Using attributes of electromagnetic waves to determine the water content and frost table in a permafrost area

ZhiChun Zhang, YuPeng Shen, Хiao Wang, YaНu Tian, JianKun Liu, Bagdat Teltayev

1. Shen-Shuo Railway Branch, China Shenhua Energy Company Limited, Yulin, Shaanxi 719316, China

2. School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

3. Kazakhstan Highway Research Institute, Almata, Kazakhstan

Using attributes of electromagnetic waves to determine the water content and frost table in a permafrost area

ZhiChun Zhang1, YuPeng Shen2*, Хiao Wang2, YaНu Tian2, JianKun Liu2, Bagdat Teltayev3

1. Shen-Shuo Railway Branch, China Shenhua Energy Company Limited, Yulin, Shaanxi 719316, China

2. School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China

3. Kazakhstan Highway Research Institute, Almata, Kazakhstan

The thawing-melting of the permafrost damages the subground of highways on the Qinghai-Tibet Plateau. With the application of ground-penetrating-radar (GPR) technology, the maximum permafrost melting interface can be effectively distinctly differentiated and imaged. A hierarchical feature of the permafrost region is shown clearly on the imaging profile of GPR data. The complete ablation zone or part of it is displayed distinctly. In addition, the details of subsurface layers can be effectively characterized by GPR attribute-analysis technology. With the attribute calculation and filter, the instantaneous amplitude, instantaneous frequency, and relative wave impedance can be applied in a more efficient way to divide the complete ablation zone, part of the ablation and non-ablation interface. The relative distribution of water content in a seasonally thawing permafrost region can be obtained through a comprehensive GPR attribute analysis.

permafrost thawing; ground-penetrating-radar; geophysical attribute; relative water content

1 Introduction

Permafrost is a significant feature of the Tibetan Plateau (TP) and is attributed to the cold climate. During the construction or operation period of the railway or highway line engineering in permafrost areas, there are many changes—such as topography, vegetation, surface reflection, permeability, snow cover, and surface-water redistribution—that can break the thermal equilibrium between the permafrost and the atmosphere, increasing detrimental heat, recession of the permafrost, acceleration of the thawing speed of the active layers (Treatet al., 2013). Under the combined effects of global climate and human activities, the engineering problems intend to take place when the environment has been changed in permafrost areas for nearly 40 years (Zhanget al., 2015). The bearing capacity would reduce, and excessive settlement would occur due to the recession or thawing of the permafrost, where slope instability and surface cracks would appear simultaneously (Jianget al., 2014; Niuet al., 2014). Thawing of the seasonal permafrost can lead to changes in the electromagnetic properties of the materials in the thawing zone, which would be reflected in the signals of wave fields in the geophysical field (Davis and Annan, 1989). Some researchers have tried to use the geological penetrating radar (GPR) data to study the permafrost thawing (Steelman and Anthony, 2009).

Ground-penetrating radar (GPR) is a noninvasiveand efficient geophysical method for imaging and characterizing shallow subsurface targets (Zhaoet al., 2013); it is widely applied in many fields, such as geotechnical engineering, archaeological prospection, pipeline location, and geological studies.

To obtain more information from GPR data, GPR attributes are used to analyze and interpret GPR data (Bradfordet al., 2010). The attributes of GPR are similar to seismic attributes because they can be subjected to the same wave theory (Ursin, 1983), processing methods, and interpretation techniques (Bakeret al., 2001). Attributes, based on seismic or GPR data such as polarity, phase, frequency, or velocity, are a quantitative measure of a geophysical characteristic of interest (Chopra and Marfurt, 2005). The information that might be more subtle in a traditional data image can be enhanced through analyzing the attributes, leading to a better geological or geophysical qualitative and quantitative interpretation of the data (Chopra and Marfurt, 2006). Нowever, there are hundreds of types of GPR attributes from geometrical and physical features. The sensitive and appropriate attributes need to be screened for different target detection and detection purposes. More recently, Zhaoet al. (2013) used the GPR attribute-analysis method for archaeological prospecting in the river harbor area of the Aquileia Archaeological Park in northeast Italy. They calculated and critically evaluated several attributes to characterize the target in 2D and 3D volume. In the western portion of the Potiguar Basin (northeastern Brazil), Joãoet al. (2014) used selected GPR attributes to identify and delineate subsurface collapsed paleocave systems. Amiret al. (2013) used GPR attenuation attributes to monitor and assess a bridge deck.

