Mining-induced ground fissures are common issues associated with mining activities, leading to significant surface damage and secondary disasters (Fathi Salmi et al. 2017; Cai et al. 2023; Malinowska 2016). This problem is particularly prominent in central and western China, where coal deposits in this area are characterized by shallow burial depth, large thickness, thin bedrock, and thick loose layers. Besides, most mines in this region adopt top-coal caving mining and large mining height technology (Wang et al. 2018; Guo et al. 2020), resulting in intensified surface damage and severe environmental disasters. For instance, ground fissures often lead to disasters such as decreased groundwater table (Bian et al. 2012), vegetation loss, reduced surface vegetation cover, and land desertification (Avera et al. 2015; Bi et al. 2019). Furthermore, fissures cause extensive damage to surface infrastructure, including houses, arable land, roads, and bridges (Diao et al. 2019; Kalogirou et al. 2014). Additionally, ground fissures pose a severe threat to coal production, particularly in the case of the shallow buried coal seams in the western region. Ground fissures can connect to the mining void area or even goaf, lead to sudden collapse of sand, coal seam spontaneous combustion and other disasters (Zhuo et al. 2018). Therefore, understanding the development and damage evaluation of mining-induced ground fissures is essential for effective management.

Most scholars have studied the mechanism of ground fissure formation based on the key strata theory. They have concluded that shallow buried coal seams with a depth-thick ratio of less than 30 tend to produce fissures at the surface by breaking of key layers (Zhou et al. 2019; Li et al. 2017). Moreover, some scholars have analyzed the mechanism of ground fissure formation using geotechnics theory and have concluded that fissures are generated when the surface deformation reaches the limit deformation value of the soil (Xu et al. 2017). Additionally, the dynamic development law and distribution of the ground fissures were analyzed concerning the working face advancement (Chen et al. 2019). Based on the location of ground fissures distribution, they are mainly divided into dynamic fissures and static fissures. Static fissures are distributed in a ring along the edge of the mining area, and static fissures do not experience self-healing. However, fissures in the middle of the working face undergo the process of opening and closing with mining (Kratzsch 1983; Hu et al. 2013). A review of current literature on the evolution of the ground fissures in the Shendong mining area was conducted to explore the timing of fissure self-healing. The ground fissures can exhibit single- or double-peak patterns depending on the mining geological conditions. The self-healing period generally lasts from 7.8 to 18 d.

Furthermore, various field observation methods have been employed to monitor ground fissures in mining fields. Mainly include leveling and total station measurements (Kharisova and Kharisov 2021; Tiwari et al. 2020), while the workload of manual surface cracks during long periods of subsidence is large and high cost. GPS/GNSS positioning technology has also been employed to measure the ground fissures by installing the equipment on both sides of the fissures (Gao and Hu 2009; Zhao et al. 2013). However, the observation systems are very complicated and expensive. Synthetic aperture radar interferometry techniques (D-InSAR / PS-InSAR / SBAS) have also been reported to monitor ground fissures (Liu et al. 2019; Yang et al. 2018, 2019). However, it accounts for phase unwrapping errors under the influence of large subsidence and ground plants. 3D laser scanning technology has advantages, such as it does not need to bury fixed observation points, and it has large scan range. However, it is known to be limited by the viewing angle or terrain (Ge et al. 2019; He et al. 2022).

UAV remote sensing technology has been widely used because it has great advantages of fast, high resolution, and a large amount of data acquired in real time (Yan et al. 2022; Zhang et al. 2021; Zhao et al. 2022). It has been widely used in environmental monitoring (Thorpe et al. 2016), modern agriculture (Zhou et al. 2021), open-pit mine slope stability monitoring, and monitoring of surface subsidence (Wang et al. 2022; Jiang et al. 2022). However, in some cases, visual observation of fissures will be blocked by plants, sand, and other ground features (Zhang et al. 2020). Objects with a temperature of more than 0 ℃ can emit infrared radiation, which could go through the ground features and be captured by an infrared imager. UAV infrared remote sensing technology for monitoring ground and fissure temperature has been known and reported. For instance, coal spontaneous combustion monitoring (Nádudvari et al. 2021), fissures and defect monitoring (Afshani et al. 2019; Baroň et al. 2014; Tavukçuoğlu et al. 2010; Xu et al. 2021).

