Propagation of ground penetrating radar waves in Chinese coals.

This paper reports an experimental study on the electrical properties of five coal samples taken from various Chinese coal mines. The dielectric permittivity and specific resistivity of grinded coal samples subjected to electromagnetic (EM) fields in a wide frequency spectrum were determined. Based on the experimental data, a set of approximating equations of the change in electric properties the 100-1000 MHz frequency region was obtained. These equations, along with EM equations for EM speed and attenuation, were used for modeling and studying radar-wave propagation in a coal seam and radar-wave reflection from the body of miners trapped in collapsed tunnels. The modeling concept assumes that a radar transducer with the dominating frequency of 500 MHz is lowered through a vertical or inclined rescue borehole to the depth of the coal seam. It is assumed that only the miner is present in the part of the tunnel that did not collapse. Thus, in the path of the radar wave from the transducer to the human body, only one geological interface reflecting the radar signals is present (coal-air) and one is connected with human body. The human (acting as the reflector) can be located at various distances from the tunnel face; this factor was included in the analyzed geometrical model. Based on the modeling results of different thickness coal seams (2, 3, 4, 5, 6, 7, 8m), conclude that a radar wave reflected from a human body can be reliably measured, when the distance between the human and the transducer is not exceed 8m.

1. Introduction

In China, coal appears to be the main energy source for the near future [1, 2]. Additionally, it constitutes a large proportion of the global primary energy sources [3-5]. Accordingly, safety issues in coal mining processes have always been a research hotspot: coal mine emergency rescue remains as the last barrier to safety and security.

In recent years, with advancements in science, technology, management, and quality of workers, the safety of coal mines has clearly improved. The total number of accidents and death tolls have dropped significantly. However, major accidents in coal mines continue to occur occasionally. For example, two particularly significant coal mine accidents and eight major accidents occurred in 2016 [6]. Coal mine accidents inflict large numbers of casualties, along with severe economic losses and adverse social impacts [7]. The earlier the location and condition of the people caught in mining accidents are known, the greater the possibility of them being rescued. The ground rescue method must be considered on encountering the following two scenarios: (a) the roadway could not be used because of coal or rock fall, high temperature, and presence of water, among others adverse conditions. (b) the lives of the rescue workers are threatened. To date, some successful drilling rescue cases have been recorded. For example, a water permeable accident occurred in the Kuixi Coal Mine in USA in July 2002. A mine collapse accident occurred in San Jose copper mine in Chile in August 2010 [8]. A mine collapse accident occurred in Yurong gypsum mine in China in December 2015 [9]. Long-distance drilling could be deflected during construction under the current technical conditions (Fig 1). Under these circumstances, life information detection technologies, such as video or audio, infrared, and gas, would not be available because they cannot penetrate the coal seam [10]. Among numerous non-contact life information detection technologies, ground penetrating radar (GPR) radar is considered to as the most effective because of its high resolution, deep penetration, and low power radiation, making it suitable for life rescue systems [11, 12]; thus it has been used for life detection and rescue [13]. Many works have been conducted on life detection based on radar [14-18]. Based on eigenvalues using UWB impulse radarremote sensing, Minhhuy studied the heart rate extraction [19]. Using UWB impulse radar, Liang et al studied the improved denoising method for through-wall vital sign detection [20]. Therefore, we believe that the GPR radar technology could be the direction of future development of life information detection through coal seams. This study shows that the propagation of GPR radar waves in coal, and the related influencing factors have very important scientific and practical significance.

Fig 1

Schematic concept of and application of borehole GPR method for location of miner trapped in underground tunnel in coal mine.

A is a three-dimensional diagram of rescue drilling during the tunnel collapse; B is a schematic diagram of the drilling offset.

Numerous scholars have extensively studied the electromagnetic properties of coal and the propagation of electromagnetic waves in coal. Fan et al. studied the dielectric properties of coals in the low THz frequency region [21]. Fan et al. studied the dielectric properties of coal in the THz frequency region of 100–500 GHz [22]. Qin and Wei studied the relative dielectric constant sweep measurement based on a vector network analyzer [23]. Brach studied the real part of the permittivity of a series of coals as a function of frequency (100 Hz–1.3×107 Hz) and temperature (200–400 K); the results showed that certain parameters are related to the microstructure of the carbonaceous phase [24]. Marland et al. tested the dielectric properties of coal using the microwave cavity method in the laboratory, and qualitatively discussed the effects of temperature, frequency, coal rank, minerals, and water content on the coal dielectric properties [25]. Valentina studied the influence of polyethylene terephthalate on the carbonization of bituminous coals and on the modification of their electric and dielectric properties [26]. The relationship between interior moisture distribution and coal dielectric properties has been constructed by Wang. Furthermore, the characteristics of permittivity variation with moisture content has been studied [27]. Peng et al studied the dielectric characteristics of an Indonesian low-rank coal by determining its permittivity from 21°C to approximately 1000 oC at 915 MHz and 2450 MHz in argon [28]. Tao et al. analyzed the effect of sulfur content on the dielectric properties of coal [29]. Xu et al. studied the influence of the crystallite structure on the dielectric properties of coal chars [30]. Liu studied the effect of coal rank on the dielectric properties through structural differences in various coal chars [31]. Emslie and Lagace. put forward the model of a coal seam waveguide, for the first time, when studying radio wave communication in coal mines [32]. Hill derived the theoretical formula of electromagnetic wave propagation under the layered model and studied the propagation of electromagnetic waves in uneven coal seams [33, 34]. Greenfield used finite difference simulations to analyze the attenuation characteristics of electromagnetic waves in discontinuous coal seams [35]. Luo et al. analyzed the effect of electrical properties of coal seams on the attenuation characteristics of electromagnetic waves [36-39]. Zhu et al. reviewed the research progress on the measurement technique of coal dielectric constants and analyzed the dielectric models of several coals [40]. Clearly, only a few have investigated the electrical parameters of coals of different ranks in the frequency range of 100–1000 MHz.