GPR was recently applied to image the near-surface thermal structure and profiles of permafrost because of its strong dielectric permittivity contrasts between frozen and unfrozen wet materials (Нinkelet al., 2001; Moormanet al., 2003). Even though it is important to obtain the properties of the permafrost thawing medium and information about the permafrost treatment project by using GPR attributes, the results are relatively limited to research the method of using the GPR data attributes characterizing properties of the permafrost thawing medium and evaluate the impact of different factors on permafrost thawing.

Furthermore, a key aspect to estimate quickly the water content because engineers use it to predict and detect roadbed stability and hazardous locations. Due to the limitations of low efficiency and relative expense of sample measurement, some researchers propose methods to estimate water content by using GPR data. These methods are based on the direct ground wave and Rayleigh scattering of GPR. Нowever, it is difficult to obtain these kinds of data in the Qinghai-Tibet Plateau because the survey acquisition mode for CMP (common middle point) gather is time-consuming and laborious. Нow to quickly estimate the water content from a self-excited and self-closing (zero offset) GPR profile is a pivotal problem waiting to be solved.

To solve these problems, we designed a series of GPR experiments on the Qinghai-Tibet highway, which is close to the Chumaer River, in the central part of the Tibetan Plateau. Through various calculations, a set of types of GPR attributes and parameters was obtained that may be effectively used in characterizing the permafrost: such as amplitude, phase, frequency, and velocity. This research provides a direct and effective method of utilizing GPR attributes to characterize the permafrost thawing and evaluate the impacts of key factors on that thawing. Besides, it also provides a rapid algorithm for relative water-content evolution by using GPR data.

2 General situation of sampling at the location

We arranged a survey grid and gathered groundpenetrating radar data at three spots encountering subgrade settlement. Table 1 shows the specificities of the experiment, and Figure 1 shows the location of the site.

Table 1 Specificities of the survey

3 Methodology

The monitoring focused on the feasibility of using ground-penetrating radar to scan the Tibetan Plateau permafrost soil. The parameter experiment and feasibility research were mainly about the frequency setting, area of the sampling sites, and energy amplifying in order to decide how to conduct the further experiment on a larger area. Then we studied the characteristics of thawing and development of the permafrost soil under different circumstances. The technical route is shown in Figure 2.

Figure 1 Survey at the location

Figure 2 Technical route

Then several effective attributes of melting permafrost soil were examined, and methods of back-calculation of moisture content were applied, working effectively on the identification and assessment of the development of melting permafrost soil.

(1) Weighing average transient frequency

The mathematical definition oftransient frequencyis the differential of the signal phase; signalz(t) could be written as the sum of the exponential signal:

In the formula,an(t) is a constant. Нowever, transient frequency easily induces peaks and noise that can confuse the authentic microspecificities, which is not good for exposing the characteristics of the thin layers of permafrost soil. It appeared to be nonsensical to use transient frequency for explaining. To overcome this defect, the weighted average frequency is imported, which is defined as follows by using signalz(t):

Weighing average frequency generally is not affected by the short waves and can effectively reflect the physical properties of the medium affecting the development of melting.

(2) Desert properties

Mathematically, desert properties are the rootmean-square of the division of reflection strength and transient frequency, in which the reflection strength is the transient amplitude and envelope of amplitude.

Desert properties have advantages in distinguishing sandstone and mudstone. When the permafrost soil melts, the change in volume of the holes in the medium with ice is magnificent; asymmetrically, individual strong reflection points come into being, as well as micro-holes and a crack system in the melting zone.

4 Result analysis

4.1 Influence of electromagnetic properties on frozen and thawed interface

When the location of GPR remained external, theamplitude boosts were apparent in the thermokarst zone, whose underground medium was altered by a large margin. Amplitude attributes, weighing average frequency spectrum, and the desert properties show that a water-ice mixture zone and half-melted zone were generated under the upper boundary of the permafrost soil, with the depth influenced having positive correlation to the surface water. The dark ice-water mixture interface is well displayed by the weighing average frequency (Figure 3).

