**1. Introduction**

Since the founding of New China in 1949, nationwide construction has spurred demand for coal and the exploitation of coal resources. Currently, coal production in China accounts for almost half of the world's total production [1]. Since 2005, to improve the efficient utilization and safe mining of coal resources, China has carried out the effective integration of coal resources [2]. However, most coal mines with integrated resources exhibit problems, such as a serious lack of data on the old goaf, unclear distribution of the boundary area, unclear water area and quantity in the goaf, and difficulty carrying out geophysical prospecting work underground in small closed-pit coal mines, which have exposed serious hidden dangers to coal mine safety. The Pingshuo Dong open-cast coal mine is a typical example. Due to long-term disorderly mining, no-filling mining, rich mining, and poor abandonment of surrounding small coal mines, many underground goafs exist in the Pingshuo Dong open-cut coal mine, and most of these goafs have no recorded specific information, such as location and size. If the roof thickness of the goaf is less than the minimum safe thickness, a roof collapse can occur, which can cause surface subsidence

**Citation:** Zhang, S.; Guan, W.; Chang, S.; Meng, Q.; Dong, Y.; Chen, Q. Integrated Geophysical Prediction of Goaf and Water Accumulation in Pingshuo Dong Open-Cut Mine with Ultrashallow and High Drops. *Processes* **2023**, *11*, 1653. https:// doi.org/10.3390/pr11061653

Academic Editors: Feng Du, Aitao Zhou and Bo Li

Received: 16 March 2023 Revised: 15 May 2023 Accepted: 19 May 2023 Published: 29 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

or cracking, seriously threatening the safety of personnel and equipment working above the roof [3,4].

Geophysical prospecting methods commonly used in goaf and water accumulation include the shallow seismic method [5,6], transient electromagnetic method [7–9], highdensity electrical method [10,11], magnetotelluric method [12,13], and ground-penetrating radar [14,15]. Various geophysical methods have achieved certain effects in detecting goaf and water accumulation, but each geophysical method has its own applicable conditions and advantages. The shallow seismic exploration method allows continuous measurement, and the range of goaf detected is more accurate and reliable, but it can hardly reflect the accumulated water of the goaf area [16]. The transient electromagnetic method is especially sensitive to low resistance and has a good effect in detecting goafs with accumulated water, and the detection depth is relatively large, but it is greatly affected by electromagnetic interference. The high-density resistivity method has a better detection effect on goaf water accumulation, but it is affected by topography and grounding resistance [17]. The magnetotelluric method is effective at detecting water accumulation in goafs, and the detection depth is large, but it can be affected by static effects and near-field effects. Because a single geophysical method is limited, the geophysical interpretation often exhibits multiple solutions and low reliability [18,19]. In recent years, many studies and applications have been carried out for integrated geophysical methods [20–23]. Integrated geophysical prospecting can obtain more physical information, verify and supplement other methods, reduce the multiple solutions of a single geophysical prospecting method, and improve the resolution of exploration and the reliability of interpretation results. However, for open-pit mining areas, the terrain conditions are more complicated, the coal seams are very shallow, and there are high-drop steps (transport lines). For goaf distribution and water accumulation prediction, the high-density electric method cannot meet the condition of the line layout, the ground-penetrating radar detection depth is insufficient, and the shallow seismic acquisition coverage is uneven. How to achieve higher resolution and better exploration results has not been systematically discussed in depth. This paper takes the prediction of the goaf distribution and water accumulation of the Pingshuo Dong open-cast coal mine as an example. Aiming at this type of problem, this paper proposes combining small-array high-density acquisition, multidomain joint denoising, and bin homogenization, weakening the influence of high-drop steps on shallow seismic exploration, and then using the multiple-attribute method to predict the goaf. Based on the electromagnetic environment analysis and topographical features of the study area, the transient electromagnetic method was used to identify goafs. The practical application shows that the high-density 3D seismic and transient electromagnetic comprehensive interpretation can effectively identify the goaf and water accumulation characteristics of an ultrashallow high-altitude open-pit mine. This proves the feasibility and application of the proposed method and can provide a meaningful reference for research in similar areas.

#### **2. Geological Background and Engineering Geological Conditions**

The study area is located in the northeastern margin of the Ningwu coalfield in Shanxi, China. Most of the surface is covered by Cenozoic strata, which are typical loess hilly landforms. There are sporadic strata bedrock outcrops near the coal-seam outcrops in the northeastern part of this area and at the bottoms of major gullies in this area. The study area is located on the southwest side of the eastern open-cast coal mine and its surrounding area, and the exploration area is approximately 0.35 km2, the scope is delineated by a red circled. as shown in Figure 1.

The Pingshuo Dong open-cast coal mine is located in the low hills of the Pingshuo Platform in the Shanxi Loess Plateau. The whole area is mostly covered with loess. The loess plateau in this area has been strongly eroded and cut. The valley is relatively developed in a "V" shape with a cutting depth of 40–70 m. The highest point in the area is located in the northern part of Shifeng, with an altitude of 1537.99 m. The lowest point is located at the Maguan River in western Xuegaodeng village, with an altitude of 1213.8 m, and the maximum relative height difference is 324.19 m. The ground surface in the study area has been stripped, showing a step-like shape, with an elevation of 1190–1380 m, a minimum of 1190 m in the northeast corner, a height of 1380 m in the south, and a relative height difference of 190 m. Some areas are the main roads in the southern part of the mine, and the traffic is more intensive, as shown in Figure 2.

**Figure 1.** Topographic map of the Pingshuo Dong open-cast coal mine.

**Figure 2.** Topographical photos of the study area, (**a**) high drop, (**b**) narrow platform, (**c**) high voltage line, (**d**) complex noise environment.