In this paper, therefore, we report an experimental study on the dielectric properties of five coal samples of different ranks, with frequencies ranging from 100–1000 MHz. Furthermore, we construct the approximation equations for propagation of radar waves in coal, and simulate and solve the propagation process. Finally, the effective range of penetration of radar waves in coal is given.

2. Experiment and methods

2.1. Measurement of dielectric properties

In this study, five different coal samples were used based on their ranks: lignite (1#), Dananhu coal mine, Xinjiang; long flame coal (2#), Tangjiahui coal mine, Inner Mongolia; gas coal (3#), Baodian coal mine, Shandong; lean coal (4#), Sangshuping coal mine, Shaanxi; anthracite (5#), Huangyanhui coal mine, Shanxi. The proximate analysis and elemental analysis results are provided in Table 1. Fresh coal samples, which were not subjected to water injection, spraying, and other treatments were sealed and packed with multilayer plastic and nylon bags, and transported to the laboratory. Each experiment was conducted four times to ensure reproducibility. The samples were prepared as follows. A fresh coal sample was crushed, and pulverized coal having a diameter of less than 0.074 mm was screened out. A certain amount of pulverized coal was weighed into a tableting machine to prepare a circular coal sample piece of diameter 1.28 cm and thickness 1.0 mm. Then conductive silver colloid was evenly coated on both sides of the coal sample, and could be tested after the silver gel dries. The preparation process of the sample is illustrated in Fig 2.

Table 1

Proximate and elemental analyses of coal.

Coal	Proximate analysis (%)				Elemental analysis (%)					Porosity (%)
	Mad	Aad	Vad	FCad	C	H	O	N	S
1#	8.80	36.62	24.68	29.90	38.94	4.01	22.00	1.15	0.27	10.11
2#	5.77	11.93	32.32	49.98	64.86	5.63	19.20	1.73	0.58	13.34
3#	2.33	15.92	35.45	46.30	63.92	4.85	17.40	2.37	0.51	6.16
4#	0.53	24.49	12.95	62.03	62.51	3.76	17.00	1.61	3.41	9.19
5#	1.20	8.84	7.85	82.11	81.82	3.79	4.90	0.85	1.49	16.20

Fig 2

Flow chart of processing test of coal samples.

The Concept-80 broadband dielectric spectrum test system was employed in the equipment, which mainly included the control software, an E4991A impedance analyzer and test fixture (Fig 3).

Fig 3

Concept-80 broadband dielectric spectrum test system.

2.2. Propagation equations of radar waves in coal

2.2.1. Dielectric properties of tested coal samples. The relative dielectric constant values of the five tested coals are shown in Fig 4, and the resistivity values are shown in Fig 5.

Fig 4

Changes of the relative dielectric permittivity with frequency for various coals.

A: lignite; B: long flame coal; C: gas coal; D: lean coal; E: anthracite; F: collective plot for all tested samples.

Fig 5

Variations of tested coals resistivity with frequency.

A: lignite; B: long flame coal; C: gas coal; D: lean coal; E: anthracite; F: collective plot for all tested samples.

Fig 4 shows the trend of the relative dielectric constants of the five coal samples were similar. They decreased first and then increased with the increasing test frequency. The minimum values of the dielectric constant occurred at approximately 300 MHz, whereas the largest values occurred at approximately 1000 MHz. At the same test frequency, lean coal had the highest relative dielectric constant, followed by lignite, gas coal, long flame coal, and anthracite (Fig 4(F)). The lignite contained a large number of hydrophilic polar functional groups, which could adsorb a large amount of intrinsic moisture. Polar functional groups and moisture had a high relative dielectric constant. Therefore, the dielectric constant of lignite was higher. The order of relative dielectric constants of the other four ranks of coal should be determined based on the carbon content in coal. The relative dielectric constant of lean coal was higher than that of lignite because it was influenced by other factors, such as porosity [24]. The relative dielectric constants and frequencies were fitted and were found to satisfy the cubic polynomial relationship. The results are given by Eq (1), and the correlation coefficients are listed in Table 2. where ε is the relative dielectric permittivity, f denotes the frequency, and C is a coefficient (i = 0, 1, 2, 3).

Table 2

Coefficients of fitting equation.