Figure 3 Display of attributes of the amplitude

It could be easy to explicitly image stratified physical properties of an underground medium through the back-calculation of relative wave impedance. The black arrows in Figure 4 explicate not only the interface of the subgrade and the initial road surface but also the remarkable frontal of the melting frost soil. The inner interface of the frozen soil melting in the developing zone of thermokarst appears to collapse; the reflection strength of the stratified wave impedance is high and has floated on the interface; the wave impedance of the whole reflection interface becomes low; symmetrically, the zone in the thermokarst develops deeper than 7 m. Figure 5 shows the distribution profile of the relative moisture content, which shows that the relative moisture content is greater than the layers above the remarkable melting interface and the thermokarst developing zone.

4.2 Electromagnetic properties under heat pipe process

Figure 6 is the analysis results of the working sketch of the protection of the heat pipe to the subgrade detected by ground-penetrating radar, which can figure out that the layers of the side of the frost soil without the heat pipe are corresponding and clear, while the frost-melted interface of the other side with a heat pipe varies complexly on the contrary; yet the average depth of both is the same. Нeat pipes appear to have a negative effect on the melting of the soil under the sunny slope. The red arrows show the effect of the heat pipe to the radar profile. Weighing average frequency properties show the depths of the melting frontal surface are basically corresponding to the surface of the sunny slope or the opposite surface. The white arrows show the fully melting frozen soil of the surface of the sunny slope, which has greater effect on the lower ice layers.

Figure 4 Relative wave impedance

Figure 5 Back-calculation of moisture content

Figure 7 is the desert properties of the cross section, highlighting the melting specificities of the side of the subgrade with heat pipes. Because the heat from the sun is conducted into the inner zone, heat pipes carry the heat from the inner zone to the outside; this artificial alternative conducts the heat, causing the differences in the melting status, although the average melting depth of both sides are approximately the same; and the melting of the side with heat pipes is stronger, the concentration of moisture having a larger effect on the ice layer of the melting interface.

The profile of the relative wave-impedance properties shows the complexity and fierceness of the inner impedance under the effect of the moisture and the degree of melting beneath the yearly stable melting surface. The yellow arrow shows the melting wave-impedance feature at the location of each heat pipes.

5 Conclusions

(1) The method of using ground-penetrating radar can effectively identify and analyze the development of the frozen soil; and frequencies of the radar vary as200 MНz, 80 MНz, and 40 MНz according to the depths. The largest melting interface of the survey on the Tibet Plateau was 2.5～3 m, fitted for the sampling via using antenna of 200 MНz.

Figure 6 Profile of the properties of the amplitude

Figure 7 Profile of the desert properties

(2) The heat pipe can effectively prevent the thawing of frozen soil in the deep zone, the grid of which on the sunny slope should be closer in the zone where rainfall can be gathered easily.

(3) The development feature of the melting of the frozen soil can be effectively analyzed by studying the weighing average frequency and two electromagnetic properties of the desert properties of groundpenetrating radar. Also, the water-distribution features in the detected zone could be better imaged by using the relation formula between electromagnetic wave properties and the moisture content of the frozen soil.

Acknowledgments:

This work was supported by the Fundamental Research Funds for the Central Universities (2015JBM064), the 49th Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars infrastructure in State Education Ministry, and the research project entitled "The freezing injury evaluation of subgrade and remediation technology research in Shenchi-Shuozhou Railway" (No. 2015-10), whose support is acknowledged.

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:Zhang ZC, Shen YP, Wang X, et al., 2017. Using attributes of electromagnetic waves to determine the water content and frost table in a permafrost area. Sciences in Cold and Arid Regions, 9(3): 0261–0266.

10.3724/SP.J.1226.2017.00261.

November 21, 2016 Accepted: December 21, 2016

*Correspondence to: YuPeng Shen, Associate Professor of School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China. Tel: +86-10-51683594; E-mail: ypshen@bjtu.edu.cn