However, none of these studies have focused on the relationship between overburden movement and ground feature’s (sand, fissures, and plants) temperature evolution as the working face advances. The ground fissures are in a dynamic development process of ‘formation-expansion-restoration’ due to the influence of mining, resulting in varying ground fissure temperatures at different locations of the lagging working face. The development of ground fissures inevitably leads to the loss of water vapor within the fracture of the broken overburden, resulting in a change in fissure temperature (Zhao et al. 2024). This phenomenon provides an opportunity for the observation of fissures using infrared technology. For instance, visible and thermal infrared cameras were used to study the law of ground feature’s temperature change in specific ground surface areas and certify the efficacy of infrared monitoring as an effective observation method (Zhao et al. 2021a, b). Therefore, this study aims to study the temperature evolution of ground features under the influence of mining advancement and to evaluate ground damage by combining the temperature evolution of fissure and overburden movement. It increases the understanding of the temperature evolution of ground features under the influence of shallowly buried coal seam mining.

This study was conducted at the 12203 working face of the Huojitu colliery, situated in Shenmu City, Yulin City, China. The Huojitu mine is a large modern facility with an annual production capacity of 10 million tonnes. The mine adopts the longwall top-coal caving mining method, with 12301 and 12305 working faces on the east and west sides, respectively. The adjacent 12201 working face has been completely mined out, while the 12205 working face remains unexplored. The working face is arranged along the dip direction of the coal seam, with a strike length of 1577.8 m and a working face length of 251.4 m. The coal seam has a simple structure with an average thickness of 6.0 m (4.8 to 10.6 m). The elevation of the coal seam floor ranges from 1108.6 m to 1124.4 m, while the ground surface elevation varies between 1167.5 m and 1217.06 m. For a comprehensive overview of the 12203 working face layout, refer to (Figs. 1a-b). The generalized borehole columnar section ((Fig. 1(c)) illustrates that the burial depth of the 12203 working face is approximately 64.74 m. The advancing distance is 481 m by 23 October 2020. Specific parameters of the working face are listed in Table 1.

Overview of No.12203 working face. a Roadway layout of No. 12203 working face b Bird’s-eye view of the ground surface of No.12203 working face c The borehole histogram of No.12203 working face

The field measurements study is mainly divided into two components: manual field measurement and UAV infrared photography. The manual field measurement involves monitoring ground fissures and hydraulic support resistance, as well as conducting UAV infrared aerial photography. The location of O-A1, O-A2, working face, and the location of ground fissures was determined using an RTK-GNSS receiver. The evaluation of the ground damage is based on the strata movement and infrared aerial remote sensing, and follows the flowchart in Fig. 2.

Flow chart of the evaluation method for mining ground fissures and overburden movement

To observe the development of ground fissures and the evolution of ground features temperature during underground mining, an observation station was established on the ground above the 12203 working face. This station consisted of two observation areas, O-A1 and O-A2, as depicted in Fig. 3. O-A1 is a rectangular area measuring 40 × 80 m, located above the boundary of the subsidence area. Its strike direction exceeds 10 m (+ 10 m) beyond the working face and lags the working face by 70 m (− 70 m). Additionally, the dip direction extends 20 m from the boundary of the haulage roadway to the side of the mining goaf and coal pillar, respectively. Alternatively, O-A2 is a rectangular area measuring 40 × 80 m, situated above the middle of the subsidence area. Its length direction exceeds the working face by 10 m (+ 10 m) and lags the working face by 70 m (− 70 m). The strike center line of the O-A2 observation area is offset by 82.5 m from the boundary of the haulage roadway to the side of the mining goaf.

Ground fissures monitoring method a Equipment for ground fissures monitoring b Distribution of surface observation area c The size and the shape of the observation area A1 and A2, temperature measurement line layout

3.1 Equipment and data collections

A Trimble GEO 7X RTK-GNSS receiver was employed to locate the working face No. 12203 and ground fissures in O-A1 and O-A2. A DJI M600Pro Unmanned aerial vehicle (UAV) equipped with a dual-real time kinetic (D-RTK) GNSS served as a platform for carrying the FLIR Duo Pro R camera, used to capture ground surface images. One of the key features of the FLIR Duo Pro R is scalable thermal fusion, which involves blending thermal and digital images and enabling picture-in-picture fusion to display thermal images superimposed on simultaneously recorded digital images. The main technical parameters and specifications of the UAV and FLIR Duo Pro camera are presented in Tables 2 and 3, respectively. To verify and calibrate the accuracy of the infrared images, a thermal hydrograph and an anemograph were used to measure the fissures, sand, atmospheric temperature, atmospheric humidity, and wind speed at various ground locations, taken 10 min before each aerial photograph.