Coals	C3	C2	C1	C0	R2 (%)
1#	-9.24×10−10	1.84×10−6	-9.14×10−4	3.62	98.98
2#	-5.82×10−10	1.17×10−6	-5.43×10−4	3.07	98.77
3#	-6.34×10−10	1.28×10−6	-5.96×10−4	3.35	99.21
4#	-7.90×10−10	1.52×10−6	-6.58×10−4	3.78	99.40
5#	-6.42×10−10	1.34×10−6	-6.86×10−4	3.05	98.49

Fig 7 shows the resistivity of coal with a different degree of metamorphism had an overall decreasing trend with the increasing test frequency. A convex transformation point occurred near 500 MHz, which could divide the trend of resistivity with frequency into two stages. It was found, through fitting, that part Ⅰ conformed to a cubic polynomial and part Ⅱ conformed to a quadratic polynomial. The results are given in Eq (2) and Table 3. Beyond 700 MHz, the resistivity of the tested coals tended to be flat and the differences between them were small. As the test frequency increases, the electrons in a bundled state of the coal structure group become freely excited electrons, causing a significant increase in free radicals (i.e., unpaired electrons) of coals; therefore, the resistivity would decrease. Previous studies have shown that anthracite is a resistive absorbing material [41], and the resistivity of its mineral impurities is higher than that of organic matter [42]. These factors could make the resistivity of anthracite relatively large.

Fig 7

Schematic diagram of types of reflection and refraction.

Radar waves travel from one object to another. (1): air→coal; (2): coal→air; (3): air→person; (4): person→air; (5): coal→person; (6): person→coal.

Table 3

Coefficients of fitting equation.

Part	Coefficientntt	1#	2#	3#	4#	5#
Ⅰ	C13	-8.00×10−5	-5.45×10−5	-4.96×10−5	-4.08×10−5	-7.48×10−5
	C12	0.10	0.07	0.06	0.05	0.10
	C11	-43.06	-30.19	-27.52	-23.03	-40.16
	C10	6.13×103	4.45×103	4.06×103	3.43×103	5.78×103
	R2	98.40%	99.25%	99.25%	99.24%	98.95%
Ⅱ	C22	1.28×10−3	9.50×10−4	8.74×10−4	7.99×10−4	1.12×10−3
	C21	-2.36	-1.81	-1.66	-1.48	-2.22
	C20	1.21×103	9.60×103	8.80×102	7.86×102	1.10×103
	R2	97.94%	99.11%	98.84%	98.58%	98.91%

2.2.2. Equation of radar wave propagation in coal

To simplify the model, we assumed that the coal body was a homogeneous medium. Because radar waves are a type of electromagnetic waves, their propagation velocity in coal could be expressed by Eq (3) [43] and Fig 6, and the results are summarized in Table 4. where v is the propagation velocity of radar waves in coal (m/s), and ε is the specific relative dielectric permittivity of coal.

Fig 6

Changes of attenuation coefficient with frequency.

A: lignite; B: long flame coal; C: gas coal; D: lean coal; E: anthracite; F: collective plot for all tested samples.

Table 4

Propagation speed of radar waves in coal (m/s).

Frequency (MHz)	1#	2#	3#	4#	5#
100	15.70×107	17.23×107	16.14×107	15.53×107	17.32×107
200	16.01×107	17.29×107	16.64×107	15.57×107	17.43×107
300	16.05×107	17.32×107	16.90×107	15.59×107	17.46×107
400	16.03×107	17.29×107	16.64×107	15.55×107	17.46×107
500	16.01×107	17.23×107	16.14×107	15.53×107	17.39×107
600	15.94×107	17.20×107	16.64×107	15.45×107	17.37×107
700	15.87×107	17.12×107	16.15×107	15.38×107	17.29×107
800	15.81×107	17.66×107	16.42×107	15.32×107	17.26×107
900	15.76×107	17.14×107	16.29×107	15.30×107	17.17×107
1000	15.74×107	16.84×107	16.26×107	15.26×107	17.14×107

According to the literature [44], the attenuation coefficient of electromagnetic wave propagation in coal could be characterized by Eq (4). Combined with the dielectric constant and electrical conductivity of coal, the variation of attenuation coefficient with frequency could be obtained (Fig 6). Where α is the attenuation coefficient of radar waves in coal, f is the frequency, ε is the relative dielectric permittivity of coal, ρ is the specific resistivity of coal, and μ is the magnetic permeability of coal.

Fig 6 shows the propagation attenuation coefficient of electromagnetic waves in five types of coal followed a similar trend in relation to frequency. It could be divided into two parts, based on the boundary line near 500 MHz (Ⅰ and Ⅱ). In part Ⅰ, the attenuation coefficient was a cubic function of frequency: it increased first and then decreased with the increasing frequency. At the same frequency, attenuation coefficients of coals decrease in the following order: lean coal, gas coal, long flame coal, anthracite, and lignite. In part Ⅱ, the attenuation coefficient was a quadratic function of frequency: it increased with the increasing frequency. In ① of part Ⅱ, at the same frequency, the attenuation coefficients decrease in the following order: anthracite, long flame coal (gas coal, lean coal), and brown coal. Among them, the attenuation coefficients of long flame coal, gas coal and lean coal had low differences. In ② of part Ⅱ, at the same frequency, the attenuation coefficients decreased in the following order: anthracite (long flame coal, gas coal), lean coal, and brown coal. Among them, the attenuation coefficients of anthracite, long flame coal, and gas coal were similar.

The attenuation coefficient and frequency were fitted, and the results are given by in Eq (5) and in Table 5.

Table 5

Coefficients of fitting equation.

