Keywords

1 Introduction

With the evolution of long-term complex geological environment and the development of human activities, Natural geo-bodies and man-made buried bodies appear [1]. The demand for energy leads to the continuous development of land, and the existence of buried bodies will adversely affect the construction and operation of infrastructure and buildings.

GPR is a non-destructive geophysical detection technology. Because of its high detection accuracy, it is widely used in many fields [2]. Underground archaeological investigation is a typical representative [3]. For hydraulic engineering, the erosion inside the dam retaining walls, structurally hidden defects and groundwater distribution characteristics are all applied [4]. In the process of geological survey, it is used to locate buried geological bodies and survey near-surface faults [5], including detection of distribution characteristics of Karst caves and sinkholes, changes in rock and soil properties and water content [6]. In civil engineering, the drawing of internal cracks in rock slopes, the evaluation of the stability of existing buildings and road foundations, and the investigation of scour pits around underwater piers have achieved good results [7].

Through the above elaboration, it can be seen that the application of GPR in practical engineering is quite extensive, and has achieved fruitful research results and economic benefits. However, through the forwarding analysis of two-dimensional GPR images, the shape of the buried object, especially the size range, is relatively less defined. Therefore, forward simulation on geometric characteristics of three typical culverts in Chongqing were carried out in this paper. The response characteristics of (ground penetrating radar) GPR profiles caused by geometric characteristics and filling materials change were summarized, and the apexes of diffraction hyperbolas and lateral changes in the reflection pattern were used to determine the dimension and boundary of buried bodies. The comparison between GPR profiles interpretation result and the measured data in the field was used to verify the validity.

2 The Basic Principle of GPR

GPR is a geophysical method propagating through high frequency electromagnetic wave. A typical GPR system consists of a laptop, a DAD control unit, and an antenna. During the whole exploration process, the transmitted electromagnetic wave propagates in the medium. Due to the different properties of the medium, the interface between different materials is reflected. The reflected signal is received and used for image storage and processing.

3 Acquisition and Interpretation of GPR Image

3.1 GPR Image Acquisition

According to the size difference of buried body, 80 and 400 MHz antennas were employed to complete the continuous survey along the measurement line parallel to the road direction, and the supporting data was obtained using RIS software. To ensure the accuracy of the image acquisition, GresWin2 software is used to process the two-dimensional GPR image.

3.2 Interpretation of GPR Image

Figure 1a is the picture of culvert 1, and the stratum structure and engineering geological environment characteristics are shown in Fig. 1b.

Fig. 1
2 illustrations. a. A photograph of a culvert with coaxial dislocation and damage that is marked as A, B, and C. b. A diagram presents a semi-circle-shaped culvert 1, which has sandstone and concrete pavement on the top, with its corresponding dimensions.

Culvert 1: a is the picture of culvert 1 in the field and b is the diagram of culvert 1’s dimension

Full size image

Figure 2 is a two-dimensional GPR image of culvert section 1. There is a discontinuity in the two-dimensional image at a depth of 0.2 m, so that the existence of the interface L1 can be determined. The coaxial dislocation in the marked B and C regions in the figure indicates the damage on the actual structure (regions B and C in Fig. 1a). The diffraction hyperbola is formed at the top of the culvert (red curve in Fig. 2). Due to the presence of air between the culvert and the water flow, a diffraction independent of the top of the culvert is formed (green curve in Fig. 2). It can be seen from the diffraction curve vertex marking data that the buried depth is 1.24 m below the ground. Due to the high relative permittivity of water, the electromagnetic wave has a high degree of attenuation during propagation (region D in Fig. 2). In addition, due to the relatively developed vegetation on the surface of the bedrock, and the GPR image manifests the coaxial dislocation of the local area (Region A in Fig. 2).

Fig. 2
A plot of depth versus X. It has several wavy patterns on the top with several fluctuations that are labeled A, B, C, and D. Regions B and C represent the damage with a diffraction hyperbola on the top of the square bounding box. Region D represents electromagnetic waves.

The labeled GPR profile of culvert 1 after processing

Full size image

The bedrock of culvert 2 is dense sandstone, and the shape of culvert is irregular quadrilateral. Through the actual investigation, the horizontal distance, buried depth and specific size from the starting point of the test point are shown in Fig. 3b. From the measured two-dimensional GPR image, the abnormal region E can be delineated (Fig. 4a). Through the processing of the image, the diffraction hyperbola will be formed at the inflection point of the irregular area, and the vertex of the hyperbola will be marked (Fig. 4b): F (1.8, 1.04), G (2.15, 1.04), H (1.75, 1.6), I (2.23, 1.6). By connecting these four points, the range of abnormal section can be determined.

Fig. 3
2 illustrations. a. A photograph of a field, which has a culvert that is surrounded by several dry crops and bushes, with 3 people standing on the field. b. A diagram presents a rectangular-shaped culvert with dimensions of 56, 46, and 32 centimeters and a field of 106 by 180 centimeters.

Culvert 2: a is the picture of culvert 2 in the field and b is the diagram of culvert 2’s dimension

Full size image
Fig. 4
2 plots of depth versus X. It has several wavy patterns on the top with fluctuations that are labeled E, F, G, H, and I, while several fluctuations at the bottom represent electromagnetic waves, in a gradient of colors.

GPR profile of culvert 2: a is the image after processing and b is the labeled image after processing

Full size image

Culvert 3 is a 0.35 m diameter concrete pipe buried 0.5 m below the soil level, surrounded by vegetation (Fig. 5a, b). Due to the difference in permittivity between air, concrete and rock and soil, the diffraction image appears black-white-black hyperbolic alternation. Ignoring the polarization and diffusion effects of the medium, the distance between the hyperbolic vertices in pairs represents the diameter of the concrete tube. According to the coordinates of the peak points L (1.2, 0.5) and M (1.2, 0.85), the buried body is buried 0.5 m deep, the diameter is 0.35 m, and the distance from the starting point of the measurement is 1.2 m. The position and size obtained by image analysis are consistent with the spatial position of the cross-section distribution. In addition, Fig. 6 shows an anomalous region K at a depth of 0.95 m, and the cross-sectional shape of the buried body is circular and the cross-sectional diameter is 0.24 m.

Fig. 5
2 illustrations. a. A photograph of a field with a hollow-shaped pipe that is surrounded by several dry crops and bushes. b. A diagram presents a circular-shaped pipe of 35 centimeters and a field of 120 by 50 centimeters.

Culvert 3: a is the picture in the field and b is the diagram of actual dimension

Full size image
Fig. 6
2 plots of depth versus X. It has several wavy patterns on the top with fluctuations that are labeled G, K, L, M, N, and Q, while several fluctuations at the bottom represent electromagnetic waves, in a gradient of colors.

GPR profile of culvert 3: a is the radargram after processing and b is the labeled image after processing

Full size image

4 Conclusion

The shape of sections were corresponding to different diffraction images. An arcuate section appear a single diffraction hyperbolic. For an circle section, a pair of diffraction hyperbolas are formed at the top and bottom of the section. When the cross-section is an irregular quadrilateral, a diffraction single hyperbolic will be formed at the turning point of the irregular pattern.

Based on the vertex coordinates of the image diffraction curve and the change of the horizontal reflection image, geometric characteristics for the cross-section of buried bodies can be determined. The results indicate that the consistency of the regular graphic results is high, and there is a certain error for irregular quadrilateral, but the error value is only 2 mm.

Combing the measured data in the field with orthogonal two-dimensional GPR images is a future development direction from the perspective of engineering practice.