3.2 Ground fissures observation

In O-A1 and O-A2, the width and fissures height difference of the fissures were measured using a steel ruler with a precision of 1 mm. Additionally, a hand-held RTK-GNSS receiver was utilized to accurately record ground fissures’ location and extension track. Combining the location of the working face and daily advanced distance, the relationship between the overburden fracture and ground fissures evolution was analysed. For O-A1, dynamic monitoring points were placed at 1 m intervals along the main fissure b1, amounting to 70 monitoring points. In O-A2, 14 main fissures, including one forefront fissure k1 and 13 dynamic fissures (k1, m1-m13) were identified. Each fissure along the strike midline of O-A2 was assigned one monitoring point.

3.3 Hydraulic support resistance monitoring

The hydraulic support resistance at the working face can reflect the overburden movement and the overburden fractures. By collecting and analyzing the resistance curve of the 12203 working face, we obtained information on the weighting position, periodic weighting step distance, and weighting duration distance at O-A1 and O-A2 of the shallowly buried coal seam, as well as the corresponding ground fissures position and fissure temperature were obtained. A total of 140 hydraulic supports are arranged along the working face. We collected mine pressure data at 10# support and 55# support. The hydraulic support resistance was collected within 75 m of the lagging workings in O-A1 and O-A2, respectively. Based on the above data, we analyzed the location of the overburden fractures, support weighting intensity, and weighting duration at the working face.

3.4 UAV field survey measurement

UAV infrared photography measurements were conducted at 13:00 on 23 October 2020, under sunny weather with stable atmospheric temperature and air speed. The flight height was maintained at 20 m. The ground resolution was 1.3 cm/pix. Two observation areas, O-A1 and O-A2, were selected for UAV photography, obtained 39 images and 77 images, respectively.

Figure 3 illustrates the distribution of the observation areas and the layout of the temperature measurement lines for the 12203 working face. The center line of the strike direction in the middle of O-A1 and O-A2 is used as the coordinate axis to establish the fissure temperature extraction line, respectively. The zero points correspond to the position of the working face. The positive direction along with the advance direction of the working face. Both O-A1 and O-A2 had a temperature measurement line extending 70 m. Along these lines, the temperature of fissures was extracted at various positions.

3.5 Temperature inversion and calibration

After the infrared camera receives the infrared radiation, it will be converted into a voltage signal based on the radiation intensity. Subsequently, the voltage signal will be transformed into a digital signal, capturing the infrared brightness images. Through temperature inversion, the actual temperature of the object can be obtained. The infrared radiation received by the infrared camera typically comprises three components: the thermal radiation of the object, thermal radiation of the atmospheric environment, and thermal radiation received by the infrared camera after the atmospheric reflection by the ground surface. Using the brightness temperature images obtained by UAV infrared remote sensing, the temperature of the object is inverted using the radiation conduction equation (Jiménez Muñoz and Sobrino 2003; Wu et al. 2016; Zhao et al. 2022):

where L_λ is the thermal infrared radiation energy received by the infrared camera, \({\varepsilon _\lambda }\) is the ground radiance, \(B\left( {\lambda ,{T_{\text{S}}}} \right)\)is the blackbody radiation brightness at temperature T_s, T_s is the true temperature of the ground features, T is the transmittance of the atmospheric environment, which can be acquired from NASA’s official website by providing the imaging time, central latitude and longitude. \({L_ \uparrow }\), \({L_ \downarrow }\)is the up-welling and down-welling radiance. The expression for \(B\left( {\lambda ,{T_{\text{S}}}} \right)\) can be given by the Planck’s law:

where c₁ = 1.19104·10⁸ w µm⁴ sr^− 1 and c₂ = 1.43877·10⁴ μm K. A relationship between radiance temperature and true temperature of ground features can be calculated using Eq. (3):

where, K₁, K₂ is a constant, respectively. The temperature inversion was performed using FLIR Tools in this paper, a professional infrared processing software. The infrared temperature map can be directly obtained by inputting the temperature and humidity of the mining fissure, along with atmospheric temperature, humidity, wind speed and the specific emissivity parameters of the ground material. The field manual observation data can also be calibrated to match the UAV infrared observation results.

4.1 Dynamic development pattern of ground fissures

4.1.1 Ground fissures distribution characteristic

The distribution of mining-induced ground fissures in the 12203 working face is shown in Fig. 4. The ground surface of O-A1 is affected by the coal pillar of the adjacent working face, leading to the development of permanent static fissures parallel to the strike direction of the working face advancement. The fissures in areas O-A1 remain in a tension and shear state. The static fissure length, fissure height difference and fissure width increased with working face advancement. The maximum fissure height difference of the static fissures reaches nearly 1.78 m, and the maximum width is approximately 1.2 m. Many tension-type fissures, parallel to the strike direction, emerge around the primary static fissure. The distribution of tension-type fissures along the dip direction ranges from 30.0 to 32.0 m. Consequently, ground fissures in O-A1 are the primary focus of fissure management.