Part	Coefficientntt	1#	2#	3#	4#	5#
Ⅰ	C'13	-8.31×10−14	-6.70×10−14	-7.01×10−14	-7.58×10−14	-6.91×10−14
	C'12	6.36×10−11	4.95×10−11	5.17×10−11	5.55×10−11	5.21×10−11
	C'11	-1.03×10−8	-6.20×10−9	-6.46×10−9	-6.90×10−9	-7.25×10−9
	C'10	4.71×10−7	1.97×10−7	2.05×10−7	2.25×10−7	2.52×10−7
	R2	99.36%	99.72%	99.72%	99.71%	99.69%
Ⅱ	C'22	-3.31×10−12	-1.31×10−12	-2.22×10−12	-4.63×10−12	-6.46×10−11
	C'21	8.44×10−9	6.78×10−9	7.90×10−9	1.04×10−8	6.12×10−9
	C'20	-2.61×10−6	2.04×10−6	-2.34×10−6	-2.94×10−6	-1.97×10−6
	R2	98.88%	99.52%	99.37%	98.66%	99.58%

According to the literature [45], the reflection (R) and refraction coefficients (T) of radar waves propagating in coal on the boundary interfaces could be characterized by Eqs (6) and (7), respectively (Fig 7). where R is the reflection coefficient of radar waves at the interface between media i and j. T is the refraction coefficient of radar waves at the interface between media i and j. ε is the relative dielectric constant of medium i, and ε is the relative dielectric constant of medium j.

The reflection and refraction coefficients of the interfaces were calculated by taking the relative dielectric constant of coal at 500 MHz as an example (Table 6). Clearly, a negative reflection coefficient appears, because the radar wave from the side with a relatively small dielectric constant moves to the side with a relatively high dielectric constant, indicating that the reflected wave was opposite in phase to the incident wave.

Table 6

Reflection and refraction coefficients.

Coal	Type	(1)	(2)	(3)	(4)	(5)	(6)
		air→coal	coal→air	air→person	person→air	coal→person	person→coal
1#	Rij	-0.31	0.31	-0.75	0.75	-0.58	0.58
	Tij	0.698	1.31	0.25	1.75	0.42	1.58
2#	Rij	-0.27	0.27	-0.75	0.75	-0.60	0.60
	Tij	0.28	1.27	0.25	1.75	0.40	1.60
3#	Rij	-0.29	0.29	-0.75	0.75	-0.59	0.59
	Tij	0.71	1.29	0.25	1.75	0.41	1.59
4#	Rij	-0.32	0.32	-0.75	0.75	-0.57	0.57
	Tij	0.68	1.32	0.25	1.75	0.43	1.57
5#	Rij	-0.27	0.27	-0.75	0.75	-0.61	0.67
	Tij	0.737	1.27	0.25	1.75	0.39	1.67

The propagation distance of the radar waves in coal was determined by the attenuation coefficient. Eq (4) shows that the independent variables determining the variable α (attenuation coefficient) are ƒ (frequency), ε (relatively dielectric constant), and ρ (resistivity), i.e., . From Eq (1), (relatively dielectric constant) is a function of ƒ (frequency), i.e., . From Eq (5), ƒ (frequency) is a function of α (attenuation coefficient), i.e., . Thus, the propagation attenuation law of radar waves in coal is given by where α is the attenuation coefficient of radar wave propagation in coal; F, ε, and ρ represent the frequency, relative dielectric constant, and resistivity, respectively. , C, are the coefficients of the corresponding function, respectively.

The characteristics of radar waves must be analyzed to find if someone is present behind the coal, when using radar for rescue. The characteristics of the waves were determined by the laws of refraction and reflection, consisting of Eqs (6) and (7): where ε and ε are the relative dielectric constants of substances i and j, respectively. R and T are the reflection and refraction coefficients, respectively.

Based on the relationship between distance, speed, and time, the distance between the radar and a person in distress can be calculated: where v is the propagation speed of radar waves in coal (m/s).

Eq (11), formed by combining Eqs (8), (9), and (10), is the control equations for the propagation of radar waves in coal:

3. Results and discussion

Eq (11) could be solved using gprMax code. gprMax is a simulation software based on the finite-difference time-domain theory, which was developed by Dr. Antonis Giannopoulos of the University of Edinburgh [46]. gprMax has been widely used for simulation and modelling of radar wave propagation in GPR applications such as underground pipeline surveys, roadbed inspections, ancient building surveys, military surveys, quality inspections, and hidden grave surveys [47-51]. The simulation calculation process is shown in Fig 8. The simulated physical model is shown in Fig 9(Ⅰ).

Fig 8

Simulation calculation flow.

Fig 9

Geometrical model of rock strata applied in numerical simulation.

Ⅰ: Schematic illustration of a disaster; Ⅱ: Schematic physical model; Ⅲ: Display of physical model in GPR software.

As shown in Fig 9(Ⅱ), the thickness of a coal body between a trapped person and the radar wave is L1, distance between a person and coal wall is L2, width of the model is L3, length of the model is L4, and dimensions of the human body is 1.6 m ×0.4 m. The specific settings of different models are listed in Table 7. Then, the simulated data was post-processed using MATLAB, and the results are shown in Fig 9(Ⅲ).

Table 7

Size of model (m).

Model	M1	M2	M3	M4	M5	M6	M7
L1	2	3	4	5	6	7	8
L2	1.3	1.3	1.3	1.3	1.3	1.3	1.3
L3	3.3	4.3	5.3	6.3	7.3	8.3	9.3
L4	10	10	10	10	10	10	10

It was assumed that coal is a semi-infinite continuous space with homogeneous isotropy. The dielectric constant of human body is 50 [52], and that of air is 1; the antenna stepping distance was 0.06 m, and the transmitting and receiving antenna spacing was 0.065 m. The number of measuring lines was 115, spatial grid step was 0.005 m × 0.005 m, permeability of all media in the model was 1.0, boundary condition was a perfect matching layer (PML), excitation function of the radar was Ricker, and the center frequency was 500 MHz. The partial electrical parameters of the model were set as given in Table 8. Due to the large number of graphs of simulation results, only lignite was considered for analysis in this day (Fig 10).

