the travel time of electromagnetic wave passing through the grouting became shorter than the previous one. It may be attributed to the loss of moisture in the solidification process of concrete, which means that the technique we proposed can monitor the change of the grouting layer and provide decision-maker with reliable information of the shield tunnel quality.

Ground penetrating radar detection of hidden defects in the seawall

For the existence of the potential safety risk, it is necessary to reveal the distribution of these hidden defects in the seawall body. Based on the analysis of seawall engineering and foundation characteristics, ground penetrating radar (GPR) has been used to identify the hidden defects and quality status of loosening weak layers in the seawall body (clay). The detection results are in good agreement with the drilling results, and it can be used as an important basis for the seawall safety evaluation.

Stratigraphic absorption compensation of GPR signal based on improved S-transform

The loss of the higher frequency components of electromagnetic wave due to soil absorption and attenuation results in GPR signal decrease rapidly in deep areas. High frequency compensation is one of most important ways to improve GPR data resolution. The S-Transform (or ST) has been widely used in digital signal processing since it was proposed. Here, improved S-transform to GPR data has been designed to enhance the deep reflection events. Firstly, GPR signals are transformed to S-transform coefficients trace by trace; then at each time sampling point, the frequency factors depended attenuation are extracted according to the soil layer feature; the S-transform coefficients are weighed by the factors to ensure energy equalization at different time sampling point. Numerical simulated and real GPR data are used for testing the proposal and its validity. According to the time-frequency spectrum analysis before and after ST treatment, almost the same energy distribution in lower frequency range is obtained, and the higher frequency components are obviously increased at deep area. The comparison demonstrates that the reconstructed GPR data from S-transform with appropriate weighting method can implement compensation absorption very well without Q value.

Analysis of hollow area beneath concrete slab of seawall by means of ground penetration radar

Seawall is an important public facility which can protect areas of human habitation from the destructive aspects of tides or waves. Maintenance and monitoring of seawall structures plays an essential role in public safety and prevention of flooding of areas behind the seawall. The presence of hollow area beneath the concrete slab of riverside slope can result in the settlement and distortion of face slab. It has been one of the most important factors seriously influencing the safety of seawall body. Here, GPR survey for hollow areas beneath the concrete slab of the seawall of Qiantang River has been implemented. The bottom of concrete slab and top of the soil layer can be clearly identified from GPR profile. The gap between these two interfaces is regarded as the hollow area beneath the concrete slab. Also, time slice obtained from grid survey lines. Excavation data at some points demonstrated that the height determination error for the hollow areas from GPR profile is less than 10%. Also, the distribution of the hollow area determined by GPR is consistent with the real information revealed by excavation on site.

The potential use of GPR in the exploration for riprap layers in tidal flat embankment

This study focuses on the use of ground penetrating radar (GPR) to delineate riprap layers in cross sections of tidal flat embankments. Foundation treatments are an important consideration in the reclamation of tidal flats and for offshore engineering in coastal areas with soft soils. Such treatments generally consist of explosive riprap fill. The bottom depth and cross-sectional shape of the riprap, which are important components of the body of an embankment, determine the effectiveness of the fill in foundation treatments and embankment stabilization efforts. Thus, identification of the bottom depth and the cross-sectional shape are important steps in the effective design and implementation of riprap fill projects. Here, we focus on a case study of a tidal flat embankment in the Zhejiang area. A comparison of the dielectric properties of a riprap layer and its surrounding soils indicates that this layer can be detected using GPR. We also present new drillhole-verified radar wave propagation velocity data for different materials obtained in this study. The velocity analysis enabled the reconstruction of the cross-section of riprap using GPR profiles. This study indicates that effective and economic riprap detection can be achieved by GPR with additional corroboration from only a few drillholes.

Application research of potential failure zone detection for seawall using ground penetrating radar

The potential failure zone in a seawall is a hidden danger that requires exact and immediate detection. Of all the different geophysical exploration methods, ground penetrating radar (GPR) has proven to be the most effective, non-destructive, highly efficiency detection method. It has been successfully used to investigate the structure and condition of many seawalls in Eastern China. In this study, GPR investigation was performed as part of a seawall integrity assessment for a maintenance plan. GPR data were collected along the seawall crest and the inner slope with the central frequency of 100 MHz and 400 MHz. Traditional drilling was also performed to explore the main structural layers of the seawall. Combined with the drilling investigation results, the GPR results reveal the distribution of heterogeneous areas, gaps and loose zones in the seawall. It must be noted that GPR interpretation results are consistent with visual observation.

GPR images reconstruction with Maxwell curl equation datuming based on Kirchhoff integral solution

In 2D ground penetrating radar (GPR) data, the reflection from the detection targets in depth are severely obscured by the strong scattering generated from near-surface non-target structures. For using GPR as a non-destructive testing device, how to eliminate the strong scattering caused by near-surface rebars in the tunnel liner to image and assess the grouting condition behind liner is still an unsolved problem. This study proposed a method for the reconstruction of deep GPR, termed the Maxwell curl equation datuming. To eliminate the deleterious effect caused by near-surface diffractive scattering, we have redefined the reference surface to an actual geologic interface by using Maxwell curl equation datuming method based on Kirchhoff integral solution. Maxwell curl equation datuming procedure can redefine the reference surface into deeper horizon on which the GPR transmitters and receivers appear to be located. Case studies were presented for synthetic examples and real GPR data for assessments of tunnel grouting. The results show that the datuming technique is able to eliminate the strong scattering related to near-surface rebars in tunnel liners, and improve the quality of deeper images beneath the tunnel liners. The Maxwell curl equation datuming is also applicable to other GPR testing situations that depend on the elimination of scattering effects caused by near-surface structures.