Diagrams of mining induced ground fissures of No.12203 working face

In the O-A2, the ground surface experiences full subsidence with mining advancement, causing rotary destabilization and overburden fractures. The ground fissures in this area follow a ‘formation-expansion-restoration’ development pattern of with working face advancement, while the ground surface deformation remains relatively moderate. Dynamic fissures develop in O-A2, perpendicular to the direction of mining advancement, with fissure spacing ranging from 5 to 6 m and a maximum fissure width of 15 cm. Furthermore, many small tensile fissures are formed between two dynamic fissures along the length of the working face.

4.1.2 Fissures dynamic developmental features

The relationship between the location of the working face and the forefront dynamic fissures is provided in Table 4. These data are combined with the daily advancing distance and location of the working face, as well as the main fissure location on the surface during the corresponding monitoring period. The overburden experiences mining damage, leading to a notable trend of high-angle development of the average forefront fissure angle. During each monitoring period, there was a significant difference between the new fissures and the position of the underground working face, with a distance ranging from 4.1 m to 6.8 m. The average angle of the forefront fissures reached 85.38°. The distance to the forefront fissures showed a negative correlation with increasing daily advancing distance.

4.1.3 Fissure dynamic variation characteristic

The main characteristic of the dynamic variation of the fissure width are summarized in Table 5. The fissures in O-A1 demonstrate a significant parabolic developmental process. With working face advancement, the fissure width and height difference continue to increase before stabilizing. The dynamic development time of fissures is approximately 4 to 5 days. Figure 5 illustrates the dynamic variation characteristic of the main fissure width in OA-2. The fissure development process is closely related to overburden fracture and surface subsidence. Fissures at different locations on the ground surface in O-A2 exhibit an evident single-peak dynamic development process. Besides, the average development time of the fissures from generation to stability at seven monitoring points (m1 to m7) is 8.8 d. Moreover, the time of fissure width increase is 4 d on average, and the time of width reduction until stability is 4.3 d on average. This suggests that the time needed for the two dynamic change states of the fissure width from increase to decrease are approximately equal.

Development of dynamic fissure width in the region of O-A2

4.2 Rules of the ground features temperature evolution in O-A1 and O-A2

4.2.1 Infrared images of the observation area

The infrared images obtained in the different observation areas and locations are shown in Fig. 6. In observation areas O-A1 and O-A2, sand appears as bright red or yellow, signifying high temperature (Fig. 6a). Fissures, plants, and ground feature shadows are represented by green or even blue, indicating lower temperatures. Additionally, the colour of fissures is darker than that of plants. In observation area O-A1, fissures with larger widths are more easily distinguishable than those with smaller width. Conversely, in O-A2, the colour of the fissures closely resembles that of plants, making it difficult to differentiate fissures from the surrounding objects (Fig. 6b).

Infrared temperature characteristic at different locations on the surface of No. 12203 working face. a In the region of static fissures zone (O-A1) b In the region dynamic fissures zone (O-A2)

4.2.2 Quality evaluation of radiative temperatures

The ground surface is mainly covered by sand and sparse plants. However, there is still no effective manual equipment to directly measure plants temperature. Hence, only the errors between the infrared temperatures and the manually measured temperatures of sand and fissures were considered in this study. In each observation area, 30 pixel points were randomly selected from the infrared radiation images for each ground feature at each fissure location along the temperature measurement line, and the corresponding temperature values were extracted to assess the accuracy of the radiative temperatures.

According to Table 6, the average relative errors between the radiative temperature and the manual temperature of sand in O-A1 are -3.40%, -3.49%, 0.19%, -3.37%, -3.85%, -0.29% and -1.59% at locations of +10, 0, -10, -20, -30, -40 and -50 m, respectively. The average relative errors of fissure in O-A1 are -6.88%, 10.51%, -5.66%, 2.42%, 18.25%, -6.89% and -4.39%, respectively. Similarly, the average relative errors between radiative temperature and manual temperature of sand in O-A2 are -3.40%, -5.52%, -6.14%, 4.44%, -4.69%, 3.92% and -1.78% at locations of +10, 0, -10, -20, -30, -40 and -50 m, respectively. The average relative errors of fissure in O-A2 are 4.16%, 2.71%, 7.77%, 3.74%, 6.16%, 4.23%, and 4.62%, respectively.