Table 8

Dielectric characterization of coal.

Coal	1#	2#	3#	4#	5#
ε	3.51	3.03	3.30	3.73	2.99
ρ (Ω·m)	328.95	290.07	267.78	242.41	318.53

Fig 10

A plot of signal amplitudes versus travel time.

Mx are the thickness of the coal body between the radar and the trapped person. M1: 2 m; M2: 3 m; M3: 4 m; M4: 5 m; M5: 6 m; M6: 7 m; M7: 8 m. h is two-way travel time of radar wave between two objects [52]. The meaning of B and C is given in Fig 9.

From Fig 10, we found that the amplitudes of the return wave from the interface between the coal pillar and the roadway in seven models were 12.51, 5.48, 2.56, 1.23, 0.63, 0.30, and 0.15 mV/m, and the amplitudes of the return wave reflected from the human body were -20.63, -10.24, -5.30, -2.72, -1.37, -0.69, and -0.35 mV/m. Clearly, the amplitude gradually decreased with the increase in L1 (thickness of coal pillar). The larger the value of L1, the longer the electromagnetic wave travels in coal. Moreover, the longer the electromagnetic wave travels, the higher its energy loss; therefore, the intensity of the target return wave amplitude gradually became smaller. By fitting, it was found that the amplitude of the wave passing through the human body decreases with the increase in L1 as an exponential function (Fig 11). The effective propagation distance of radar waves in coal was 6–8 m for the transducer with the central frequency of 500 MHz.

Fig 11

Relationship between amplitude of a radar signal reflected from a person and a coal seam thickness L1.

4. Conclusions

The dielectric properties of five different coal samples obtained from various Chinese coal deposits were studied using the alternating current impedance method and the Concept-80 broadband dielectric Spectrum test system for electromagnetic waves, in the frequency range of 100–1000 MHz. Additionally, the propagation mechanism of radar waves in coal was analyzed.

The experimental results showed that the dielectric constants of the five samples follow similar trends. The values of relative dielectric constant of coals decreased first and then increased with the increasing frequency. At the same frequency, the relative permittivity of coal in the descending order was lean coal, lignite, gas coal, long flame coal, and anthracite. The specific resistivity of coal decreased considerably with the increasing frequency, but an upward bump was observed near 500 MHz.

Furthermore, the key parameters of the radar waves propagating through coal were analyzed. The propagation velocities values of radar waves through coals of different ranks and the reflection and refraction coefficients at different interfaces were calculated. The attenuation coefficient of the radar waves in all coal samples was found to follow a similar trend.

With 500 MHz as the dividing point, the relationship between the attenuation coefficient of radar waves and the frequency was divided into two parts. In part I, the frequency was a cubic function of the attenuation coefficient. The attenuation coefficient initially increased to a certain point and then decreased with the increasing frequency. In part II, the frequency was a quadratic function of the attenuation coefficient, and the attenuation coefficient increased with the increasing frequency. A set of equations for the propagation of radar waves in coal were established, based on the above findings. By analyzing the governing equations, it was found that the effective penetration distance of radar waves with the dominant frequency of 500 MHz in coal is 6–8 m.

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23 Dec 2019

PONE-D-19-29325

Propagation Law of Ultra-wideband Radar Waves in Coal

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Partly

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: No

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: See attached for completed comments.

The key contribution of this paper is that it reports the effects of EM wave frequencies on two fundamental parameters or properties: resistivity and dielectric permitivity for different rank coal samples. I am not quite sure one can claim the outcome/results of the lab testing of these parameter can be declared as “Law”. The claim of the Ultra-wideband radar wave is also arguable as the radar wave used is not necessarily ultra-wideband but the frequency was varied from 100 Mhz – 1000 MHz. Otherwise they would not be able to obtain the parameter variations with frequencies. Based on these measurements, it also derived the EM wave attenuation and speed variations with frequencies for different rank coal samples.

The speed is computed by using the simplified version of the wave speed formula (3). I suggest you may also compute with a more accurate formula such as (Zhou and Fullagar, 2001):

So that it may get different values to the simplified one. Surely the results may do not change much as the frequencies used are relatively high and the approximated version could provide sufficient accuracy.

One of the objectives of the paper is to use GPR for detection of mine trapped personnel after a coal mine disaster. With a frequency ranging from 100 MHz to 1000 MHz, I wonder what kind of penetration capability it will be although the paper claims, in Introduction, the GPR has “strong penetration”. The detection examples in Figures 11 & 12 are simplified too much with no practical use at all. At most, you only illustrate you can estimate the distances of reflection objects without knowing which direction they come from. Such ability is well illustrated in seismic and GPR textbooks. GPR is relatively matured technique. Is there any successful examples such as from controlled experiments in coal mine environment? I guess not. Even if you can receive the reflections from trapped personnel, you need to address the issue such as how to differentiate the human reflections from many other reflections from imhomogeinities of the strata, underground equipment and objects. If you cannot resolve this issue, your GPR measurements have no use to this particular applications.