Notably, the largest large errors frequently happened under high wind speed, especially at the location of +10, -30 m in O-A1 and -30 m in O-A2. The max relative error can reach at -3.40%, -3.85%, and -4.69% for sand, and -6.88%, 18.25%, and 6.16% for fissures. However, the errors of ground fissures are generally larger than that of sand. It is likely that the sand is under more stable situation than ground fissures. The ground fissures are intended to be influenced by the vapour transfer in overburden fractures under the mining advancement. while the minimal is -5.57 ℃ in this study (Table 6). The largest absolute must less than the minimum temperature difference. It is apparently that the largest absolute error (-1.93 ℃) can meet the minimal value of temperature difference. Moreover, based on the data analysis the temperature evolution trend still can be clearly figured out.

4.2.3 Radiative temperature characteristic and variation pattern of typical ground features

The surface temperature of the working face in O-A1 and O-A2 exhibited different variation patterns. At each location, infrared radiative temperature images with typical features were extracted before, during, and after the influence of mining advancement. The temperature of three main ground features (sand, plants, ground fissures) was extracted (Fig. 7) to analyse the temperature evolution pattern. The temperature changes from the range +10 m to -60 m in O-A1 and O-A2 followed an approximately quadratic function curve, with the ground temperature first decreasing and then increasing. During the observation period, sand temperature > plant temperature > fissure temperature in both O-A1 and O-A2.

Temperature evolution patterns of different features on the ground under the influence of mining advancement a In O-A1 (static fissures zone) b In O-A2 (dynamic fissures zone)

The temperature evolution patterns of the static fissure at different ground locations of O-A1 are shown in Fig. 7a. The temperatures of the fissures, sand and plants followed a quadratic function variation pattern in O-A1. They gradually decreased within the surface range from +10 to 0 m directly above the working face, reaching the lowest temperature of 9.0, 17.4, and 15.9 ℃ at approximately 0 m, respectively. Within the surface range of 0 to -31 m, the features in the O-A1 exhibited a significant decreasing trend, with the lowest temperature of ground features at -31 m reaching 5.8, 24.2, and 12.1 ℃ for fissures, sand, and plants, respectively. The temperature of surface fissures, plants, and sand rapidly increased from -31 to -38 m. Within the range of -38 to -58 m behind the working face, the temperature of the ground features returned to a stable value, with fissure, sand, and plant temperatures of approximately 20.6, 32.56, and 26.5 ℃, respectively.

The temperature of ground features within the range of +10 to -60 m in O-A2 is shown in Fig. 7b. The average radiative temperature of ground fissures, sand, and plants in O-A2 followed a quadratic change pattern of first decreasing, then increasing and eventually stabilizing. The average radiative temperature of sand was higher than that of plants and fissures during the field measurements. The temperature of the fissures, sand, and plants gradually decreased from + 10 to 0 m, reaching the lowest value at 0 m of the working face: 17.7, 24.7, and 16.6 ℃, respectively. The temperature of the ground fissures, sand, and plants gradually increased from 0 to -35 m, and the temperature of the surface fissures, sand, and plants reached relative stable value at -35 m: 20.5, 33.5, and 24.5 ℃, respectively. Afterwards, the temperature of the surface fissures, sand and plants gradually stabilized between -35 and -56 m, with fissures, sand, and plant temperatures approximately 21.3, 33.52, and 24.5 ℃, respectively. Moreover, dynamic surface fissures with relatively small widths and high temperatures developed in O-A2, and multiple small high-temperature fissures developed between each main dynamic fissures.

The distribution characteristic of the surface temperature range are shown in Fig. 8. The surface temperature affected by mining can generally be divided into three zones: the temperature stabilization zone before mining, the temperature reduction zone during mining, and the temperature recovery zone after mining. The surface temperature in O-A1 and O-A2 exhibited significant differences with obvious zoning characteristic. OA-1 was characterized by a large temperature variation range and low fissure temperature, while OA-2 was characterized by a small temperature variation range and high fissure temperature. Water vapor transport within fissures caused by mining advancement may be the main cause of the observed surface temperature variations.

Temperature range in different areas of the working surface a In O-A1 (static fissures zone) b In O-A2 (dynamic fissures zone)

The range of surface temperature corresponding to the static fissure zone in O-A1 is 24.0 to 27.0 ℃ at +10 m. The surface temperature range in the dynamic fissure zone of O-A2 is 18.4 to 26.5 ℃ at +10 m. O-A1 and O-A2 shown a high-temperature zone, and the corresponding temperature range difference is small.

The ground surfaces corresponding to O-A1 and O-A2 under the influence of mining exhibited an obvious cooling trend at 0 m. The highest and lowest temperatures of the ground surface in O-A1 were 9.0 and 17.4 ℃, respectively. The highest and lowest temperatures of the ground surface in O-A2 were 16.6 and 24.7 ℃, respectively. The temperature range in OA-1 is 13.2 to 23.0 ℃ at -10 m, with the lowest to highest temperature of 16.7 to 35.3 ℃ in O-A2 at -10 m.

The range of surface temperature in OA-2 at -30 m ranges from 16.8 to 35.4 ℃, and the surface temperature range in OA-1 is ranges from 6.8 to 25.2 ℃. The surface temperature in the middle of the mining area is higher than that in the haulage roadway area, and it shows an obvious low temperature zone at -30 m. The surface temperature at -40, -50, and -60 m in OA-1 and OA-2 rebounds and stabilizes, with the surface temperature in O-A1 ranges from 21.2 to 34. 8 ℃, while the surface temperature in O-A2 ranges from 20.5 to 33.9 ℃.

4.2.4 Effect of the fissure size on the temperature evolution in O-A1

Ground fissures connected to the mining goaf lead to the flow of water vapour, which further causes surface temperature changes. To analyse the mechanism of temperature variation based on the surface fissure size, we extracted the width and the height difference of the main static fissure in O-A1. We compared and analyzed the influence of fissure size on the fissure temperature evolution.

4.2.4.1 Influence trend of the fissure width on the temperature evolution

As shown in Fig. 9, the fissure surface temperature in O-A1 exhibited a temperature reducing zone, a temperature increase zone, and a temperature stabilization zone. The fissure width gradually increased and then stabilized with the working face advancement. Firstly, the ground fissure was generated 3 m ahead of the working face, then the fissure width gradually increased from 0 to -20 m, reaching a maximum of 121.5 cm at -20 m and remaining stable between -20 m and -58 m in OA-1.

Temperature evolution influenced by fissure width and fissure difference in O-A1

A comparison of the fissure width and fissure temperature curves in O-A1 reveals that the fissure temperature gradually decreased with increasing fissure width within the +10 to -20 m range. The fissure width remained constant, and the fissure temperature continued to decrease within the -20 to -30 m range. The fissure temperature rapidly recovered from -30 to -38 m, while the fissure width remained stable. However, within the range of -38 to -60 m, the fissure width was not the primary factor lead to the decrease in fissure temperature. Although the fissure width influenced the fissure temperature, the transportation of water vapor in fissures with different fissure widths removed some of the water vapour, leading to a temperature decrease. When the fissure width remained stable, as observed within the ranges of -20 to -30 m and − 30 to -38 m, the fissures continued to cool rapidly and abnormally.

4.2.4.2 Influence pattern of the fissure difference on the temperature evolution

The main fissure height difference in the surface area of O-A1 showed an initial increasing and subsequent decreasing trend. At + 3 m, the fissure height difference was -6.15 cm, and it gradually increased from +3 to -38 m in OA-1, reaching a maximum of -178 cm at -38 m. From -38 to -58 m, there was a gradual decrease in the fissure height difference. The soil layer near the surface of the road is hard within the -38 to -58 m, and the fissure width is larger. At -58 m, the fissure height difference was -43 cm, the overburden was rotated but did not experience integral cut-down, and the fissure height difference was small.

Via comparing of the fissure height difference and fissure temperature, the fissure temperature gradually decreased with increasing fissure height difference within +3 to -30 m. At -4 and -18 m, the fissure temperature abruptly decreases due to the sudden increase in the fissure width and height difference. The temperature reached a minimum of 6.8 ℃ at 30 m. This finding shows that the fissure width was not the main factor influencing the reduction in the fissure temperature. Conversely, the continuous cooling in OA-1, with a slight change in the fissure width from -20 to -30 m, was primarily due to the fissure penetrating the water vapour circulation channel between the mining goaf and overburden fracture. The stratum settlement did not cease after cutting along the coal pillar. The increasing trend of the fissure height was moderate within the -30 to -35 m. However, the fissure temperature increased rapidly at this stage, reaching 20.6 ℃ at -38 m. This increase was influenced mainly by the later stage, which exhibited less mining influence and self-healing of the surface fissures within -35 to -60 m. The damage to the overburden gradually decreased, and the surface fissure temperature fluctuated with 5 ℃.

The upmentioned finds indicated that the fissure temperature was closely related to the fissure difference at the early stage of mining, with the fissure temperature continuously decreasing with increasing fissure difference. In O-A1, the fissure temperature characteristic included a small width, a slight drop, and a low temperature within 0 to -30 m. The main reason for the temperature decrease was the continuous movement of the broken strata, which caused the fissure conduction channel to remain open, and connected with the stable underground water source and the underground ventilation system. During the mining influence period, the main factor affecting the temperature was the fissure difference resulting from overburden cut-off movement. During the later period of mining, the temperature was mainly impacted by the evolution of ground fissures.