Based on my above comments, I suggest the author concentrated on the property measurement report rather than try also to address the trapped human detection. Properly understanding the property of coal samples for different frequencies is a good contribution to the literature.

In addition, please improve your English expression of the paper to ensure communicate your finding to your reader effectively. This can be achieved by asking a native English speaker or English Editing services to help you before submit your paper.

Reference

Zhou, B. and Fullagar, P.K., 2001, Delineation of sulphide ore-zones by borehole radar tomography at Hellyer Mine, Australia: Journal of Applied Geophysics, 47, No 3-4, pp.261-269.

Reviewer #2: General remarks: The paper presents data from laboratory testing of electrical properties of various coal samples taken from Chinese mines. Test were performed on a commercial Broadband Dielectric Spectrum Test System -Concept-80. It allows to determine the values of dielectric permittivity and specific resistivity of grinded coal samples subjected to the electromagnetic fields in wide frequency spectrum. On the base of laboratory data a set of approximating equations showing the change of electric properties in frequency domain ranging from 100-1000 MHz was obtained. These equations together with known from EM theory formulas for EM waves speed and attenuation were used for modeling and studying the process of radar waves propagation in a coal seam and reflection from the miners entrapped in collapsed tunnels. The modeling concept assumes that a radar transducer with dominating frequency MHz will be lowered through a vertical or inclined rescue borehole to the depth of coal seam with mining opening. It is assumed that in not collapsed part of tunnel only miner is present. Thus on the path of radar wave from transducer to a human we have only one geological interface reflecting the radar signals (coal-air) and one connected with human body. The human reflector can be located in various distances from the tunnel face. This factor was included in analyzed geometrical model. On the base of modeling results the authors of the paper conclude that a reliable human origin reflected radar signal can be measured when a distance between human and transducer do not exceed 5 m.

Although the concept and research data presented in the manuscript are very interesting and valuable scientifically it needs a thorough revision. There are a lot of deficiencies. Starting from the title, the next parts of manuscript (abstract, introduction and conclusions) in my opinion should be completely re written (my general remarks can be used as a new abstract in the paper). The rest of them need to be corrected. I suggest to remove figures 1 and 2 from the paper. They do not bring nothing and are detached from the content of the paper. Some other figures also need corrections. I recommend to send the manuscript back to authors for major revision. My detailed remarks are included in a word processor file, attached to the revision. I hope that they help the authors to correct the manuscript

**********

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

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Submitted filename: Review Comments.pdf

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Submitted filename: PONE-D-19-29325_reviewer-ak7.docx

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6 Jan 2020

We thank the anonymous reviewers for taking precious time to review our manuscript. We appreciate the reviewers giving us valuable comments that are helpful to improve our manuscript. Here we attempt to reply where and how we revised our manuscript in the light of reviewers’ comments. Revised contents of our manuscript and our replies are marked in red font.

Reviewer #1

Thanks for the reviewer’s comments. In the revised manuscript, we have modified the title as well as removed the word of “Law”. The controversial term UWB to GPR. As shown in Figure 1, we have developed a prototype radar device that can penetrate the coal seam to detect life information. During the research, we tested the dielectric constant of coal and analyzed the propagation of radar waves in the coal. At the initial stage of research, we have assumed some basic physical models. Experts from the China National Key R & D Project team tested our radar prototype at Jining No. 2 Coal Mine. The results show that the radar can penetrate 5m coal seam. We use a combination of the delay band intersection algorithm and the gyroscope at the bottom of the radar to determine the direction of the target. The electrical parameters of strata and human beings are different. Also the directions of reflected waves are different. Therefore, it is possible to distinguish between reflected waves of people and strata.

Underground equipments and objects bring enormous interference waves during the detection process. We use the following method to screen effective waves. FIR low-pass filter is made use of eliminating direct waves and some background noises. The adaptive minimum mean square algorithm is used to eliminate periodic interference waves and narrowband waves in order to improve the signal-to-noise ratio.

The speed formula was simplified from the accurate formula suggested by the reviewer. We find that the difference between the simplified formula and theaccurate formula was very small. To the certain accuracy, the difference between the results of the two formulas can no longer be seen. For the rigor of the manuscript, we decided to accept the reviewer's opinion that the exact formula was chosen.

The article has been edited by academic editing with the native language.

Reviewer #2

We thank the reviewer carefully checked on this part. The title has been modified to Thank you very much for your detailed suggestions and comments on the manuscript. In particular, the reviewers' general remarks can be directly used as a new abstract of the paper. We have carefully revised the manuscript based on the comments. For example, Figures 1 and 2 have been removed from the paper. We have checked and corrected the words, descriptions, spelling and grammar in the draft in detail.

Submitted filename: Response to reviewers.docx

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21 Jan 2020

PONE-D-19-29325R1

Propagation of Ground Penetrating Radar Waves in Chinese Coals

PLOS ONE

Dear Dr. Zhang,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Kind regards,

Marco Lepidi

--------------------------------------------------------------------------------------

Marco Lepidi, PhD

Associate Professor, Academic Editor of PLOS ONE

Dipartimento di Ingegneria Civile, Chimica ed Ambientale

Università degli Studi di Genova (Italy)

Mail: Marco.Lepidi@unige.it

Website: http://www3.dicca.unige.it/mlepidi/index.html

Google Scholar: http://scholar.google.it/citations?user=uePAdVUAAAAJ&hl=it

Additional Editor Comments (if provided):

Dear Authors,

two reports have been collected for your submission from the same reviewers of the first round. Both the reports are generally positive, but one of the report still recommends major improvements.