4.3 Mechanism of overburden fractures on the fissure temperature evolution

4.3.1 Mechanism of overburden fracture and ground fissure generation

The relationship between the temperature evolution and ground fissures provides an ideal prospective to evaluate the mining ground damage. Moreover, the breakage of overburden strata and ground fissures are closely related. At a mining depth to mining height ratio of H/M ≤ 30, overburden fractures generally develop from the goaf to the surface with the working face advancement, resulting in the formation of ‘two zones’, namely the caved zone and the fractured zone (He et al. 2021).

Field measurement of hydraulic support resistance a 10# support b 55# support

The change pattern of the hydraulic support resistance can reflect the periodic breaking characteristic of the overburden. When the overburden strata break, weighting on the working face support emerges. The field measurement of 10# support resistance is shown in Fig. 10. As the working face is advanced and the overburden breaks, the working face experiences weightings within 0 to -58.0 m: at locations of -0.7, -8.2 to -10.8, -16.5 to -20.3, -27.8, -33.0 to -34.8 and -46 m.

The field measurement of 55# support resistance is shown in Fig. 10. The working face experiences weightings within 0 to -51.2 m, specifically at locations of -0.7 to -2.1, -7.2 to -10.8, -17.2, -24.9, -33.4 to -34.8, and -41.8 m. The locations of ground fissure can be determined from the corresponding weighting position at -5.2, -10.8, -19.6, -29.7, -38.5, and -43.7 m, respectively. The periodic distance of ground fissures is generally equal to the periodic breaking distance of the key strata, which verifies that the periodic breaking of key strata in the working surface causes overburden fracture directly reach at the ground surface.

4.3.2 Relationship between overburden fractures and ground fissure temperature

The locations of the ground fissures and fissure temperatures in OA-1 are provided in Table 7. The fissure temperatures are 13.4, 8.9 to 13.5, 11.25 to 15.5, 8.9 , 6.3 to 16.8, and 19.85 ℃ at the weighting locations of -0.7, -8.2 to -10.8, -16.5 to -20.3 -27.8, -33.0 to -34.8, and -46 m on the ground, respectively. Notably, a significant weighting occurs at the working face at − 0.7 m due to mining influence. The length of the advanced temperature influence range is + 5 m. The ground features are influenced by the static fissures at O-A1, and the temperature of the surface fissures, plants, and sand starts to decrease at + 5 m to 0 m. The temperature reduction area extends from 0 to − 34.8 m, and a total of 5 weighting cycles occurred in the working face.

The surface temperature measurement line in OA-2 corresponds to the 55# support. The fissure temperatures are 16.73, 18.0, 18.05, 23.75, 20.58, and 22.5 ℃ at the ground fissure location of -0.7 to -2.1, -7.2 to -10.8, -17.2 m, -24.9, -33.4 to -34.8, and − 41.8 m, respectively. One major weighting occurs at the working face within the 0.7 to 2.1 m, and 5 cycles of weightings occurs at the workings points within 0 to − 41.8 m range in the temperature reduction zone.

Previous research shown that the fissure development process is closely tied to periodic overburden breakage, especially during mining of shallow buried coal seams. Based on field measurement, the periodical weighting spacing ranges from approximately 5.0 to 6.0 m. The periodical ground fissure distance is approximately equivalent to the periodical weighting spacing of overburden. Combining the surface observation data of the overburden movement and dynamic fissure monitoring results, a quantitative model of the dynamic development time of fissures in the 12203 working face was established. The spatiotemporal relationship between ground fissure temperature development and overburden fracture in shallow coal seam mining can be modeled as shown in Fig. 11. p₁ to p₅ are different positions during advancement of the working face. S_c denotes the horizontal distance between the forefront fissure and the boundary of the working face, and can be calculated using Eq. (4).

where H is the burial depth of the coal seam and α is the forefront fissure angle.

The distance of the first dynamic fissure from the working face S_f can be calculated by Eq. (5):

where φ is the overburden fracture angle.

The range of surface temperature recovery zone, denoted as S_r is intricately linked to both the number of overburden weighting cycles and the overburden breakage step, according to the mine pressure theory (Qian et al. 2010), S_r can be expressed by Eq. (6):

where k denotes the number of key strata weighting cycles for temperature recovery, L denotes the periodical weighting distance, h is the thickness of the key strata, R_T is the ultimate tensile strength of the key strata and q is the overburden rock load on key strata.

The range of surface temperature reduction zone S can be expressed by Eq. (7):

With a working face advancement speed of 9 to 12 m/d, we can calculate the influence cycle T of the surface temperature using Eq. (8):

where v presents the working face advancement speed, v = 10 m/d in this study.

Spatiotemporal relationship between ground temperature development and overburden fracture in shallow buried coal seam mining

When the working face advanced to p₁, overburden weighting occurs at the working face, and the overburden fracture spreads directly to the ground. The distance of each main ground fissure is related to the weighting step of the overburden, and each weighting results in a step-like fissure at the surface. As the working face advances to the next cycle of weighting, ground fissures are periodically generated. After the working surface advanced more than 5 weighting cycles (position p₅), the fissures reach the max width and height difference, and the fissure temperature become stable.

4.3.3 Evaluation on temperature evolution of ground fissures

The temperature of the surface features has the characteristic of self-recovery as the working face advancing. The length of the surface temperature reduction zone in the 12203 working face under the influence of mining is 40 m, from +5 m to -35 m of the working face. The temperature recovery cycle of the dynamic fissure in OA-2 is calculated at an advancement rate of 10 m/d for about 4.0 d. The working face in OA-2 exhibits short pressure cycles and weighting steps. The temperature influence range indicates a small temperature influence range, and fast temperature recovery.

The temperature of the surface feature in O-A1 recovers from 5 cycles of overburden weighting, with the working face advanced 41.8 m. Additionally, the fissure temperature starts to decrease at +4.8 m due to the influence of the forefront fissure, and a recovery cycle of the static fissure temperature is calculated at an advancement rate of 9 m/d for about 4.6 d. Alternatively, the surface features in OA-1 displays contrasting feature of longer temperature influence distance, larger temperature influence range and self-healing time than O-A2. Notably, the self-healing time in O-A1 exceeds that of O-A2 by 13.7%.

The fissure evolution characteristic of ground fissures in different coal mines are listed in Table 8. It indicates that the duration of reaching max width and height difference generally varies from 1 to 7 days for different fissures, most of them from 4 to 6 days, our results are generally consisted with their findings. This finding prove the liability to assess fissure evolution by radiative temperature monitoring.

This study demonstrates that low altitude infrared UAV photography can effectively identify temperature abnormal areas, such as low-temperature zones. Additionally, the UAV infrared monitoring facilitate the identification of the working face’s mining position, advance direction, haulage roadway’s distribution position, the surface boundary fissure area, and ranges of dynamic fissure area. These findings provide new insights for damage evaluation of the mining area.

4.3.4 Shortcomings and prospects

This study investigated the temperature evolution of ground features under the influence of underground mining. Furthermore, we evaluate the fissures self-healing effect based on the fissure evolution, ground feature’s temperature, and overburden fracture. However, the radiative temperature accuracy of ground features is influenced by environmental conditions, for instance, atmospheric temperature and moisture, thermal conductivity, sunshine intensity, and among other factors. The effective method for quantitatively evaluating the impact of these factors on data errors need to further study. In addition, the fractures of overlying rock generally do not reach the surface in deep coal seam mining. The evolution of ground fissures temperature in different geological conditions still need to be studied. Furthermore, the observation of dynamic evolution of overburden fracture with mining advancement is still a great challenge. In future work, more on-site measurements and summary analysis are needed to conduct.

In this study, UAV infrared remote sensing technology and a hydraulic support resistance monitoring method were integrated to measure the temperature of ground fissure, sand, and plant in two observation areas, spatial and temporally. In addition, the self-healing of ground fissure was evaluated. The following conclusions can be drawn:

1. (1)

   The surface temperature in O-A1 and O-A2, within the range of +10 to -60 m, experienced three zones: a temperature stabilization zone before mining, a temperature cooling zone during mining and a temperature recovery zone after mining. The temperature decrease in O-A2 was smaller than that in O-A1, and the ground feature’s temperature in O-A2 was higher than that in O-A1.

2. (2)

   The temperatures of fissures, sand, and plants exhibited a quadratic curve change trend from +10 to -60 m in O-A1 and O-A2. The temperature reduction zone in area O-A1 was 41.8 m, with 5 weighting cycles, while the temperature reduction zone in O-A2 was 40.0 m, with 5 weighting cycles. The temperature recovery periods were 4.0 d and 4.6 d, respectively.

3. (3)

   Low-altitude UAV infrared photography can be used to detect the surface temperature anomalies, identify the mining position of the working face, and determine the type of ground fissures. This study can provide a basis for evaluating ground fissures in the mine areas.