Please, take this last opportunity to carefully revise your manuscript according to the questions and/or suggestions provided by the reviewers.

Best regards

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: My concern on how to differentiate the radar reflections from many other reflections from inhomogeneities of the strata, underground equipment and objects has not been properly addressed. I fully understand that human is different from strata and equipment. But there are so many situations that you can observe similar reflections to those from human bodies. For example, will you be able to tell the difference of human reflections from the waterbody reflections in the strata? This is related to multi-solutions of geophysical problems – the same observation can be caused by many different situations. In addition, I have not been convinced by the authors’ response that their prototype radar system with a gyro can tell the direction of received radar waves, unless their radar system is a directional system. Can you provide an example of the testing results for trapped humans using your prototype radar system. Otherwise, there is a 360 degrees ambiguity. If you want to use the radar method for detection of the trapped miners in disasters, you have to address and discuss these issues.

I don’t think your paper has been properly edited by an academic editing people as there are many Chinglish expressions remained – some of them are in the attached marked manuscript. It is interesting to see that you just copied and pasted the reviewer’s comments as your abstract without changing a word.

Reviewer #2: The manuscript has been deeply improved and all my suggestions have been considered. Nevertheless, I've noticed some minor errors or "bugs" which have to be removed before publication. I attach a revised manuscript indicating noticed "bugs". After removal of them the manuscript can be accepted for publishing.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

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Submitted filename: PONE-D-19-29325_R2_reviewer2.docx

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Submitted filename: PONE-D-19-29325_R1_reviewer.pdf

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23 Feb 2020

The author has responded to all reviewer questions in "Response to reviewers".

Submitted filename: Response to reviewers.docx

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16 Mar 2020

PONE-D-19-29325R2

Propagation of Ground Penetrating Radar Waves in Chinese Coals

PLOS ONE

Dear Dr. Zhang,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

We would appreciate receiving your revised manuscript by Apr 30 2020 11:59PM. When you are ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

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An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Marco Lepidi

--------------------------------------------------------------------------------------

Marco Lepidi, PhD

Associate Professor, Academic Editor of PLOS ONE

Dipartimento di Ingegneria Civile, Chimica ed Ambientale

Università degli Studi di Genova (Italy)

Mail: Marco.Lepidi@unige.it

Website: http://www3.dicca.unige.it/mlepidi/index.html

Google Scholar: http://scholar.google.it/citations?user=uePAdVUAAAAJ&hl=it

XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

Dear Authors,

The two reviewers have reconsidered your submission. One of them is fully satisfied, the other is still asking for some minor improvements.

Please, reply (and/or carefully revise your manuscript according) to the questions and/or suggestions provided by the second reviewers.

Best regards

Marco Lepidi

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: No

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: See attached comments

I don’t think the authors have addressed my previous comments properly.

The authors replied

1> We can distinguish the return wave is reflected by the human or the formation water. There are two reasons for this. The first is that the electrical parameters of water are significantly different from the electrical parameters of the human body. The second is that the human's reflected waves are loaded with regular heartbeat signals.

Theoretically what you said is correct. However, in practice, how do you differentiate the human reflection from the formation water body reflection from recorded radargrams? Please provide your method in the paper. Can the radar record show or detect human’s heartbeats? What is the resolution of the GPR system? Can you please provide any evidence or references on how your GPR can detect heartbeating? Please write your paper more scientifically, not with statements without support.

2> As the reviewer said, our radar system is a directional system. As shown in Figure 1, the radar structure includes the transmit and receive window. The size of the window is 70 mm × 50 mm. Experts from the China National Key R & D Project team tested our radar prototype at Jining No. 2 Coal Mine. One of the test scenarios is shown in Figure 2, and the test results are shown in Figure 3.

You only provide numerical examples which prove nothing as any different objects can cause radar reflections theoretically. Your Figure 3 in your reply also shows nothing – why are there no actual radar data presented to support your conclusion? How good is the directionality of your radar system? What is the radiation pattern? Why don’t you include a real data example in this paper for human detection or event to see the heartbeating?

3> The article has been edited by academic editing with the native language. We have provided the certificate of english editing in the previous reply. If required, we can invite academic editors to make it more standard. After reading the reviewers' comments, we all think that it is very perfect. It is more suitable as an abstract for a dissertation. Therefore, we have completely used the reviewer's comments as a new abstract.

I have made some suggestions in the last review but you totally ignored my suggestions and believe that your English is fine! You may pay some attention to the following comments/suggestions (I am sure there are more in the paper) :

1. Line 29: Prefer to change “the authors of the paper conclude” to “, it is concluded”. The reviewer’s writing is ok, but I don’t think the authors of the paper should use similar expression.

2. Line 31: Not quite sure what exact you mean by “do not exceed 5-8 m”! Do you mean it should not exceed 5m or you really mean it should not exceed 8m. Or you mean the distances between 5m and 8m are ok. How did you derive these number? Or is this because you only modelled the data up to 8m?

3. Line 43: I am not sure you have used “particularly significant” and “major” correctly for these events – please check your original Chinese meaning with the corresponding English.

4. Line 67/8: “Fan and Jia et al.” -> “Fan et al.”

5. Line 69: “Fan and Chang et al.” -> “Fan et al.”

6. Line 82: “Peng” -> “Peng et al.”

7. Line 85: “Xu” -> “Xu et al.”

8. Line 88: “Emslie et al.” -> “Emslie and Lagace”

9. Line 94: “Luo and Stolarczyk et al.” -> “Luo et al.”

10. Line 106: “Dielectric properties measurement” ->”Dielectric property measurement”

11. Line 108-110: Change “in” to “from”.

12. Line 130: “followed similar regularities” -> “follow a similar trend”. Using “regularity” is very Chinese!

13. Line 139: “larger” -> “higher”. The constant is “higher” not “larger”.

14. Line 160: “large” -> “high”. We normally say the attenuation is high or low not large or small in English!

15. Line 184, 188, 191: “large”->”high”; “small” ->”low”.

Reviewer #2: I would response in point 5 "yes" but I have noticed some, following typographical errors in the manuscript:

line 102 ........ radar waves in coal "are" constructed (should be "is" constructed),

line 132 ........ largest "one" (should be plural "ones"),

line 216 ......... is relatively (should be "relative").

Figure captions:

Fig.5 variations of tesed coals (should be "tested"),

Fig.9 I - schematic "of" illustration of disaster (should be schematic illustration of disaster),

Fig.9 II - Schematic "of simplified" of physical model (should be schematic physical model),

Fig.11 - Relationship between (or a plot) between............. Dual expression. Should be only one eg. Relationship between amplitudes........ (remove "or a plot").

Notice to editor:

Although the current title of manuscript has been my proposal (revision 1) I suggest the editor to consider the following change "A concept of borehole radar for rescuing miners trapped in deep coal mines". After revisions made this title reflects better the content of the paper. In my opinion it could also increase the interest in the article. I leave decision upon the editor.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files to be viewed.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org. Please note that Supporting Information files do not need this step.

Submitted filename: R2 Comments.pdf

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22 Apr 2020

Reviewer #1

1.Thanks for the reviewer’s comments. The beating of the heart can cause micro-undulation vibrations in the chest cavity at approximately periodic frequencies. The purpose of human body detection can be achieved form identifying the frequency of these micro-motion signals. Obstacles such as formation water bodies have no periodic frequency characteristics. But it still causes interference to radar waves. Therefore, it is necessary to effectively process the radar echoes in order to extract and identify signals containing human micro-motion information features. The method of identifying the characteristics of the micro-motion signal of radar echo information is as follows. The first is the preprocessing of the echo signal. It mainly includes DC offset removal, background removal, time-varying gain, and mean denoising of the echo signal. The second is the selection of sliding time window function and energy integration. The third is to use wavelet transform to process the energy integration results. The number of radar signal sampling points is 4096, and the scanning speed is 16Hz. In particular, radar can be used to obtain heartbeat and respiration signals, which has been employed in many areas. Such as: “Xiaolin Liang, Hao Zhang, Shengbo Ye, et al. Improved denoising method for through-wall vital sign detection using UWB impulse radar, Digital Signal Processing 74(2018) 72–93”, “Minhhuy Le. Heart rate extraction based on eigenvalues using UWB impulse radar remote sensing, Sensors and Actuators A 303 (2020) 111689”, “F. JalaliBidgoli, S. Moghadami, S. Ardalan, A compact portable microwave life-detection device for finding survivors, IEEE Embed. Syst. Lett. 8(1) (2016) 10–13”.

2. We thank the reviewer for this valuable comment. He is correct that any different object can cause radar reflections in theory. However, the slight fluctuation of the chest cavity caused by heartbeats are periodic and can be identified from radar echoes through a series of algorithm processing. In this article, we focus on establishing an approximate equation of radar wave propagation in coal and try to solve it. So there are no examples of measured data in this article. As a response to this comment, we add a set of actual radar data in the attachment. The schematic diagram of the experiment is shown in Figure 1 (In the file of "Response to reviewers"). In future works, we will research on measured data and try to propose micro-vibration recognition algorithms for the life information. The detection direction of the radar system is shown in Figure 2 (In the file of "Response to reviewers"). The angle between the detection direction and the horizontal direction is smaller than 120 oC, while the angle between the detection direction and the vertical direction is smaller than 90 oC. According to the detection angle, this detection scheme can be realised by adjusting direction of the radar window.

3.We thank the reviewer’s comments. We found a lot of problems about English expression. We improved it according to the suggestion. The article was edited again by academic editing with the native language. Thank you again for your meaningful suggestions and comments.

Reviewer #2

We thank the reviewer carefully checked on this part. Thank you very much for your detailed suggestions and comments on the manuscript. We have carefully revised the manuscript based on the comments. Thank you again for your meaningful suggestions and comments.

Submitted filename: Response to reviewers.docx

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6 May 2020

Propagation of Ground Penetrating Radar Waves in Chinese Coals

PONE-D-19-29325R3

Dear Dr. Zhang,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

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--------------------------------------------------------------------------------------

Marco Lepidi, PhD

Associate Professor, Academic Editor of PLOS ONE

Dipartimento di Ingegneria Civile, Chimica ed Ambientale

Università degli Studi di Genova (Italy)

Mail: Marco.Lepidi@unige.it

Website: http://www3.dicca.unige.it/mlepidi/index.html

8 May 2020

PONE-D-19-29325R3

Propagation of Ground Penetrating Radar Waves in Chinese Coals

Dear Dr. Zhang:

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on behalf of

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Academic Editor

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